,,the ,,thought ,,is ,,in ,,the ,,question ,,the ,,information ,,is ,,in ,,the ,,answer ,illustration3 ,,hawkins ,,electrical ,,guide ,,number ,,six ,,questions ,,answers ,,illustrations ,a ,,progressive ,,course ,,of ,,study ,,for ,,engineers1 ,,electricians1 ,,students ,,and ,,those ,,desiring ,,to ,,acquire ,a ,,working ,,knowledge ,,of ,,electricity ,,and ,,its ,,applications ,a ,,practical ,,treatise by ,,hawkins ,,and ,,staff ,illustration3 ,,theo4 ,,audel ,,co4 #gb ,,fifth ,,ave4 ,,new ,,york4 ,,copyrighted1 #aiad1 ,,by ,,theo4 ,,audel ,,co41 ,,new ,,york4 ,printed in the ,united ,states4 ,,table ,,of ,,contents ,,guide ,,no4 #f4 ,,alternating ,,current ,,motors #a1bfg to #a1cgf ,classification--synchronous motors--essential parts--synchronous motor principles3 condition for starting2 effective pressure2 dead centers2 speed2 limit of lag2 effect of load changes--effect of altering the field strength--disadvantages of synchronous motors2 advantages--the ,v curve--adaptation--efficiency--hunting of synchronous motors2 mechanical analogy--use as condenser--surging--characteristics of synchronous motors3 starting2 running2 stopping2 effect upon circuit2 power factor2 auxiliary apparatus2 adaptation--induction 7asynchronous7 motors--essential parts--types--oscillating magnetic field--rotating magnetic field--operation of single phase motor2 why not self starting2 provision for starting--operation of polyphase induction motor2 why called asynchronous--speed2 classification according to speed--the terms primary and secondary--why polyphase induction motors are explained before single phase--polyphase induction motors--features--essential parts--principles--production of rotating field--,tesla's rotating field--method of obtaining resultant flux of ,tesla's field--,arago's rotations2 explanation--,faraday's experiment--production of two phase rotating field2 resultant poles--six and eight pole two phase rotating fields--physical conception of two phase rotating field--production of three phase rotating field2 with ring winding--physical conception of three phase rotating field--three phase six pole winding--slip--copper cylinder illustrating principle of operation of induction motor--calculation of slip--table of synchronous speeds--variation of slip2 why so small2 variation with load2 table of variation--sector method of measuring slip--evolution of the squirrel cage armature2 construction--the field magnets2 parts2 construction--field windings for induction motors--calculation for revolutions of rotating field2 objection to high speed of field--difficulty with low frequency currents--general character of field winding--formation of poles--grouping of coils--starting of induction motors3 external resistance1 auto-transformer1 internal resistance methods--internal resistance induction motors2 adaptation--how resistance is cut out--why not desirable for large sizes--external resistance or slip ring motors--operation--armature connections--single phase induction motors--service suitable for--disadvantage--parts--why not self-starting--how started--phase splitting2 production of rotating field from oscillating field--methods--starting coils--shading coils--character of the starting torque--modification of armature for starting with heavy load--clutch type of single phase induction motor2 its action in starting--commutator motors--classification--action of closed coil rotating in alternating field--the transformer pressure--generated pressure--self-induction pressure--local armature currents2 reason for sparking2 how reduced--high resistance connectors--effect of low power factor--effect of frequency--series motor--features--adaptation--neutralized series motor--conductive method--inductive method--shunt motors--repulsion motors--difficulty with early motors--means employed to stop sparking--essentials of single repulsion motors--the term repulsion induction motor--compensated repulsion motor--power factor of induction motors--its importance--false ideas in regard to power factor--speed and torque of motors4 ,,transformers #a1cgg to #a1def ,their use--essential parts--basic principles--the primary winding--the secondary winding--magnetic leakage--the induced voltage--no load current--magnetizing current--action of transformer with load--classification--step up transformers--use--construction--copper economy--step down transformers--use--construction--core transformers--construction--advantages-- shell transformers--comparison of core and shell types--choice--combined core and shell transformers--economy of construction--single and polyphase transformers--features of each type--choice of types for polyphase currents--operation of three phase transformer with one phase damaged--transformer losses--hysteresis--what governs the loss--how reduced--eddy currents--lamination--thickness of lamin--importance of iron losses--how to reduce iron losses--copper losses--how caused--effect on power factor--effect of resistance--cooling of transformers--cooling mediums employed--heating of transformers--objection to heating--dry transformers--air cooled transformers--natural draught type--forced draught or air blast type--construction of coils for air cooling--requirements with respect to air supply--quantity of air used--oil cooled transformers--circulation of the oil--action of the oil--objection to oil--kind of oil used--oil requirements--moisture in oil--water cooled transformers--internal coil type--external coil type--thermo-circulation--quantity of circulating water required--transformer insulation--the major and minor insulation--mica--outdoor transformers for irrigation service--oil insulated transformers--efficiency of transformers--efficiency curve--all day efficiency of transformers--transformer fuse blocks--auto-transformers--constant current transformers for series arc lighting2 elementary diagram illustrating principles--regulation--transformer connections--single phase connections--combining transformers--precautions--operating secondaries in parallel--connections for different voltages--precautions--two phase connections--three phase connections3 delta1 star1 delta star1 star-delta--comparison of star and delta connections--three phase transformers--comparison of air blast1 water cooled1 and oil cooled transformers--standard transformer connections--how to test transformers--transformer operation with grounded secondary--transformer capacity for motors--transformer connections for motors--arc lamp transformer--transformer installed on pole--static booster or regulating transformer4 ,,converters #a1deg to #a1did ,where used--kinds of converter--,a4,i4,e4,e4 classification--rotary converters--operation--speed--principles --relation between input and output pressures--single and polyphase types--advantage of polyphase converters--armature connections of polyphase converter--pressure relation--voltage variation--advantage of unity power factor--effect of field too strong--compounding of rotary converters--ratio of conversion--voltage regulation--split pole method--regulating pole method--best location of regulating poles--reactance method--multi-tap transformer method--synchronous booster method--winding connections--field connections--adaptation--motor generator sets--classification--standard practice--behavior of rotary when hunting2 comparison with motor generator sets--racing--frequency changing sets--parallel operation of frequency changers--cascade converter--speed--action in motor armature winding--advantages--how started--comparison of cascade converter with synchronous converter4 ,,rectifiers #a1die to #a1ecj ,classification--mechanical rectifiers--essential features--construction--application--ele ctrolytic rectifiers--principles of operation--,mohawk rectifier--the term valve--metals for electrodes--electrolyte--,nodon valve--,audion valve--,buttner valve--,churcher valve--,de ,faria valve--,fleming oscillation valve--,grisson valve--,pawlowski valve--,giles electric valve--,buttner valve--mercury vapor rectifiers--principles--the terms arc and vapor--three phase mercury vapor rectifier--construction--auxiliary apparatus--series mercury arc rectifier--dissipation of heat from bulb--replacement of bulb--advantages of rectifier--precautions in installing--electromagnetic rectifiers--construction and operation4 ,,chapter ,,li ,,alternating ,,current ,,motors ,the almost universal adoption of the alternating current system of distribution of electrical energy for light and power1 and the many inherent advantages of the alternating current motor1 have created the wide field of application now covered by this type of apparatus4 ,as many central stations furnish only alternating current1 it has become necessary for motor manufacturers to perfect types of alternating current motor suitable for all classes of industrial drive and which are adapted for use on the kinds of alternating circuit employed4 ,this has naturally resulted in a multiplicity of types and a classification1 to be comprehensive1 must1 as in the case of alternators1 divide the motors into groups as regarded from several points of view4 ,accordingly1 alternating current motors may be classified3 #a4 ,with respect to their principle of operation1 as a4 ,,synchronous ,,motors2 b4 ,,asynchronous ,,motors3 #a4 ,induction motors2 series2 #b4 ,commutator motors compensated2 shunt2 repulsion4 #b4 ,with respect to the current as a4 ,single phase2 b4 ,polyphase2 ,illustration3 ,,figs4 #a1ehe to #a1ehh4--,synchronous motor principles3 ,i4 ,a single phase synchronous motor is not self-starting4 ,the figures show an elementary alternator and an elementary synchronous motor1 the construction of each being identical as shown4 ,if the alternator be started1 during the first half of a revolution1 beginning at the initial position ,,abcd1 fig4 #a1ehe1 current will flow in the direction indicated by the arrows1 passing through the external circuit and armature of the motor1 fig4 #a1ehf1 inducing magnetic poles in the latter as shown by the vertical arrows4 ,these poles are attracted by unlike poles of the field magnets1 which tend to turn the motor armature in a counter-clockwise direction4 ,now1 before the torque thus set up has time to overcome the inertia of the motor armature and cause it to rotate1 the alternator armature has completed the half revolution1 and beginning the second half of the revolution1 as in fig4 #a1ehg1 the current is reversed and consequently the induced magnetic poles in the motor armature are reversed also4 ,this tends to rotate the armature in the reverse direction1 as in fig4 #a1ehh4 ,these reversals of current occur with such frequency that the force does not act long enough in either direction to overcome the inertia of the armature2 consequently it remains at rest1 or to be exact1 it vibrates4 ,hence1 a single phase synchronous motor must be started by some external force and brought up to a speed that gives the same frequency as the alternator before it will operate4 ,a single phase synchronous motor1 then1 is not self-starting1 which is one of its disadvantages2 the reason it will operate after being speeded up to synchronism with the alternator and then connected in the circuit is explained in figs4 #a1ehi to #a1eib4 #c4 ,with respect to speed1 as a4 ,constant speed2 b4 ,variable speed4 #d4 ,with respect to structural features1 as a4 ,enclosed2 b4 ,semi-enclosed2 c4 ,open2 d4 ,pipe ventilated2 e4 ,back geared2 f4 ,skeleton frame2 g4 ,riveted frame2 h4 ,ventilated2 etc4 ,of the above divisions and sub-divisions some are self-defining and need little or no explanation2 the others1 however1 will be considered in detail1 with explanations of the principles of operation and construction4 ,synchronous ,motors4--,the term synchronous means in unison1 that is1 in step4 ,a so called synchronous motor1 then1 as generally defined1 is one which rotates in unison or in step with the phase of the alternating current which operates it4 ,strictly speaking1 however1 it should be noted that this condition of operation is only approximately realized as will be later shown4 ,any single or polyphase alternator will operate as a synchronous motor when supplied with current at the same pressure and frequency as it produces as a generator1 the essential condition1 in the case of a single phase machine1 being that it be speeded up to so called synchronism before being put in the circuit4 ,in construction1 synchronous motors are almost identical with the corresponding alternator1 and consist essentially of two elements3 #a4 ,an armature1 #b4 ,a field4 ,illustration3 ,,figs4 #a1ehi to #a1eib4--,synchronous motor principles3 ,,ii4 ,the condition necessary for synchronous motor operation is that the motor be speeded up until it rotates in synchronism1 that is1 in step with the alternator4 ,this means that the motor must be run at the same frequency as the alternator 7not necessarily at the same speed74 ,in the figures it is assumed that the motor has been brought up to synchronism with the alternator and connected in the circuit as shown4 ,in figs4 #a1ehi and #a1eij the arrows indicate the direction of the current for the armature position shown4 ,the current flowing through the motor armature induces magnetic poles which are attracted by the field poles1 thus producing a torque in the direction in which the armature is rotating4 ,after the alternator coil passes the vertical position1 the current reverses as in fig4 #a1eia1 and the current flows through the motor armature in the opposite direction1 thus reversing the induced poles as in fig4 #a1eib4 ,this brings like poles near each other1 and since the motor coil has rotated beyond the vertical position the repelling action of the like poles1 and also the attraction of unlike poles1 produces a torque acting in the direction in which the motor is rotating4 ,hence1 when the two armatures move synchronously1 the torque produced by the action of the induced poles upon the field poles is always in the direction in which the motor is running1 and accordingly1 tends to keep it in operation4 either of which may revolve4 ,the field is separately excited with direct current4 ,illustration3 ,,figs4 #a1eic and #a1eid4--,synchronous motor principles3 ,,iii4 ,the current which flows through the armature of a synchronous motor is that due to the effective pressure4 ,since the motor rotates in a magnetic field1 a pressure is induced in its armature in a direction opposite to that induced in the armature of the alternator1 and called the reverse pressure1 as distinguished from the pressure generated by the alternator called the impressed pressure4 ,at any instant1 the pressure available to cause current to flow through the two armatures1 called the effective pressure1 is equal to the difference between the pressure generated by the alternator or impressed pressure and the reverse pressure induced in the motor4 ,now if the motor be perfectly free to turn1 that is1 without load or friction1 the reverse pressure will equal the impressed pressure and no current will flow4 ,this is the case of real synchronous operation1 that is1 not only is the frequency of motor and alternator the same1 but the coils rotate without phase difference4 ,in figs4 #a1eic and #a1eid1 the impressed and reverse pressures are represented by the dotted arrows ,p and ,pr1 respectively4 ,since in this case these opposing pressures are equal1 the resultant or effective pressure is zero2 hence1 there is no current4 ,in actual machines this condition is impossible1 because even if the motors have no external load1 there is always more or less friction present2 hence1 in operation there must be more or less current flowing through the motor armature to induce magnetic poles so as to produce sufficient torque to carry the load4 ,the action of the motor in automatically adjusting the effective pressure to suit the load is explained in figs4 #a1eie and #a1eif4 ,the principles upon which such motors operate may be explained by considering the action of two elementary alternators connected in circuit1 as illustrated in the accompanying illustrations1 one alternator being used as a generator and the other as a synchronous motor4 ,suppose the motor1 as in figs4 #a1ehe and #a1ehf1 be at rest when it is connected in circuit with the alternator4 ,the alternating current will flow through the motor armature and produce a reaction upon the field tending to rotate the motor armature first in one direction1 then in another4 ,illustration3 ,,figs4 #a1eie and #a1eif4--,synchronous motor principles3 ,,iv--,a synchronous motor adjusts itself to changes of load by changing the phase difference between current and pressure4 ,if there be no load and no friction1 the motor when speeded up and connected in the circuit1 will run in true synchronism with the alternator1 that is1 at any instant1 the coils ,a ,b ,c ,d and ,a,b,c,d will be in parallel planes4 ,when this condition obtains1 no current will flow and no torque will be required 7as explained in figs4 #a1eic and #a1eid74 ,if a load be put on the motor1 the effect will be to cause ,a,b,c,d to lag behind the alternator coil to some position ,a,b,c,d and current to flow4 ,the reverse pressure will lag behind the impressed pressure equally with the coil1 and the current which has now started will ordinarily take an intermediate phase so that it is behind the impressed pressure but in advance of the reverse pressure4 ,these phase relations may be represented in the figure by the armature positions shown1 viz43 #a1 the synchronous position ,a,b,c,d representing the impressed pressure1 #b1 the intermediate position ,a',b',c',d'1 the current1 #c1 the actual position ,a,b,c,d 7corresponding to mechanical lag71 the reverse pressure4 ,from the figure it will be seen that the current phase represented by ,a',b',c',d' is in advance of the reverse pressure phase represented by ,a,b,c,d4 ,hence1 by armature reaction1 the current leading the reverse pressure weakens the motor field and reduces the reverse pressure1 thus establishing equilibrium between current and load4 ,as the load is increased1 the mechanical lag of the alternator coil becomes greater and likewise the current lead with respect to the reverse pressure1 which intensifies the armature reaction and allows more current to flow4 ,in this way equilibrium is maintained for variations in load within the limits of zero and #ij mechanical lag4 ,the effect of armature reaction on motors is just the reverse to its effect on alternators1 which results in marked automatic adjustment between the machines especially when a single motor is operated from an alternator of about the same size4 ,in other words1 the current which weakens or strengthens the motor field1 strengthens or weakens respectively the alternator field as the load is varied4 ,because of the very rapid reversals in direction of the torque thus set up1 there is not sufficient time to overcome the inertia of the armature before the current reverses and produces a torque in the opposite direction1 hence1 the armature remains stationary or1 strictly speaking1 it vibrates4 ,illustration3 ,,figs4 #a1eig and #a1eih4--,synchronous motor principles3 ,v4 ,the effectiveness of armature reaction in weakening the field is proportional to the sine of the angle by which the current lags behind the impressed pressure4 ,if a motor be without load or friction1 its armature will revolve synchronously 7in parallel planes7 with the alternator armature4 ,in the figures let ,,abcd represent an instantaneous position of the motor armature when this condition obtains2 it will then represent the phase relationship of impressed and reverse pressures for the same condition of no load1 no friction1 operation4 ,now1 if a light load be placed on the motor for the same instantaneous position of alternator armature1 the motor coil will drop behind to some position as ,a1 fig4 #a1eig 7part of the coil only being shown74 ,the reverse pressure will also lag an equal amount and its phase with respect to the impressed pressure will be represented by ,a4 ,the armature current will ordinarily take an intermediate phase1 represented by coil position ,a',b',c',d'1 inducing a field strength corresponding to the #i lines of force ,,of1 ,o',f'1 etc4 ,the current being in advance of the phase of the reverse pressure ,a1 the armature reaction weakens the field1 thus reducing the reverse pressure and allowing the proper current to flow to balance the load4 ,the amount by which the field is weakened may be determined by resolving the induced magnetic lines ,,of1 ,o',f'1 ,o,f1 etc41 into components ,,og1 ,,gf1 ,o',g'1 ,g',f'1 ,o,g1 ,g,f1 etc41 respectively parallel and at right angles to the lines of force of the main field4 ,of these components1 the field is weakened only by ,,og1 ,o',g'1 ,o,g1 etc4 ,since by construction1 angle ,,ofg ,,aoa'1 and calling ,,of unity length1 ,,og sine of angle by which the current lags behind the impressed pressure4 ,the construction is shown better in the enlarged diagram4 ,for a heavier load the armature coil will drop back further to some position as ,a'1 fig4 #a1eih1 and the lag of the current increase to some intermediate phase as ,a,b,c,d4 ,by similar construction it is seen that the component ,,og 7fig4 #a1eig7 has increased to ,,oj 7fig4 #a1eih71 this component thus further weakening the main field1 by an amount proportional to the sine of the angle by which the current lags behind the impressed pressure4 ,the increased current which is now permitted to flow1 causes the induced field to be strengthened 7as indicated by the dotted magnetic lines ,m1 ,m'1 ,m1 etc471 thus increasing the torque to balance the additional load4 ,now if the motor armature be first brought up to a speed corresponding in frequency to that of the alternator before connecting the motor in the circuit1 the armature will continue revolving at the same frequency as the alternator4 ,the armature continues revolving1 because1 at synchronous speed1 the field flux and armature current are always in the same relative position1 producing a torque which always pulls the armature around in the same direction4 ,a polyphase synchronous motor is self starting1 because1 before the current has died out in the coils of one phase1 it is increasing in those of the other phase or phases1 so that there is always some turning effort exerted on the armature4 ,the speed of a synchronous motor is that at which it would have to run1 if driven as an alternator1 to deliver the number of cycles which is given by the supply alternator4 ,illustration3 ,,figs4 #a1eii and #a1fjj4--,synchronous motor principles3 ,,vi4 ,a single phase synchronous motor has dead centers1 just the same as a one cylinder steam engine4 ,two diagrams of the motor are here shown illustrating the effect of the current in both directions4 ,when the plane of the coil is perpendicular to the field1 the poles induced in the armature are parallel to field for either direction of the current2 that is to say1 the field lines of force and the induced lines of force acting in parallel or opposite directions1 no turning effect is produced1 just as in analogy when an engine is on the dead center1 the piston rod 7field line of force7 and connecting rod 7induced line of force7 being in a straight line1 the force exerted by the steam on the piston produces no torque4 ,for instance a #ab pole alternator running at #fjj revolutions per minute will deliver current at a frequency of #fj cycles a second2 an #h pole synchronous motor supplied from that circuit will run at #ijj revolutions per minute1 which is the speed at which it would have to be driven as an alternator to give #fj cycles a second--the frequency of the #ab pole alternator4 ,illustration3 ,,figs4 #a1fja to #a1fjd4--,synchronous motor principles ,,vii4 ,an essential condition for synchronous motor operation is that the mechanical lag be less than #ij4 ,figs4 #a1fja and #a1fjb represent the conditions which prevail when the lag of the motor armature ,a',b',c',d' is anything less than #ij4 ,as shown1 the lag is almost #ij4 ,the direction of the current and induced poles are indicated by the arrows4 ,the inclination of the motor coil is such that the repulsion of like poles produces a torque in the direction of rotation1 thus tending to keep motor in operation4 ,now1 in figs4 #a1fjc and #a1fjd1 for the same position of the alternator coil ,,abcd1 if the lag be greater than #ij1 the inclination of the motor coil ,a',b',c',d' is such that at this instant the repulsion of like poles produces a torque in a direction opposite to that of the rotation1 thus tending to stop the motor4 ,in actual operation this quickly brings the motor to rest1 having the same effect as a strong brake in overcoming the momentum of a revolving wheel4 ,illustration3 ,,figs4 #a1fje to #a1fjh4--,synchronous motor principles3 ,,viii4 ,if the torque and current through the motor armature be kept constant1 strengthening the field will increase the mechanical lag1 and the lead of the current with respect to the reverse pressure4 ,in the figures1 let ,a be an instantaneous position of the alternator coil1 ,a1 synchronous position of motor coil1 ,a'1 position corresponding to current phase1 ,a1 actual position or mechanical lag of motor coil behind alternator coil necessary to maintain equilibrium4 ,in fig4 #a1fjf1 let ,a' and ,a represent respectively the relation of current phase and mechanical lag corresponding to a certain load and field strength4 ,for these conditions ,,og1 ,o',g'1 ,o,g1 etc41 will represent the components of the induced lines of force in opposition to the motor field1 that is1 they indicate the intensity of the armature reaction at the instant depicted4 ,now1 assume the field strength to be doubled1 as in fig4 #a1fjh1 the motor load and current being maintained constant4 ,under these conditions1 the armature reaction must be doubled to maintain equilibrium2 that is1 the components ,,og1 ,o',g'1 etc41 fig4 #a1fjh1 must be twice the length of ,,og1 ,o',g'1 etc41 fig4 #a1fje4 ,also since the current is maintained constant1 the induced magnetic lines ,,of1 ,o',f' are of same length in both figures4 ,hence1 in fig4 #a1fjh the plane of these components is such that their extremities touch perpendiculars from ,g1 ,g'1 etc41 giving the other components ,,fg1 ,f',g'1 etc4 ,the plane ,a'1 normal to ,,of1 ,o',f'1 etc41 gives the current phase4 ,by construction1 the phase difference between ,a and ,a' is such that sin ,a,,oa' 7fig4 #a1fjh7 #b sin ,a,,oa' 7fig4 #a1fjf74 ,that is1 doubling the field strength causes an increase of current lag such that the sine of the angle of this lag is doubled4 ,since the intensity of the armature reaction depends on the lead of the current with respect to the reverse pressure1 the mechanical lag of the coil must be increased to some position as ,a 7fig4 #a1fjh71 such as will give an armature reaction of an intensity indicated by the components ,,og1 ,o',g'1 etc4 ,the following simple formula gives the speed relations between generators and motors connected to the same circuit and having different numbers of poles4 ,p ,s s ----- p in which s4 ,revolutions per minute of the motor2 p4 ,number of poles of the motor2 ,s4 ,revolutions per minute of the alternator2 ,p4 ,number of poles of the alternator4 ,question4 ,if the field strength of a synchronous motor be altered1 what effect does this have on the speed1 and why8 ,ans4 ,the speed does not change 7save for a momentary variation to establish the phase relation corresponding to equilibrium71 because the motor has to run at the same frequency as the alternator4 ,ques4 ,how does a synchronous motor adjust itself to changes of load and field strength8 ,ans4 ,by changing the phase difference between the current and pressure4 ,if1 on connecting a synchronous motor to the mains1 the excitation be too weak1 so that the voltage is lower than that of the supply1 this phase difference will appear resulting in wattless current1 since the missing magnetization has1 as it were1 to be supplied from an external source4 ,a phase difference also appears when the magnetization is too strong4 ,ques4 ,state the disadvantages of synchronous motors4 ,ans4 ,a synchronous motor requires an auxiliary power for starting1 and will stop if1 for any reason1 the synchronism be destroyed2 collector rings and brushes are required4 ,for some purposes synchronous motors are not desirable1 as for driving shafts in small workshops having no other power available for starting1 and in cases where frequent starting1 or a strong torque at starting is necessary4 ,a synchronous motor has a tendency to hunt#a and requires intelligent attention2 also an exciting current which must be supplied from an external source4 #a ,,note4--,see ,hunting of synchronous motors1 page #a1bhj4 ,ques4 ,state the advantage of synchronous motors4 ,ans4 ,the synchronous motor is desirable for large powers where starting under load is not necessary4 ,its power factor may be controlled by varying the field strength4 ,the power factor can be made unity and1 further1 the current can be made to lead the pressure4 ,illustration3 ,,fig4 #a1fji4--;,diagram illustrating method of representing the performance of synchronous motors4 ,the ,v shaped curve is obtained by plotting the current taken by motor under different degrees of excitation1 the power developed by the motor remaining constant4 ,the current may be made to lag or lead while the load remains constant1 by varying the excitation4 ,by varying the excitation1 a certain value may be reached which will give a minimum current in the armature2 this is the condition of unity power factor4 ,if now the excitation be diminished the current will lag and increase in value to obtain the same power2 if the excitation be increased the current will lead and increase in value to obtain the same power4 ,the results plotted for several values of the excitation current will give the ,v curve as shown4 ,this is an actual curve obtained by ,mordey on a #ej kw4 machine running unloaded as a motor4 ,other curves situated above this one may be obtained for various loadings of the motor4 ,a synchronous motor is frequently connected in a circuit solely to improve the power factor4 ,in such cases it is often called a condenser motor for the reason that its action is similar to that of a condenser4 ,the design of synchronous motors proceeds on the same lines as that of alternators1 and the question of voltage regulation in the latter becomes a question of power factor regulation in the former4 ,ques4 ,for what service are they especially suited8 ,ans4 ,for high pressure service4 ,high voltage current supplied to the armature does not pass through a commutator or slip rings2 the field current which passes through slip rings being of low pressure does not give any trouble4 ,illustration3 ,,fig4 #a1faj4--,westinghouse self-starting synchronous motor4 ,motors of this type are suitable for constant speed service where starting conditions are moderate1 such as driving compressors1 pumps1 and large blowers4 ,synchronous motors can be made to operate not only as motors but as synchronous condensers to improve the power factor of the circuit4 ,the field is provided with a combined starting and damper or amorlisseur winding so proportioned that the necessary starting torque is developed by the minimum current consistent with satisfactory synchronous running without hunting4 ,the armature slots are open and the coils form wound1 impregnated1 and interchangeable4 ,malleable iron finger plates at each end of the core support the teeth4 ,ventilating finger plates assembled with the laminations form air ducts4 ,the frames are of cast iron1 box section with openings for ventilation2 shoes and slide rails permit adjustment of position4 ,the brush holders are of the standard sliding shunt type4 ,two or more brushes are provided for each ring4 ,ques4 ,how do synchronous and induction motors compare as to efficiency8 ,ans4 ,synchronous motors are usually the more efficient4 ,illustration3 ,,fig4 #a1faa4--,mechanical analogy illustrating hunting4 ,the figure represents two flywheels connected by a spring susceptible to torsion in either direction of rotation4 ,if the wheels ,a and ,b be rotating at the same speed and a brake be applied1 say to ,b1 its speed will diminish and the spring will coil up1 and if fairly flexible1 more than the necessary amount to balance the load imposed by the brake2 because when the position of proper torque is reached1 ,b is still rotating slightly slower than ,a1 and an additional torque is required to overcome the inertia of ,b and bring its speed up to synchronism with ,a4 ,now before the spring stops coiling up the wheels must be rotating at the same speed4 ,when this occurs the spring has reached a position of too great torque1 and therefore exerting more turning force on ,b than is necessary to drive it against the brake4 ,accordingly ,b is accelerated and the spring uncoils4 ,the velocity of ,b thus oscillates above and below that of ,a when a load is put on and taken off4 ,owing to friction1 the oscillations gradually die out and the second wheel takes up a steady speed4 ,a similar action takes place in a synchronous motor when the load is varied4 ,hunting of ,synchronous ,motors4--,since a synchronous motor runs practically in step with the alternator supplying it with current when they both have the same number of poles1 or some multiple of the ratio of the number of poles on each machine1 it will take an increasing current from the line as its speed drops behind the alternator1 but will supply current to the line as a generator if for any reason the speed of the alternator should drop behind that of the motor1 or the current wave lag behind1 which produces the same effect1 and due to additional self-induction or inductance produced by starting up or overloading some other motor or rotary converter in the circuit4 ,when the motor is first taking current1 then giving current back to the line1 and this action is continued periodically1 the motor is said to be hunting4 ,illustration3 ,,fig4 #a1fab4--;,diagram illustrating the use of a synchronous motor as a condenser4 ,if a synchronous motor be sufficiently excited the current will lead4 ,hence1 if it be connected across an inductive circuit as in the figure and the field be over excited it will compensate for the lagging current in the main1 thus increasing the power factor4 ,if the motor be sufficiently over excited the power factor may be made unity1 the minimum current being thus obtained that will suffice to transmit the power in the main circuit4 ,a synchronous motor used in this way is called a rotary condenser or synchronous compensator4 ,this is especially useful on long lines containing transformers and induction motors4 ,ques4 ,what term is applied to describe the behavior of the current when hunting occurs8 ,ans4 ,the term surging is given to describe the current fluctuations produced by hunting4 ,the mechanical analogy of hunting illustrated in fig4 #a1faa will help to an understanding of this phenomenon4 ,in alternating current circuits a precisely similar action takes place between the alternators and synchronous motors1 or even between the alternators themselves4 ,,characteristics ,,of ,,synchronous ,,motors ,starting4--,the motor must be brought up to synchronous speed without load1 a starting compensator being used4 ,if provided with a self-starting device1 the latter must be cut out of circuit at the proper time4 ,the starting torque of motor with self-starting device is very small4 ,running4--,the motor runs at synchronous speed4 ,the maximum torque is several times full load torque and occurs at synchronous speed4 ,stopping4--,if the motor receive a sudden overload sufficient to momentarily reduce its speed1 it will stop2 this may be brought about by momentary interruption of the current1 sufficient to cause a loss of synchronism4 ,effect upon ,circuit4--,in case of short circuit in the line the motor acts as a generator and thus increases the intensity of the short circuit4 ,the motor impresses its own wave form upon the circuit4 ,over excitation will give to the circuit the effect of capacity1 and under excitation1 that of inductance4 ,power ,factor4--,this depends upon the field current1 wave form and hunting4 ,the power factor may be controlled by varying the field excitation4 ,necessary ,auxiliary ,apparatus4--,power for starting1 or if self-starting1 means of reducing the voltage while starting2 also1 field exciter1 rheostat1 friction clutch1 main switch and exciter switch1 instruments for indicating when the field current is properly adjusted4 ,adaptation4--,if induction motors be connected to the same line with a synchronous motor that has a steady load1 then the field of the synchronous motor can be over excited to produce a leading current1 which will counteract the effect of the lagging currents induced by the induction motors4 ,owing to the weak starting torque1 skilled attendance required1 and the liability of the motor to stop under abnormal working conditions1 the synchronous motor is not adapted to general power distribution1 but rather to large units which operate under a steady load and do not require frequent starting and stopping4 ,illustration3 ,,figs4 #a1fac to #a1fbe4--;,disassembled view of ,western ,electric three phase squirrel cage skeleton frame induction motor4 ,induction 7,asynchronous7 ,motors4-- ,an induction motor consists essentially of an armature and a field magnet1 there being1 in the simplest and most usual types1 no electrical connection between these two parts4#b #b ,,note4--,the author prefers the terms armature and field magnet1 instead of primary1 secondary1 stator1 rotor1 etc41 as used by other writers1 the armature being the part in which currents are induced and the field magnet 7or magnets7 that part furnishing the field in which the induction takes place4 ,according to the kind of current that an induction motor is designed to operate on1 it may be classified as3 #a4 ,single phase2 #b4 ,polyphase4 ,the operation of an induction motor depends on the production of a magnetic field by passing an alternating current through field magnets4 ,the character of this field is either #a4 ,oscillating#c1 or #b4 ,rotating1 according as single phase or polyphase current is used4 #c ,,note4--,the word oscillating is becoming specialized in its application to those currents and fields whose oscillations are being damped out1 as in electric 'oscillations4' ,but for this1 we should have spoken of an oscillating field4--,s4 ,p4 ,thompson4 ,the author believes the word oscillating1 notwithstanding its other usage1 best describes the single phase field1 and should be here used4 ,illustration3 ,,figs4 #a1fbf to #a1fbh4--;,general ,electric base construction for polyphase induction motors4 ,the base is made of cast iron4 ,adjusting gear is provided to slide the motor along the base as shown in the illustrations1 the movement being from #f to #ab inches according to size4 ,with this design of base1 motors are securely held in position under all conditions and may be run with an upward pull on the belt4 ,close fitting guides moving in an accurately machined slot on the base preserve a correct alignment of the motor when adjustment of the latter is required4 ,the same base can be used whether the motor be supported from the wall or ceiling or located on the floor4 ,a single adjusting screw is placed under the center line of the motor frame1 which produces an even and balanced draw in either direction on all parts of the motor when the belt tension is altered4 ,this screw can be located at either end of the base4 ,the base can be omitted when the motor is direct connected or when provision for belt adjustment is not required4 ,ques4 ,describe briefly the operation of a single phase motor4 ,ans4 ,a single phase current being supplied to the field magnets1 an oscillating field is set up4 ,a single phase motor is not self-starting2 but when the armature has been set in motion by external means1 the reaction between the magnetic field and the induced currents in the armature being no longer zero1 a torque is produced tending to turn the armature4 ,the current flowing through the armature produces an alternating polarity such that the attraction between the unlike armature and field poles is always in one direction1 thus producing the torque4 ,illustration3 ,,fig4 #a1fbi4--,richmond three phase induction motor on base fitted with screw adjusting gear for shifting the position of the motor on the base to take up slack of belt4 ,ques4 ,why is a single phase induction motor not self-starting8 ,ans4 ,when the armature is at the rest1 the currents induced therein are at a maximum in a plane at right angles to the magnetic field1 hence there is no initial torque to start the motor4 ,ques4 ,what provision is made for starting single phase induction motors8 ,ans4 ,apparatus is supplied for splitting the phase 7later described in detail7 of the single phase current furnished1 converting it temporarily into a two phase current1 so as to obtain a rotating field which is maintained till the motor is brought up to speed4 ,the phase splitting device is then cut out and the motor operated with the oscillating field produced by the single phase current4 ,illustration3 ,,figs4 #a1fcj to #a1fda4--,terminals for ,general ,electric polyphase induction motors4 ,in order to prevent any mechanical strain on the leads being transmitted to the motor windings1 the terminal cables are clamped in insulated bushings with a connector for each cable4 ,ques4 ,describe briefly the operation of a polyphase induction motor4 ,ans4 ,its operation is due to the production of a rotating magnetic field by the polyphase current furnished4 ,this field rotating in space about the axis of the armature induces currents in the latter4 ,the reaction between these currents and the rotating field creates a torque which tends to turn the armature1 whether the latter be at rest or in motion4 ,illustration3 ,,figs4 #afdb and #afdc4--,western ,electric end flange rivets and punchings of riveted frame induction motor4 ,the riveted frame is constructed of two cast iron flanges between which the stator laminations of sheet steel are securely clamped and riveted under hydraulic pressure4 ,this construction exposes the laminations directly to the air and improves the radiation1 thus insuring high overload capacity and low operating temperatures4 ,the field slots are overhung or partially closed1 affording mechanical protection to the coils4 ,ques4 ,why are induction motors called asynchronous8 ,ans4 ,because the armature does not turn in synchronism with the rotating field1 or1 in the case of a single phase induction motor1 with the oscillating field 7considering the latter in the light of a rotating field74 ,ques4 ,how does the speed vary8 ,ans4 ,it is slower 7more or less according to load7 than the field speed1 that is1 than synchronism or the synchronous speed4 ,illustration3 ,,figs4 #a1fdd to #a1fdi4--;,construction of ,general ,electric drawn shell fractional horse power motors4 ,the distinguishing feature of drawn shell motors is the field construction which consists of a steel shell or cylinder supporting and clamping together the stator or field punchings4 ,this method avoids the cast frame work outside the active magnetic material4 ,a disc is first punched or blanked out of soft steel1 fig4 #a1fdd1 this disc being faced into the shape1 fig4 #a1fde1 with one end closed4 ,the other end of the shell is then cut out1 leaving the small flange as in fig4 #a1fdf4 ,it is now ready to receive the core punchings4 ,in the next operation a suitable number of spacing rings1 fig4 #a1fdg1 are forced into the shell and seated against the retaining lip1 which may be seen in fig4 #a1fdf4 ,the field punchings or lamin1 fig4 #a1fdh1 are now assembled1 after which a second and equal set of spacing rings are put into place to center the active field iron4 ,the open edge of the shell is then rolled over the punchings under heavy pressure1 thus preparing the field structure for the machining and fitting of the end heads and base4 ,fig4 #a1fdi shows a section of the completely assembled field structure1 the parts being cut away to indicate the relation between the field punchings1 spacing rings and shell4 ,after the spacing rings at both frame ends have been turned true and grooved1 the bearing heads1 fig4 #a1fdi1 are ready for fastening in place by four fillister headed screws4 ,a complete wound field is shown in fig4 #a1heh1 with flat base casting attached4 ,ques4 ,what is the difference of speed called8 ,ans4 ,the slip4 ,this is a vital factor in the operation of an induction motor1 since there must be slip in order that the armature inductors shall cut magnetic lines to induce 7hence the name induction motor7 currents therein so as to create a driving torque4 ,illustration3 ,,fig4 #a1fej4--;,ideal fifteen horse power two phase induction motor4 ,the armature core is supported by a cast iron frame carried on a base1 with sliding ways and screw adjustment for tightening the belt4 ,the armature core is provided with ventilating apertures1 with metal spacers between each tooth4 ,the revolving field is a steel casting with radially projecting poles1 to which the pole shoes are bolted4 ,the overhanging pole tips retain the field coils4 ,all coils of the smaller sizes are wound with insulated copper wire of square section1 and of the larger sizes1 with flat copper1 wound on edge1 each turn being insulated by sheet insulation4 ,motors of this type are adapted for use in small power plants and isolated plants4 ,the relatively high speed for which they are designed1 reduces considerably the weight and overall dimensions1 and likewise the cost4 ,the exciter is belt driven4 ,the normal kw4 capacity of the exciter usually exceeds the kw4 required for the excitation under normal load conditions to permit of station lighting4 ,all exciters are built as compound wound dynamos1 capable of delivering the exciter current up to #abe volts1 which is sufficient margin in the field to control the alternating current line voltage on circuits of unusually low power factor4 ,ques4 ,what is the extent of the slip8 ,ans4 ,it varies from about #b to #e per cent4 of synchronous speed depending upon the size4 ,ques4 ,why are induction motors sometimes called constant speed motors8 ,ans4 ,they are erroneously and ill advisedly1 yet conveniently so called by builders to distinguish them from induction motors fitted with special devices to obtain widely varying speeds1 and which are known as variable speed induction motors4 ,the term adjustable would be better4 ,motor1 ,constant ,speed4--,a motor in which the speed is either constant or does not materially vary2 such as synchronous motors1 induction motors with small slip1 and ordinary direct current shunt motors4--,paragraph #df of #aijg ,standardization ,rules of the ,a4,i4,e4,e4 ,motor1 ,variable ,speed4--,a motor in which provision is made for varying the speed as desired4 ,the ,a4,i4,e4,e4 has unfortunately introduced the term varying speed motor1 to designate motors in which the speed varies with the load1 decreasing when the load increases1 such as series motors4 ,the term is objectionable1 since by the expression variable speed motor a much more general meaning is intended4 ,illustration3 ,,fig4 #a1fea4--,western ,electric core construction and method of winding field of skeleton frame induction motor4 ,the coils are wound on forms to give them exact shape and dimensions required4 ,they are pressed into hot moulds to remove any irregularities and then the coils are impregnated with hot cement1 to bind the layers together in their permanent shape4 ,the portion of the coil which fits into the slot is wrapped with varnished cloth and a layer of dry tape is wound over the entire coil4 ,the coils are then impregnated with an insulating compound and baked1 the process being repeated six times4 ,coils for #a1ajj and #b1bjj volt motors have an extra covering of insulation and double the amount of impregnating and baking4 ,the coils may be furnished with special insulation and treatment for exceptionally severe service conditions1 such as exposure to excessive moisture1 extreme heat1 acid or alkaline fumes1 etc4 ,the coils are accessible and for the final finish are sprayed with black varnish4 ,ques4 ,why do some writers call the field magnets and armature the primary and secondary1 respectively8 ,ans4 ,because1 in one sense1 the induction motor is a species of transformer1 that is1 it acts in many respects like a transformer1 the primary winding of which is on the field and the secondary winding on the armature4 ,in the motor the function of the secondary circuit is to furnish energy to produce a torque1 instead of producing light and heat as in the case of the transformer4 ,such comparisons are ill advised when made for the purpose of supplying names for motor parts4 ,there can be no confusion by employing the simple terms armature and field magnets1 remembering that the latter is that part that produces the oscillating or rotating field 7according as the motor is single or polyphase71 and the former1 that part in which currents are induced4 ,illustration3 ,,fig4 #a1feb4--;,armature of ,allis-,chalmers squirrel cage induction motor4 ,the frame casting is of the box type and has large cored openings for ventilation4 ,lugs are cast on the interior surface of the frame to support the core1 leaving a large air space between4 ,ques4 ,why are polyphase induction motors usually presented in text books before single phase motors8 ,ans4 ,because the latter must start with a rotating field and come up to speed before the oscillating field can be employed4 ,a knowledge then of the production of a rotating field is necessary to understand the action of the single phase motor at starting4 ,polyphase ,induction ,motors4--,as many central stations put out only alternating current circuits1 it has become necessary for motor builders to perfect types of alternating current motor suitable for all classes of industrial drive and which are adapted for use on these commercial circuits4 ,three phase induction motors are slightly more efficient at all loads than two phase motors of corresponding size1 due to the superior distribution of the field windings4 ,the power factor is higher1 especially at light loads1 and the starting torque with full load current is also greater4 ,furthermore1 for given requirements of load and voltage1 the amount of copper required in the distributing system is less2 consequently1 wherever service conditions will permit1 three phase motors are preferable to two phase4 ,illustration3 ,,fig4 #a1fec4--,sectional view showing parts of ,reliance polyphase induction motor4 ,a special feature of the squirrel cage armature construction is the multiplicity of short circuiting rings4 ,the holes in the rings are bored slightly smaller than the diameter of the copper rods1 and the force fit gives good contact4 ,the rings having been forced in place are dip soldered in an alloy of tin of high melting point4 ,the motor parts are3 #a1 end yoke2 #b1 shaft2 #c1 armature short circuiting rings2 #d1 oil ring2 #e1 self-aligning bearing bushing2 #f1 spider2 #g1 armature bars2 #h1 field coils2 #i1 field lamination end plate2 #aj1 field laminations2 #aa1 eye bolt2 #ab1 stator locking key2 #ac1 armature laminations2 #ad1 armature lamination end plate2 #ae1 armature locking key2 #af1 dust cap2 #ag1 oil well cover2 #ah1 oil throws2 #ai1 field frame2 #bj1 squirrel cage armature4 ,illustration3 ,,fig4 #a1fed4--,tesla's rotating magnetic field4 ,the figure is from one of ,tesla's papers as given in ,the ,electrician1 illustrating how a rotating magnetic field may be produced with stationary magnets and polyphase currents4 ,the illustration shows a laminated iron ring overwound with four separate coils1 ,,aa1 and ,,bb1 each occupying about #ij of the periphery4 ,the opposite pairs of coils ,,aa and ,,bb respectively are connected in series and joined to the leads from a two phase alternator1 the pair of coils ,,aa being on one circuit and the coils ,,bb on the other4 ,the resultant flux may be obtained by combining the two fluxes due to coils ,,aa and ,,bb1 taking account of the phase difference of the two phase current1 as in fig4 #a1fee4 ,the construction of an induction motor is very simple1 and since there are no sliding contacts as with commutator motors1 there can be no sparks during operation--a feature which adapts the motor for use in places where fire hazards are prominent4 ,the motor consists1 as already mentioned1 simply of two parts3 an armature and field magnets1 without any electrical connection between these parts4 ,its operation depends upon3 #a4 ,the production of a rotating field2 #b4 ,induction of current in the armature2 #c4 ,reaction between the revolving field and the induced currents4 ,illustration3 ,,fig4 #a1fee4--,method of obtaining resultant flux of ,tesla's rotating magnetic field4 ,the eight small diagrams here seen show the two components and resultant for eight equivalent successive instants of time during one cycle4 ,at #a1 the vertical flux is at maximum and the horizontal is zero4 ,at #b1 the vertical flux is still but decreasing1 and the horizontal is and increasing1 the resultant is the thick line sloping at #de upwards to the right4 ,at #c1 the vertical flux is zero1 and the horizontal is at its maximum1 and similarly for the other diagrams4 ,thus at #h1 the vertical flux is and increasing1 while the horizontal is-and decreasing1 the resultant is the thick line sloping at #de upwards to the left4 ,at points #b1 #d1 #f1 and #h the increasing fluxes are denoted by full and the decreasing by dotted lines4 ,the laminated iron of the ring is indicated by the circles1 and the result is that at the instants chosen the flux across the plane of the ring is directed inwards from the points #a1 #b1 #c1 #d1 etc41 on the inner periphery of the iron4 ,there will1 therefore1 appear successively at these points effective north poles1 the corresponding south poles being simultaneously developed at the points diametrically opposite4 ,these poles travel continuously from one position to the next1 and thus the magnetic flux across the plane of the ring swings round and round1 completing a revolution without change of intensity during the cycle time of the current4 ,production of a ,rotating ,field4--,it should at once be understood that the term rotating field does not signify that part of the apparatus revolves1 the expression merely refers to the magnetic lines of force set up by the field magnets without regard to whether the latter be the stationary or rotating member4 ,a rotating field then may be defined as the resultant magnetic field produced by a system of coils symmetrically placed and supplied with polyphase currents4 ,a rotating magnetic field can1 of course1 be produced by spinning a horse shoe magnet around its longitudinal axis1 but with polyphase currents1 as will be later shown1 the rotation of the field can be produced ,without any movement of the mechanical parts of the electro magnets4 ,illustration3 ,,fig4 #a1fef4--;,arago's rotations4 ,the apparatus necessary to make the experiment consists of a copper disc ,m1 arranged to rotate around a vertical axis and operated by belt drive1 as shown4 ,by turning the large pulley by hand1 the disc ,m may be rotated with great rapidity4 ,above the disc is a glass plate on which is a small pivot supporting a magnetic needle ,n4 ,if the disc now be rotated with a slow and uniform velocity1 the needle is deflected in the direction of the motion1 and stops at an angle of from #bj to #cj with the direction of the magnetic meridian1 according to the velocity of the rotation of the disc4 ,if the velocity increase1 the needle is ultimately deflected more than #ij and then continues to follow the motion of the disc4 ,the original rotating magnetic field dates back to #ahbc1 when ,francois ,jean ,arago1 an assistant in ,davy's laboratory1 discovered that if a magnet be rotated before a metal disc1 the latter had a tendency to follow the motion of the magnet1 as shown in fig4 #bij1 page #bgj and also in fig4 #a1fef4 ,this experiment led up to the discovery which was made by ,arago in #ahbd1 when he observed that the number of oscillations which a magnetized needle makes in a given time1 under the influence of the earth's magnetism1 is very much lessened by the proximity of certain metallic masses1 and especially of copper1 which1 may reduce the number in a given time from #cjj to #d4 ,illustration3 ,,fig4 #a1feg4--;,explanation of ,arago's rotations4 ,part of fig4 #a1fef is here reproduced in plan4 ,faraday was the first to give an explanation of the phenomena of magnetism by rotation in attributing it to the induction of currents which by their electro-dynamic action1 oppose the motion producing them2 the action is mechanically analogous to friction4 ,in the figure1 let ,,ab be a needle oscillating over a copper disc1 and suppose that in one of its oscillations it goes in the direction of the arrow from ,m to ,s4 ,in approaching the point ,s1 for instance1 it develops there a current in the opposite direction1 and which therefore repels it2 in moving away from ,m it produces currents which are of the same kind1 and which therefore attract1 and both these actions concur in bringing it to rest4 ,again1 suppose the metallic mass turn from ,m towards ,s1 and that the magnet be fixed2 the magnet will repel by induction points such as ,m which are approaching ,a1 and will attract ,s which is moving away2 hence the motion of the metal stops1 as in ,faraday's experiment4 ,if in ,arago's experiment the disc be moving from ,m to ,s1 ,m approaches ,a and repels it1 while ,s1 moving away1 attracts it2 hence the needle moves in the same direction as the disc4 ,if this explanation be true1 all circumstances which favor induction will increase the dynamic action2 and those which diminish the former will also lessen the latter4 ,the explanation of ,arago's rotations is that the magnetic field cutting the disc produces eddy currents therein and the reaction between the latter and the field causes the disc to follow the rotations of the field4 ,the induction motor is a logical development of the experiment of ,arago1 which so interested ,faraday while an assistant in ,davy's laboratory and which led him to the discovery of the laws of electromagnetic induction1 which are given in ,chapter ,x4 #d;,in #ahhe1 ,professor ,ferraris1 of ,turin discovered that a rotating field could be produced from stationary coils by means of polyphase currents4 #d ,,note4--,walmsley attributes the first production of rotating fields to ,walter ,bailey in #ahgi1 who exhibited a model at a meeting of the ,physical ,society of ,london1 but very little was done1 it is stated1 until ,ferraris took up the subject4 ,illustration3 ,,fig4 #a1feh4--;,experiment made by ,faraday being the reverse of ,arago's first observation4 ,faraday assumed that since the presence of a metal at rest stops the oscillations of a magnetic needle1 the neighborhood of a magnet at rest ought to stop the motion of a rotating mass of metal4 ,he suspended a cube of copper by a twisted thread1 which was placed between the poles of a powerful electromagnet4 ,when the thread was left to itself1 it began to spin round with great velocity1 but stopped the moment a powerful current was passed through the electromagnet4 #e,this discovery was commercially applied a few years later by ,tesla1 ,brown1 and ,dobrowolsky4 #e ,,note4--,the ,tesla patents were acquired in the ,u4,s4 by the ,westinghouse ,co4 in #ahhh1 and polyphase induction motors1 as they were called1 were soon on the market4 ,brown of the ,oerlikon ,machine ,works developed the single phase system and operated a transmission plant over five miles in length at ,kassel1 ,germany1 which operated at #b1jjj volts4 ,the principles of polyphase motors can be best understood by means of elementary diagrams illustrating the action of polyphase currents in producing a rotating magnetic field1 as explained in the paragraphs following4 ,production of a ,rotating ,magnetic ,field by ,two ,phase ,currents4--,fig4 #a1fei represents an iron ring wound with coils of insulated wire1 which are supplied with a two phase current at the four points ,a1 ,b1 ,c1 ,d1 the points ,a and ,b1 and ,c and ,d1 being electrically connected4 ,illustration3 ,,fig4 #a1fei4--,production of a rotating magnetic field by two phase currents4 ,the figure represents an iron ring1 wound with coils of insulated wire1 and supplied with two phase currents at the four points ,a1 ,b1 ,c1 and ,d4 ,the action of the two phase current on the ring in producing a rotating magnetic field is explained in the accompanying text4 ,according to the principles of electromagnetic induction1 if only one current a entered the ring at ,a1 and the direction of the winding be suitable1 a negative pole 7-7 will be produced at ,a and a positive pole 77 at ,b1 so that a magnetic needle pivoted in the center of the ring would tend to point vertically upward towards ,a4 ,now suppose that at this instant1 corresponding to the beginning of an alternating current cycle1 a second current b1 differing in phase from the first by #ij degrees1 is allowed to enter the ring at ,c4 ,as shown in fig4 #a1fei1 when the pressure of the current a is at its maximum1 that of the current b is at its minimum2 therefore1 even a two phase current1 at the beginning of the cycle1 the needle will point toward ,a4 ,illustration3 ,,fig4 #a1ffj4--,production of rotating magnetic field in a two pole two phase motor4 ,the poles are numbered from #a to #d in a clockwise direction4 ,phase ,a winding is around poles #a and #c1 and phase ,b winding1 around poles #b and #d4 ,in each case the poles are wound alternately1 that is1 if #a be wound clockwise1 #c will be wound counter clockwise1 thus producing unlike polarity in opposite poles4 ,now during one cycle of the two phase current1 the following changes take place1 starting with pole #a of ,n polarity and #c1 of ,s polarity3 ---------------------------------------- ---------------------- ,one ,cycle ---------------------------------------- ------------------ ,degrees #j to #ij #ij to #ahj #ahj to #bgjbgj to #cfj ---------------------------------------- ------------------ ,polarity #a,n - #c,s #b,n - #d,s #c,n - #a,s #d,n - #b,s ---------------------------------------- ---------------------- ,illustration3 ,,fig4 #a1ffa4--;,diagram showing resultant poles due to two phase current4 ,illustration3 ,,fig4 #a1ffb4--;,diagram of two phase1 six pole field winding4 ,there are six coils in each phase1 as shown4 ,the coils of each phase are connected in series1 adjacent coils being joined in opposite senses1 thus1 for each phase1 first one coil is wound clockwise1 and the next counter clockwise4 ,as the cycle continues1 however1 the strength of a will diminish and that of b increase1 thus shifting the induced pole toward ,c1 until b attains its maximum and a falls to its minimum at #ij or the end of the first quarter of the cycle1 when the needle will point toward ,c4 ,at #ij1 the phase a current reverses in direction and produces a negative pole at ,b1 and as its strength increases from #ij to the #ahj point of the cycle1 and that of phase b diminishes1 the resultant negative pole is shifted past ,c toward ,b1 until a attains its maximum and b falls to its minimum at #ahj1 and the needle points in the direction of ,b4 ,illustration3 ,,fig4 #a1ffc4--;,diagram of two phase1 eight pole field winding4 ,the winding is divided into #af groups 7equal to the product of the number of poles multiplied by the number of phases74 ,each group such as at ,a comprises a number of coils in series1 each coil being located in a separate pair of slots1 the end of one being connected to the beginning of the next4 ,when the currents are in the same direction1 the currents circulate in the same direction in two adjacent groups1 a pole then with this arrangement being formed by two groups1 both phases contributing to the formation of the pole4 ,after cycle when the current in each phase reverses1 the pole advances the angular distance1 covered by two groups2 hence the field completes one revolution in eight alternations of current4 ,illustration3 ,,figs4 #a1ffd to #a1fhc4--,sine curves of two phase current and diagrams showing the physical conception of a two phase rotating magnetic field4 ,the alternating magnetizing current is assumed to be of such strength that1 at its maximum strength1 the field produced may be represented by #aj lines of force as indicated by the parallel lines4 ,at the beginning of the rotation1 fig4 #a1ffd1 phase ,a magnetization1 according to sine curve is zero1 indicated by the solid black poles1 while phase ,b is of strength #aj with ,illustration3 current in the direction to produce a south pole at ,b4 ,similarly1 in fig4 #a1ffe1 the strength of ,a is #d lines1 and of ,b1 #i lines1 the resultant magnetization having rotated #bb4 ,the direction of the resultant magnetization is indicated by the arrow in each figure4 ,it should be noted in fig4 #a1ffi1 that the polarity of ,b is reversed1 the current curve now being above the zero line4 ,by following the arrow through the successive positions the rotation of the resultant magnetization is clearly seen4 ,at the #ahj point of the cycle1 b reverses in direction and produces a negative pole at ,d1 and as the fluctuation of the pressure of the two currents during the second half of the cycle1 from #ahj to #cfj1 bear the same relation to each other as during the first half1 the resultant poles of the rotating magnetic field thus produced carry the needle around in continuous rotation so long as the two phase current traverses the windings of the ring4 ,illustration3 ,,fig4 #a1fhd4--,moving picture method of showing motion of a rotary magnetic field4 ,a number of sheets of paper are prepared1 each containing a drawing of the motor frame and a magnetic needle in successively advancing angular positions1 indicating resultant directions of the magnetism4 ,the sheets are bound together so that the axis of the needle on each sheet coincides4 ,when passing the sheets in one way the revolving field will be seen to rotate in one direction1 while1 when moving the sheets backward1 the rotation of the magnetic field is in the opposite direction1 showing that the reversal of the order of the coils has the effect of reversing the rotation of the magnetic field4 ,production of ,rotating ,magnetic ,field by ,three ,phase ,current4--,a rotating magnetic field is produced by the action of a three phase current in a manner quite similar to the action of a two phase current4 ,fig4 #a1fhe shows a ring suitably wound and supplied with a three phase current at three points ,a1 ,b1 ,c1 #abj of a cycle apart4 ,illustration3 ,,fig4 #a1fhe4--,production of a rotating magnetic field by three phase current4 ,a ring wound as shown is tapped at points ,a1 ,b1 and ,c1 #abj apart1 and connected with leads to a three phase alternator4 ,as described on page #a1cjd1 a rotating magnetic field is produced in a manner similar to the two phase method4 ,illustration3 ,,fig4 #a1fhf4--;,diagram of three phase1 four pole ,y connected field winding4 ,at the instant when the current a1 flowing in at ,a1 is at its maximum1 two currents b and c1 each one-half the value of a1 will flow out ,b and ,c1 thus producing a negative pole at ,a and a positive pole at ,b and at ,c4 ,the resultant of the latter will be a positive pole at ,e1 and consequently1 the magnetic needle will point towards ,a4 ,illustration3 ,,fig4 #a1fhg4--,production of a rotating magnetic field in a two pole three phase motor4 ,in order to obtain a uniformly rotating magnetic field1 it is necessary to arrange the phase windings in the direction of rotation1 in the sequence ,,acb1 not ,,abc as indicated on the magnets4 ,thus poles #a and #d are connected in series to phase ,a1 #b and #e in series to phase ,c1 and #c and #f in series to phase ,b4 ,the different phase windings are differently lined1 and it should be noted that they have a common return wire1 though this is not absolutely necessary4 ,since the phases of the three currents differ from each other by one-third of a period or cycle1 each of the phase windings will therefore set up a field between its poles1 which at any instant will differ1 both in direction and magnitude1 from the fields set up by the other phase windings4 ,hence1 the three phase windings acting together will produce a resultant field1 and if plotted out1 the directions of this field for various fractions of the period is such that in one complete period the resultant field will make one complete round of the poles in a clockwise direction1 as indicated by the curved arrow4 ,the positions of the resultant field during one complete period may be tabulated as follows3 ---------------------------------------- ---------------------------- ,one ,cycle ---------------------------------------- -------------- #j to #fj #fj to #abj to #ahj to #bdj to #cjj to #abj #ahj #bdj #cjj #cfj ---------------------------------------- -------------- ,polarity #a,n - #d,s #b,n - #e,s #c,n - #f,s #d,n - #a,s #e,n - #b,s #f,n - #c,s ---------------------------------------- ---------------------- ,as the cycle advances1 however1 the mutual relations of the fluctuations of the pressures of the three currents1 and the time of their reversals of direction will be such1 that when a maximum current is flowing at any one of the points ,a1 ,b1 and ,c1 two currents each of one-half the value of the entering current will flow out of the other two points1 and when two currents are entering at any two points1 a current of maximum value will flow out of the other point4 ,this action will produce one complete rotation of the magnetic field during each cycle of the current4 ,illustration3 ,,fig4 #a1fhh4--,production of three phase rotating magnetic field with winding on laminated iron ring4 ,the winding is divided into twelve sections1 which are connected in three groups1 ,a1 ,b1 and ,c1 of four sections each1 the sections in each group being evenly placed round the ring with the sections of the two other groups between them4 ,one end of each group is to be connected to the line wire and the other end to the common junction ,j1 from which it follows that the winding given is an example of star winding4 ,with three phase currents the winding will give at every instant four ,n poles and four ,s poles round the ring1 and in actual working these poles will be on the inner periphery because of the presence of an inner ring or cylinder of good magnetic iron placed1 with the requisite clearance to allow of rotation1 as close as is mechanically possible to the outer ring4 ,each one of these eight poles will make a complete revolution round the ring in four times the periodic time of the currents supplied4 ,thus1 if the supply current has a frequency of #ej1 a complete revolution of the field will take place in 4#jh 7_/7 of a second1 which corresponds to an angular velocity of #gej revolutions per minute in place of #c1jjj revolutions per minute1 which would be the angular velocity with a bipolar field at this periodicity4 ,similarly a continuously wound ,gramme ring tapped at twelve points1 joined in three groups of four each to the supply mains1 would give an eight pole rotary field4 ,in this case the grouping would be a mesh grouping1 with each side of the mesh formed of four coils in parallel4 ,illustration3 ,,figs4 #a1fhi to #a1gjh4--,sine curves of three phase current and diagrams showing the physical conception of a three phase rotating magnetic field4 ,the diagrams are constructed in the same manner as explained in figs4 #a1ffd to #a1fhc4 ,it should be noted that the phase windings are arranged in the direction of rotation in the sequence ,,acb1 phase ,c being wound in opposite ,illustration3 sense to ,a and ,b1 as indicated by the curves1 in that north poles are produced at ,a and ,b when the respective curves are above the zero line1 a south pole being produced at ,c when its curve is above the zero line4 ,the rotation of the resultant magnetization is clearly seen by following the arrow through its successive positions4 ,slip4--,instead of the magnetic needle as was used in the preceding figures1 a copper cylinder may be placed in a rotating magnetic field and it will be urged also to turn in the same direction as the rotation of the field4 ,illustration3 ,,fig4 #a1gji4--;,diagram of three phase1 six pole field winding4 ,there are #ah groups1 and the sequence of phases is ,,abc in a counter clockwise direction4 ,for a ,y connection1 the middle phase is reversed1 so that a pole will be formed by the three consecutive phases when the current is in the same direction in ,a and ,c1 and opposite in ,b4 ,the beginning of the middle coil ,c1 and not the end1 as with the other two1 is connected to the common point ,o4 ,in this case the pole shifts a distance equal to three groups for each alternation1 so that one revolution of the field requires three cycles4 ,the torque tending to turn the cylinder is due to the induction of currents of opposite polarity in the cylinder4 ,for simplicity1 the rotating magnetic field may be supposed to be produced by a pair of magnetic poles placed at opposite sides of the cylinder and revolved around it as in fig4 #a1gaj4 ,now1 for instance in starting1 the cylinder being at rest any element or section of the surface as the shaded area ,,ab1 will1 as it comes into the magnetic field of the rotating magnet1 cut ,illustration3 ,,fig4 #a1gaj4--;,copper cylinder and rotating magnet illustrating the principle of operation of an induction motor4 ,the rotating magnetic field which is necessary for induction motor operation is for simplicity here produced by rotating a magnet as shown4 ,in starting1 the cylinder being at rest1 any element as ,,ab1 as it is swept by the field will cut magnetic lines1 which will induce a current upward in direction as determined by applying ,fleming's rule 7fig4 #acb1 page #acc74 ,the inductive action is strongest at the center of the field hence as ,,ab passes the center the induced pressure along ,,ab is greater than along elements more or less remote on either side4 ,accordingly a pair of eddy currents will result as shown 7see fig4 #bia1 page #bga74 ,applying the right hand rule for polarity of these eddy currents 7see fig4 #aai1 page #aag7 it will be seen that a ,s pole is induced by the eddy on the side of the cylinder receding from the magnet1 and a ,n pole by the eddy on the side toward which the magnet is approaching4 ,the cylinder1 then1 is attracted in the direction of rotation of the magnet by the induced pole on the receding side1 and repelled in the same direction by the induced pole on the approaching side4 ,accordingly1 the cylinder begins to rotate4 ,the velocity with which it turns depends upon the load2 it must always turn slower than the magnet1 in order that its elements may cut magnetic lines and induce poles to produce the necessary torque to balance the load4 ,the difference in speed of the magnet and cylinder is called the slip4 ,evidently the greater the load1 the greater is the slip required to induce poles of sufficient strength to maintain equilibrium4 ,the figure is drawn somewhat distorted1 so that both eddies are visible4 magnetic lines of force inducing a current therein1 whose direction is easily determined by applying ,fleming's rule4#f #f ,,note4--,in order to avoid confusion in applying ,fleming's rule1 it may be well to regard the pole as being stationary and the cylinder as in motion2 for1 since motion is purely a relative matter 7see fig4 #a1cic71 the inductive action will be the same as if the pole stood still while the cylinder revolved from left to right1 that is1 counter clockwise1 looking down on it4 ,regarding it thus 7pole stationary and cylinder revolving counter clockwise7 ,fleming's rule 7see fig4 #acb1 page #acc7 is easily applied to ascertain the direction of the induced current1 which is found to flow upward in the shaded area as shown4 ,since the field is not uniform1 but gradually weakens1 as shown1 on either side of the shaded area 7which is just passing the center71 the pressure induced on either side will be less than that induced in the shaded area1 giving rise to eddy currents 7as illustrated in fig4 #bia1 page #bga74 ,these eddy currents induce poles as indicated at the centers of the whorls1 the polarity being determined by applying the right hand rule 7fig4 #aai1 page #aag74 ,illustration3 ,,figs4 #a1gaa to #a1gah4--,parts of ,allis-,chalmers polyphase induction motor with squirrel cage armature4 ,by inspection of fig4 #agaj1 it is seen that the induced pole toward which the magnet is moving is of the same polarity as the magnet2 therefore it is repelled1 while the induced pole from which the magnet is receding1 being of opposite polarity1 is attracted4 ,a torque is thus produced tending to rotate the cylinder4 ,it must be evident that this torque is greatest when the cylinder is at rest1 because the magnetic lines are cut by any element on the cylindrical surface at the maximum rate4 ,moreover1 as cylinder is set in motion and brought up to speed1 the torque is gradually reduced1 because the rate with which the magnetic lines are cut is gradually reduced4 ,ques4 ,what is the essential condition for the operation of an induction motor8 ,ans4 ,the armature1 or part in which currents are induced1 must rotate at a speed slower than that of the rotating magnetic field4 ,in the elementary induction motor1 fig4 #a1gaj1 the cylinder is the armature1 and the rotating magnets are the equivalent of a rotating magnetic field4 ,ques4 ,what is the difference of speed called8 ,ans4 ,the slip4 ,ques4 ,why is slip necessary in the operation of an induction motor8 ,ans4 ,if the armature had no weight and there was no friction offered by the bearings and air1 it would revolve in synchronism with the rotating magnetic field1 that is1 the slip would be zero2 but since weight and friction are always present and constitute a small load1 its speed of rotation will be a little less than that of the rotating magnetic field1 so that induction will take place1 in amount sufficient to produce a torque that will balance the load4 ,illustration3 ,,fig4 #a1gai4--;,general ,electric vertical type induction motor2 sectional view showing oiling system4 ,it is provided with ball thrust bearings and top and bottom guide bearings1 and a continuous flow of oil is maintained through all the bearings by means of a pump which is made integral with the motor4 ,the ball thrust bearings are designed to support the weight of the armature only4 ,in cases where the armature is direct connected a flexible coupling should be used to prevent additional weight coming on the thrust bearings4 ,in operation1 when the motor starts1 the oil1 revolving with the pan1 flows against the stationary nozzle and is forced by its velocity at a high pressure through the oil pipe into the reservoir on top4 ,it then flows down through the ball bearing and upper guide bearing1 through a slot in the armature spider into the lower guide bearing and thence into the oil pan4 ,thus a continuous stream of oil is delivered through all bearings4 ,ques4 ,how is slip expressed8 ,ans4 ,in terms of synchronism1 that is1 as a percentage of the speed of the rotating magnetic field4 ,the slip is obtained from the following formula3 ,slip 7rev4 per sec47 ,sf - ,s or1 expressed as a percentage of synchronism1 that is1 of the synchronous speed1 7,sf - ,s7 #ajj ,slip 77 ----------------- ,sf where ,sf ,synchronous speed1 or ,r4,p4,m4 of the rotatory magnetic field2 ,s ,speed of the armature4 ,the synchronous speed is determined the same as for synchronous motor by use of the following formula3 #b;f ,sf ---- ,p where ,sf ,synchronous speed or ,r4,p4,m4 of the rotating magnetic field2 ,p ,number of poles2 f frequency4 ,illustration3 ,,fig4 #agbj4--,triumph back geared polyphase induction motor4 ,a great many applications1 especially for direct attachment1 require the use of either a very slow or special speed motor4 ,as these are quite costly1 the preferable arrangement1 and one equally as satisfactory1 is the use of a standard speed motor combined with a back geared attachment4 ,rawhide pinions are furnished whenever possible1 insuring smooth running with a minimum of noise4 ,illustration3 ,,figs4 #a1gba to #a1gce4--,parts of ,general ,electric small polyphase induction motors4 ,a1 armature2 ,b1 key for armature shaft2 ,c1 oil ring2 ,d1 bearing lining2 ,e1 bearing head1 pulley end2 ,f1 cap screw for bearing heads2 ,g1 field1 complete with winding1 terminal plate and leads2 ,h1 motor leads2 ,i1 terminal connector for motor leads2 ,j1 soft rubber bushing for motor leads2 ,k1 terminal plate2 ,l1 screw for terminal board2 ,m1 field coils2 ,n1 wooden top sticks for field coils2 ,o1 oil filler2 ,p1 bearing head opposite pulley end2 ,q1 screw for oil well cover2 ,r1 oil well cover2 ,s1 socket pipe plug for bearing head2 ,u1 motor base2 ,v1 yoke for motor base2 ,w1 motor base adjusting screw2 ,x1 bolt for motor base and frame 7short72 ,y1 cap screw for bearing head2 ,z1 internal directive fan2 ,aa1 pulley4 ,the following table gives the synchronous speed for various frequencies and different numbers of poles3 ,table of ,synchronous ,speeds ---------------------------------------- -------------- ,r4,p4,m4 of the rotating magnetic field1 when number of poles is ,frequency------------------------------ -------------- #b #f #aj #af #bj #bd ---------------------------------------- -------- #be #a1ejj #ejj #cjj #ahh #aej #abe #fj #c1fjj #a1bjj #gbj #dej #cfj #cjj #hj #d1hjj #a1fjj #ifj #fjj #dhj #djj #ajj #f1jjj #b1jjj #a1bjj #gej #fjj #ejj #abj #g1bjj #b1djj #a1ddj #ijj #gbj #fjj #abe #g1ejj #b1ejj #a1ejj #ich #gej #fbe ---------------------------------------- -------------- ,ques4 ,how does the slip vary8 ,ans4 ,it varies from about #a per cent4 in a motor designed for very close regulation to #dj per cent4 in one badly designed1 or designed for some special purpose4 ,ques4 ,why is the slip ordinarily so small8 ,ans4 ,because of the very low resistance of the armature1 very little pressure is required to produce currents therein1 of sufficient strength to give the required torque4 ,hence1 the necessary rate of cutting the magnetic lines to induce this pressure in the armature is reached with very little difference between the field speed and armature speed1 that is1 with very little slip4 ,ques4 ,how does the slip vary with the load8 ,ans4 ,the greater the load the greater the slip4 ,in other words1 if the load increase1 the motor will run slower1 and the slip will increase4 ,with the increased slip1 the induced currents and the driving force will further increase4 ,if the motor be well designed so that the field strength is constant and the lag of the armature currents is small1 the driving force developed or torque will be proportional to the slip1 that is the slip will increase automatically as the load is increased1 so that the torque will be proportional to the load4 ,according to ,weiner1 the slip varies according to the following table3 ,,slip ,,of ,,induction ,,motors ---------------------------------------- ---------------------------- ,slip at full load ,slip at full load ,capacity per cent4 ,capacity per cent4 of motor ------------------------ of motor ------------------------ ,h4 ,p4 ,usual limits ,average ,h4 ,p4 ,usual limits ,average ---------------------------------------- -------------------------- #bj to #dj #cj #ae #e to #aa #h #aj #cj #bj #bj #d #aj #g #aj #bj #ae #cj #c #i #f #a #h #bj #ad #ej #b #h #e #b #h #ah #ac #ge #a #g #d #c #h #af #ab #ajj #a #f #c4e #e #g #ae #aa #aej #a #e #c #g #f #ad #aj #bjj #a #d #b4e #aj #g #ab #i #cjj #a #c #b ---------------------------------------- -------------------------- ,illustration3 ,,fig4 #a1gcf4--,sector method of measuring the slip of induction motors4 ,a black disc having a number of white sectors 7generally the same as the number of poles of the induction motor7 is fastened with wax to shaft of the induction motor1 and is observed through another disc having an equal number of sector shaped slits 7that is a similar disc with the white sectors cut out7 and attached to the shaft of a small self-starting synchronous motor1 which is fitted with a revolution counter that can be thrown in or out of gear at will2 then the slip 7in terms of ,nr7 ,n 7,ns ,nr71 in which3 ,n number of passages of the sectors2 ,ns number of sectors2 ,nr number of revolutions recorded by the counter during the interval of observation4 ,for large values of slip1 the observations may be simplified by using only one sector 7,ns #a71 then ,n will equal the slip in revolutions4 ,ques4 ,describe one way of measuring the slip4 ,ans4 ,a simple though rough way is to observe simultaneously the speed of the armature and the frequency1 calculating the slip from the data thus obtained1 as on page #a1cae4 ,this method is not accurate1 as1 even with the most careful readings1 large errors cannot be avoided4 ,a better way is shown in fig4 #a1gcf4 ,illustration3 ,,fig4 #a1gcg4--;,detail of ,westinghouse squirrel cage armature for induction motor4 ,this is an example of cast on construction similar to that of ,morse-,fairbanks 7see figs4 #a1geb1 #a1gec and #a1iae74 ,the inductors are embedded in a special cement4 ,evolution of the ,squirrel ,cage ,armature4--,in the early experiments with rotating magnetic fields1 copper discs were used2 in fact1 it was then discovered that a mass of copper or any conducting metal1 if placed in a rotating magnetic field1 will be urged in the direction of rotation of the field4 ,ferraris used a copper cylinder as in figs4 #a1gaj and #a1gch1 which was the first step in the evolution of the squirrel cage armature4 ,the trouble with an armature of this kind is that there is no definite path provided for the induced currents4 ,illustration3 ,,so ,,called ,,squirrel ,,cage ,illustration3 ,,figs4 #a1gch to #a1gdd4--;,evolution of the squirrel cage armature4 ,the early experiments of ,arago1 ,herschel1 ,babbage and ,baily demonstrated that a mass of copper or any conducting metal1 if placed in a revolving magnetic field1 will be urged to revolve in the direction of the revolving field4 ,they used discs1 but ,ferraris used a copper cylinder as shown in figs4 #a1gaj and #a1gch2 this was the first squirrel cage armature4 ,figs4 #a1gci to #a1gdd show the gradual development of the primitive device shown in fig4 #a1gch2 fig4 #a1gci1 ,ferraris' cylinder with slots restricting the path of induced currents2 fig4 #a1gdj1 ,dobrowolsky's so called squirrel cage which he embedded in a solid iron core1 as in fig4 #a1gda2 fig4 #a1gdb1 design with insulated bars and laminated core to prevent eddy currents in the core2 fig4 #a1gdc1 laminated core with ventilating ducts2 fig4 #a1gdd1 modern squirrel cage armature representing the latest practice as built by ,mechanical ,appliance ,co4 ,the core is built up of discs punched from ,no4 #bi gauge electrical sheet1 insulated from each other and firmly clamped between end plates locked on the shaft4 ,the slots in the discs are of the same general form as those in the core4 ,heavy fibre end pieces1 punched to match the discs are placed at each end of the core1 to prevent the bars coming in contact with the sharp edges of the teeth4 ,the winding is made up of rectangular copper bars1 passing through slots in the core1 and short circuited on each other by means of copper end rings of special design4 ,the bars are pressed into holes punched in the end rings1 and the contact is then protected from corrosion by being dipped in a solder bath4 ,the bars are insulated from the iron of the core by fibre cell projecting beyond the end of the slot4 ,to secure ventilation the short circuiting rings are set some distance from the end of the core4 ,in this way the bars between the core and the ring act as the vanes of a pressure blower1 forcing a large volume of air through the field coils and ventilating openings4 ,illustration3 ,,fig4 #a1gde4--,mechanical ,appliance ,co4 solid core discs as used on small and medium size induction motors4 ,illustration3 ,,fig4 #a1gdf4--;,allis-,chalmers squirrel cage armature construction4 ,the core lamin are mounted on a cast iron spider having arms shaped to act as fan blades for forcing air through the motor4 ,the spider is pressed on to the shaft4 ,in the smallest sizes the punchings are mounted directly on the shaft1 which is properly machined to hold them firmly4 ,copper bars are used as inductors in the larger sizes1 and copper rods in the smaller sizes4 ,the ends of the inductors are turned down somewhat smaller than the body and fit in holes in the end rings4 ,the shoulder thus formed fits firmly against the end rings4 ,good electrical contact is obtained by expanding the inductors in the end ring holes4 ,in large armatures both bars and end rings are of rectangular cross section1 the bars and rings being fastened by machine steel cap screws4 ,obviously1 a better result is obtained if1 in fig4 #a1gch1 the downward returning currents of the eddies are led into some path where they will return across a field of opposite polarity from that across which they ascended1 as in such case1 the turning effect will be doubled4 ,accordingly the design of fig4 #a1gch was modified by cutting a number of parallel slits which extended nearly to the ends1 leaving at each end an uninterrupted ring of metal4 ,this may be called the first squirrel cage armature1 and in the later development ,dobrowolsky was the first to employ a built-up construction1 using a number of bars joined together by a ring at each end1 as in fig4 #a1gdj1 and embedded in a solid mass of iron1 as in fig4 #a1gda2 he regarding the bars merely as veins of copper lying buried in the iron4 ,illustration3 ,,fig4 #a1gdg4--,triumph squirrel cage armature4 ,in construction thin sheet steel laminations1 japanned1 are built up to form the core1 and are rigidly clamped together by heavy malleable iron end plates4 ,semi-enclosed slots are punched in the outer periphery to receive the windings1 so that none of the centrifugal force is carried by the inductors4 ,these inductors are set edge on1 and are riveted and soldered into resistance rings4 ,these rings are punched to receive the inductors in such a manner that there is an unbroken strip of metal completely surrounding them4 ,moreover1 the short circuiting rings are set some distance from the end of the core1 so that the inductors between the core and ring act as vanes to force air through the coils for ventilation4 ,illustration3 ,,fig4 #a1gdh4--;,general ,electric soldered form of end ring construction on squirrel cage armatures4 ,the armature inductors or copper bars laid in the core slots are short circuited by these end rings1 which are also made of copper4 ,for the smaller sizes the rings are thin1 but of considerable radial depth and are held apart by spacing washers4 ,they have rectangular holes punched near their outer peripheries through which the bars pass4 ,lips are formed on the rings1 as shown1 to which the bars are soldered4 ,illustration3 ,,fig4 #a1gdi4--;,general ,electric welded form of end ring construction on squirrel cage armatures4 ,space limitations make it difficult to provide multiple soldered rings of sufficient area for large motors2 hence1 on such machines welding is resorted to1 as shown4 ,the ring in welded construction is placed beneath the bars at each end of the armature4 ,short radial bars are welded to the edges of these rings and to the inductors or squirrel cage bars1 thereby making good electrical contact4 ,a solid cylinder of iron will of course serve as an armature1 as it is magnetically excellent2 but the high specific resistance of iron prevents the flow of induced currents taking place sufficiently copiously2 hence a solid cylinder of iron is improved by surrounding it with a mantle of copper1 or by a squirrel cage of copper bars 7like fig4 #a1gdj71 or by embedding rods of copper 7short circuited together at their ends with rings7 in holes just beneath its surface4 ,however1 since all eddy currents that circle round1 as those sketched in fig4 #a1gch1 are not so efficient in their mechanical effect as currents confined to proper paths1 and as they consume power and spend it in heating effects1 the core was then constructed with laminations lightly insulated from each other1 and further the squirrel cage copper bar inductors were fully insulated from contact with the core4 ,tunnel slots were later replaced by designs with open tops4 ,illustration3 ,figs4 #a1gej and #a1gea4--;,built up core construction with discs punched in one piece4 ,the spider proper consists of a hub provided with four radial arms1 which fit the inner diameter of the disc4 ,the hub is bored out so that it fits very tightly on the shaft1 and a key is provided to avoid any chance of turning4 ,the core disks are clamped firmly in place by two heavy cast iron end plates which are pressed up and held by the bolts4 ,these bolts pass under the discs1 so that there is no danger of their giving rise to eddy currents4 ,the key not only prevents the discs turning on the spider but also ensures the alinement of discs1 which is necessary to make the teeth form smooth slots when the core is assembled4 ,fig4 #a1gdd shows a modern squirrel cage armature conforming to the latest practice1 other designs being illustrated in the numerous accompanying cuts4 ,in the smaller sizes1 the core lamin are of the solid type as shown in fig4 #a1gde1 but for larger motors the core consists of a spider and segmental discs as shown in figs4 #a1gej and #a1gea4 ,fig4 #a1gdh shows a soldered form of end ring construction1 and figs4 #a1geb and #a1gec the method of welding the end ring to the inductors4 ,the ,field ,magnets4--,the construction of the field magnets1 which1 when energized with alternating current produce the rotating magnetic field1 is in many respects identical with the armature construction of revolving field alternators4 ,illustration3 ,,fig4 #a1geb4--;,fairbanks-,morse squirrel cage armature with cast-on rings showing inspection grooves4 ,the method consists in fusing the ends of the inductors into an end ring of a special composition1 thereby producing a perfect electrical and mechanically strong joint4 ,in this process the armature with its bars in place is put into a mould and the molten metal poured around the inductors1 melting their ends and effectually fusing them into the body of the ring4 ,the ring is then turned down to finished size and polished4 ,an inspection groove is cut as shown to indicate that the fusion is complete and the joint perfect4 ,illustration3 ,,fig4 #a1gec4--,section of ,fairbanks-,morse cast-on joint showing union of end ring and inductor4 ,the view shows the ,v-shape inspection groove as described in fig4 #a1geb4 ,broadly1 the field magnets of induction motors consists of3 #a4 ,yoke or frame2 #b4 ,lamin1 or core stampings2 #c4 ,winding4 ,illustration3 ,,fig4 #a1ged4--,richmond field construction for polyphase induction motors1 showing style of winding for use with squirrel cage and wound armature types4 ,ques4 ,what is the construction of the yoke and lamin8 ,ans4 ,they are in every way similar to the armature frame and core construction of revolving field alternators4 ,illustration3 ,,fig4 #a1gee4--,western ,electric squirrel cage armature of high speed induction motor for centrifugal pump service4 ,this armature is an example of heavy duty construction4 ,the inductors are welded to the short circuiting end rings1 the latter being located beneath the inductors1 as shown4 ,fan vanes are provided at one end for ventilation4 ,in the field construction1 the core laminations are assembled in a closed box frame1 and clamped by heavy rings while under hydraulic pressure4 ,the stator coils are form wound and subjected to a special insulating process1 which renders them especially impervious to moisture1 and capable of operating without breakdown in locations which are too damp for ordinary motors4 ,the bearing brackets are of rigid mechanical construction1 and the pulley end bracket and bearings of all sizes are split to facilitate removal of the rotor and complete inspection4 ,these machines range in size from #ej to #bjj horse power1 the rugged construction adapting them to heavy and severe service1 such as is met with in mining1 the construction of dams1 canals1 aqueducts1 tunnels1 etc4 ,illustration3 ,,fig4 #a1gef4--,wagner squirrel cage armature for polyphase induction motor1 as employed on motors of from #e to #be horse power4 ,the features of construction as seen in the illustration are bar inductors1 ventilating passages through the core lamin1 riveted connection between inductors and end rings ventilating vanes on end plate1 extra large end rings4 ,the object of making the rings unusually large is to make the resistance of the rings lower than is desirable for some classes of service1 in order to obtain motors having minimum slip1 increased efficiency1 and maximum overload capacity under normal operation4 ,when the torque required by some very unusual and entirely abnormal installation exceeds that of the average conditions1 it is an easy matter to reduce the section of the end rings1 by turning them down in a lathe1 thereby increasing the resistance and starting torque4 ,field ,windings for ,induction ,motors4--,the field windings of induction motors are almost always made to produce more than two poles in order that the speed may not be unreasonably high4 ,this will be seen from the following3 ,if ,p be the number of pairs of poles per phase1 f1 the frequency1 and ,n1 the number of revolutions of the rotating field per minute1 then #fj f ,n -------- ,p ,thus for a frequency of #ajj and one pair of poles1 ,n #fj #ajj #a #f1jjj4 ,by increasing the number of pairs of poles to #aj1 the frequency remaining the same1 ,n #fj #ajj #aj #fjj4 ,hence1 in design1 by increasing the number of pairs of poles the speed of the motor is reduced4 ,illustration3 ,,fig4 #a1geg4--,richmond squirrel cage armature4 ,the copper bars are double riveted at either end to the resistance rings1 then dipped into a solder bath4 ,ques4 ,state an objection to very high speed of the rotating field4 ,ans4 ,the more rapid the rotation of the field1 the greater is the starting difficulty4 ,ques4 ,besides employing a multiplicity of poles1 what other means is used to reduce the speed8 ,ans4 ,reducing the frequency4 ,ques4 ,what difficulty is encountered with low frequency currents8 ,ans4 ,if the frequency be very low1 the current would not be suitable for incandescent lamp lighting1 because at low frequency the rise and fall of the current in the lamps is perceptible4 ,illustration3 ,,fig4 #a1geh4--;,field construction of ,crocker-,wheeler induction motor with magnetic bridge4 ,steel bridges are inserted in the grooves where the coils are placed1 to protect them from dirt and mechanical injury and at the same time provide a path for the magnetic flux which has a more uniform reluctance1 thereby insuring a better distribution of the flux in the air gap and at the same time retaining open slot construction from which the coils can be readily removed4 ,ques4 ,what is the general character of the field winding8 ,ans4 ,the field core slots contain a distributed winding of substantially the same character as the armature winding of a revolving field polyphase alternator4 ,ques4 ,are the poles formed in the usual way8 ,ans4 ,they are produced by properly connecting the groups of coils and not by windings concentrated at certain points on salient or separately projecting masses of iron1 as in direct current machines4 ,ques4 ,how are the coils grouped8 ,ans4 ,three phase windings are usually ,y connected4 ,illustration3 ,,fig4 #a1gei4--,western ,electric squirrel cage armature4 ,the inductors consist of solid copper bars embedded in the slots of a laminated core1 with their projecting ends securely fitted and soldered to heavy copper rings4 ,ques4 ,what other arrangement is sometimes used8 ,ans4 ,in some cases ,y grouping is used for starting and grouping for running4 99 99 99 99 99 ,starting of ,induction ,motors4--,it must be evident that if the field winding of an induction motor whose armature is at rest1 be connected directly in the circuit without using any starting device1 the machine is placed in the same condition as a transformer with the secondary short circuited and the primary connected to the supply circuit4 ,owing to the very low resistance of the armature1 the machine1 unless it be of very small size1 would probably be destroyed by the heat generated before it could come up to speed4 ,accordingly some form of starting device is necessary4 ,there are several methods of starting1 as with3 #a4 ,resistances in the field2 #b4 ,auto-transformer or compensator2 #c4 ,resistance in armature4 ,illustration3 ,,fig4 #a1gfj4--;,holzer ,cabot combination polyphase induction motor set1 consisting of wound frame and three rotors3 #a1 squirrel cage armature1 #b1 wound armature1 #c1 rotating field4 ,the set is intended for school demonstration of induction motor phenomena4 ,the motor operating with the squirrel cage armature has an inherent constant speed characteristic and on brake tests will show its exceptionally strong starting torque and ability to take excessive overloads4 ,this motor can be used as a generator also1 in the sense that if connected to the line and driven above synchronous speed by some external means1 it will act as an asynchronous generator and return power to the line4 ,for variable speed service1 an armature having a winding upon it similar to that on the frame must be used4 ,external resistances inserted in the armature circuit may be used to produce1 first1 a reduction of starting current1 second1 an increase of starting torque1 or third1 a variation of speed4 ,thus an extensive list of experiments can be performed with this phase wound armature directly along the line of present engineering practice4 ,the phase wound armature can be used as an alternator in the same sense as mentioned above for the squirrel cage machine4 ,for synchronous motor and three phase operation the revolving field with projecting poles and slip rings would be used1 the field being excited from a direct current supply4 ,ques4 ,explain the method of inserting resistances in the field4 ,ans4 ,variable resistances are inserted in the circuits leading to the field magnets and mechanically arranged so that the resistances are varied simultaneously for each phase in equal amounts4 ,these starting resistances are enclosed in a box similar to a direct current motor rheostat4 ,ques4 ,is this a good method8 ,ans4 ,it is more economical to insert a variable inductance in the circuit1 by using an auto-transformer4 ,illustration3 ,,fig4 #a1gfa4--,westinghouse auto-starter4 ,polyphase induction motors may be started by connecting them directly to the circuit with an ordinary switch1 and the smaller motors are started in this way in practice4 ,in the larger motors1 however1 the starting torque at normal voltage is several times its full load torque2 therefore1 they are started on a reduced voltage1 and the full pressure of the circuit is not applied until they have practically reached their operating speed4 ,the figure shows connections with a two phase alternating current circuit4 ,the auto-starter consists of two auto-transformers ,t and ,t'1 each having only a single winding for both primary and secondary1 which are tapped at certain points by switches1 thus dividing the winding into a number of loops1 so that one of several voltages may be applied for starting1 and the starting torque thus adjusted to the work that has to be performed4 ,at the highest points tapped by the switches ,s1 and ,s'1 the full pressure1 and at the lowest points1 the lowest pressure1 is applied to the motor by the operation of the main switch ,m4 ,this switch has four blades and three positions4 ,when thrown to the left as indicated1 it connects the auto-transformers ,t and ,t'1 across the circuits ,a and ,b respectively1 so that the pressure across the transformer coils1 as determined by the position of the switches ,s and ,s'1 is applied to the motor circuits ,a and ,b4 ,the intermediate position of the switch ,m interrupts both circuits4 ,to start the motor1 the switch ,m is thrown to the left and a reduced pressure applied2 after the motor has started and come up to speed the switch ,m is thrown to the right1 thus cutting out the transformer and connecting the motor directly to the circuit4 ,the starting device can be located at a point remote from the motor1 thus eliminating danger from fire due to possible sparks1 in case where it is necessary to install the motors in grain elevators1 woolen mills1 or in any place exposed to inflammable gases1 or floating particles of combustible matter4 ,this feature is also valuable in cases where motors are suspended from the ceiling1 or installed in places not easily accessible4 ,ques4 ,what is the auto-transformer or compensator method of starting8 ,ans4 ,it consists of reducing the pressure at the field terminals by interposing an impedance coil across the supply circuit and feeding the motor from variable points on its windings4 ,illustration3 ,,fig4 #a1gfb4--;,auto-transformer or compensator connections for three phase induction motor4 ,in operation when the double throw switch is thrown over to starting position1 the current for each phase of the motor flows through an auto-transformer1 which consists of a choking coil for each phase1 arranged so that the current may be made to pass through any portion of it 7as #a1 #b1 #c7 to reduce the voltage to the proper amount for starting4 ,after the motor has come up to speed on the reduced voltage1 the switch is thrown over to running position1 thus supplying the full line voltage to the motor4 #g;,in actual construction fuses are usually connected1 so that they will be in circuit in the running position1 but not in the starting position1 where they might be blown by the large starting current4 #g ,,note4--,the construction of starting devices for induction motors is fully explained later1 the accompanying cuts serving merely to illustrate the principles involved4 ,internal ,resistance ,induction ,motors4--,the armature of this type of induction motor differs from the squirrel cage variety in that the winding is not short circuited through copper rings1 but1 in starting1 is short circuited through a resistance mounted directly on the shaft in the interior of the armature4 ,when the motor is thrown in circuit1 a very low starting current is drawn from the line due to the added resistance in the armature4 ,as the motor comes up to speed1 this resistance is gradually cut out1 and at full speed the motor operates as a squirrel cage motor1 with short circuited winding4 _/ ,ques4 ,how is the resistance gradually cut out in internal resistance motors8 ,ans4 ,by operating a lever which engages a collar free to slide horizontally on the shaft4 ,the collar moves over the internal resistance grids 7located within the armature spider71 thus gradually reducing their value until they are cut out4 ,illustration3 ,,fig4 #a1gfc4--,view of armature interior of ,wagner polyphase induction motor with wound armature1 showing the centrifugal device which at the proper speed short circuits all the coils1 transforming the motor to the squirrel cage type4 ,the winding is connected with a vertical commutator so called4 ,inside the armature are two governor weights1 which are thrown outwards by the centrifugal force when the machine reaches the proper speed1 thus pushing a solid copper ring 7which encircles the shaft7 into contact with the inner ends of the commutator bars1 thus completely short circuiting the armature winding4 ,ques4 ,for what size motors is the internal resistance method suited8 ,ans4 ,small motors4 ,ques4 ,why is it not desirable for large motors8 ,ans4 ,the excessive ,i,r loss in the resistances1 if confined within the armature spider1 would produce considerable heating1 and on this account it is best placed external to the motor4 ,ques4 ,on what class of circuit are internal resistance motors desirable8 ,ans4 ,on circuits devoted to lighting service as well as power service1 where a high degree of voltage regulation is essential4 ,the initial rush of current when a squirrel cage motor is thrown on the line is more or less objectionable and there are central stations which allow only resistance type of induction motor to be used on their lines4 ,illustration3 ,,fig4 #a1gfd4--,western ,electric wound armature for internal resistance induction motor4 ,in starting the inductors are short circuited through a resistance which is gradually cut out as the motor comes up to speed4 ,illustration3 ,,figs4 #a1gfe ,,to #agfi4--,western ,electric wound armature for external resistance1 or slip ring induction motor1 showing brush rigging1 slip rings and bar winding4 ,external ,resistance or ,slip ,ring ,motors4--,in large machines1 and those which must run at variable speed1 such as is required in the operations of cranes1 hoists1 dredges1 etc41 it is advisable that the regulating resistances be placed externally to the motor4 ,motors having this feature are commercially known as slip ring motors1 because connections are made between the external resistances and the armature inductors by means of slip rings4 ,illustration3 ,,fig4 #a1ggj4--,richmond slip ring motor4 ,illustration3 ,,fig4 #a1gga4--,richmond slip ring armature as used on motor in fig4 #a1ggj4 ,illustration3 ,,fig4 #a1ggb4--,western ,electric riveted frame slip ring induction motor for variable speed service2 adapted either to continuous or intermittent operation4 ,as with the internal resistance motor the armature winding of a slip ring motor is not short circuited through copper rings in starting1 but through a resistance1 which in this case is located externally4 ,ques4 ,how is the armature winding connected8 ,ans4 ,it is connected in ,y grouping and the free ends connected to the slip rings1 leads going from the brushes to the variable external resistances1 these being illustrated in fig4 #a1ggi4 ,illustration3 ,,figs4 #a1ggc to #a1ggh4--,sprague skeleton type motor frame with various types of armature4 ,fig4 #a1ggg1 plain squirrel cage armature2 fig4 #a1ggh1 internal resistance armature2 fig4 #a1ggc slip ring armature4 ,in the construction of the plain squirrel cage armature1 fig4 #a1ggg1 copper bars are inserted in the slots of the core1 and are insulated from the core by enclosing tubes which project about one-half inch beyond the iron at each side4 ,the bars are short circuited at their ends by copper rings4 ,these rings are thin1 but of considerable radial depth and are held apart by spacing washers4 ,they have rectangular holes punched near their outward periphery1 through which the armature bars pass1 and to which they are soldered4 ,the internal resistance armature1 fig4 #a1ggh1 is provided with a phase winding1 starting 7internal7 resistance1 and switch located on the shaft4 ,the starting resistance is designed to give approximately full load torque with full load current at starting4 ,a greater torque than full load torque can be obtained for starting1 if required1 by cutting out resistance4 ,the resistance consists of cast iron grids enclosed in a triangular cover which is bolted to the end plates holding the armature lamin together1 and is short circuited by sliding laminated spring metal brushes along the inside surface of the grids4 ,the brushes are supported by a metal sleeve sliding on the shaft which is operated by a lever secured to the bearing bracket and located just above the bearing4 ,a rod passing through the end of the shaft operates the short circuiting arrangement in sizes up to about #be horse power4 ,the external resistance or slip ring armature1 fig4 #a1ggc1 is similar in construction to fig4 #a1ggh1 with the exception that slip rings are provided because of the external location of the resistance4 ,these rings connect the inductor through brushes to a controlling and external resistance1 two or more carbon brushes being provided for each ring1 as in fig4 #a1ggf4 ,single ,phase ,induction ,motors4--,the general utility of single phase motors1 particularly the smaller sizes1 is constantly being enlarged by the growing practice of central stations generating polyphase current1 of supplying their lighting service through single phase distribution1 and permitting the use of single phase motors of moderate capacity on the lighting circuit4 ,illustration3 ,,fig4 #a1ggi4--;,external resistance or slip ring induction motor connections4 ,the squirrel cage armature winding is not short circuited by copper end rings1 but connected in ,y grouping and the three free ends connected to three slip rings1 leads going from the brushes to three external resistances1 arranged as triplex rheostat having three arms rigidly connected as shown1 so that the three resistances may be varied simultaneously and in equal amounts4 ,illustration3 ,,fig4 #a1hjj4--;,allis-,chalmers phase wound external resistance type or slip ring armature construction4 ,the winding is for three phases and the terminals are brought out to three slip rings4 ,the front bracket is slightly modified to make room for these rings on the inside4 ,for starting duty sufficient resistance is supplied to reduce the starting current taken by the motor to #a times the normal full load current4 ,in the running position the resistance is all cut out of the circuit4 ,for speed regulation sufficient resistance is supplied to reduce the speed #ej on normal full load torque4 ,illustration3 ,,figs4 #a1hja to #a1hbh4--;,disassembled view of ,western ,electric three phase external resistance or slip ring mill type induction motor4 ,it is adapted to severe working conditions1 such as are met with in steel mills1 crane and hoist service1 etc4 ,designed for #bbj or #ddj volt1 #be cycle circuits4 ,the frame is divided horizontally into an upper and a lower steel casting1 both of which are bolted together at the corners by four heavy bolts4 ,the lower casting is provided with four feet for bolting the motor to its foundation4 ,the end of the upper frame which covers the slip rings is equipped with malleable iron covers held in place by lock bolts4 ,the field and armature are of the usual construction4 ,one end of the armature winding is protected against mechanical injury by the slip rings which are of heavy construction and of practically the same diameter as the armature1 and the other end by a detachable flange of the same diameter as the outside of the winding4 ,the slip rings are mounted on the same spider as the armature1 so that the shaft can be removed without disturbing any of the connections4 ,the brushes are equipped with riveted pigtails1 and held in brass brush boxes machined to gauge4 ,heavy coiled clock springs are used to maintain an even pressure of the brushes on the slip rings4 ,the armature leads are brought out through holes in the upper half of the frame1 and the field leads are brought through a block1 which fits in an opening in the upper edge of the lower half4 ,the simplicity of single phase systems in comparison with polyphase systems1 makes them more desirable for small alternating current plants4 ,the disadvantage of single phase motors is that they are not self-starting4 ,a single phase motor consists essentially of an armature and field magnet having a single phase winding and also some phase splitting arrangement for starting4 ,illustration3 ,,fig4 #ahbi4--;,general ,electric single phase induction motor4 ,it is suitable for constant speed service where full load torque at starting does not exceed #adj per cent41 and in general is adapted to drive all geared and belted machinery requiring constant speed with light or moderate starting torque4 ,illustration3 ,,fig4 #a1hcj4--,simplified diagram showing the principle of phase splitting for starting single phase induction motors4 ,by the use of an auxiliary set of coils connected in parallel with the main coils and having in series a resistance or condenser as shown1 the single phase current delivered by the alternator is split into two phases1 which are employed to produce a rotating field on which the motor is started4 ,illustration3 ,,figs4 #a1hca to #a1hej4--,parts of ,sprague single phase clutch type induction motor4 ,the armature is of the high resistance smooth core squirrel cage type1 the core lamin being assembled upon a steel sleeve4 ,on starting the armature revolves freely around the shaft on roller bearings until it accelerates to about #ge of its rated speed1 when a centrifugal clutch engages with an outer shell keyed directly on the shaft1 thus throwing on the load4 ,this type of motor is adapted to drive all belted1 geared1 or direct connected machinery requiring constant speed with moderate starting torque1 such as generators1 blowers1 line shafting in machine shops and factories1 drill presses1 laundry machinery1 baking machinery1 and the like4 ,when greater torque is required at the moment of starting type ,,ri motors should be used1 or clutch couplings may be installed between the motor and the machine it is to drive4 ,the parts are as follows2 ,a1 field frame2 ,b1 field coils2 ,c1 terminal block2 ,d1 terminal block screws2 ,e1 connectors2 ,f1 bearing head pulley end2 ,g1 bearing head opposite pulley end2 ,h1 motor clamping bolts2 ,i1 oil well cover2 ,j1 oil well plug2 ,k1 drain plug2 ,l1 oil filter2 ,m1 cap bolts2 ,n1 bearing lining2 ,o1 oil ring2 ,p1 belt tightener screw2 ,q1 armature core2 ,r1 latch2 ,s1 driving shell2 ,t1 driving shell set screw2 ,u1 clutch ring2 ,v1 clutch ring spring2 ,w1 spring adjusting screw2 ,x1 nut for belt tightener screw2 ,y1 shaft2 ,z1 driving shell key2 ,aa1 armature bearing2 ,ba1 pulley2 ,ca1 pulley set screw2 ,da1 pulley key2 ,ea1 sliding base2 ,fa1 yoke4 ,ques4 ,why is a single phase motor not self-starting8 ,ans4 ,because the nature of the field produced by a single phase current is oscillating and not rotating4 ,ques4 ,how is a single phase motor started8 ,ans4 ,by splitting the phase1 a field is set up normal to the axis of the armature1 and nearly #ij displaced in phase from the field in that axis4 ,this cross field produces the useful torque4 ,illustration3 ,,fig4 #a1hea4--;,general ,electric high resistance clutch type smooth core squirrel cage armature of single phase induction motor4 ,the core lamin are slotted near the circumference to retain the bar inductors1 which extend beyond the core at either end where they are permanently connected to heavy short circuiting rings4 ,illustration3 ,,figs4 #a1heb to #a1hee4--,parts of ,general ,electric centrifugal clutch pulley as used on clutch type1 single phase induction motor4 ,a1 clutch2 ,b1 friction band2 ,c1 adjusting spring2 ,d1 outer clutch shell with pulley sleeve2 ,e1 solid removable pulley2 ,f1 internal mechanism comprising parts ,a1 ,b1 and ,c2 ,g1 outer shell and pulley comprising parts ,d and ,e4 ,illustration3 ,,figs4 #a1hef and #a1heg4--,partly assembled clutch pulley4 ,f1 internal mechanism comprising parts ,a1 ,b1 ,c1 of fig4 #a1heb4 ,g1 outer shell and pulley1 comprising parts ,d and ,e of fig4 #a1heb4 ,phase ,splitting2 ,production of ,rotating ,field from ,oscillating ,field4--,as previously stated1 an oscillating field1 that is1 one due to a single phase current1 does not furnish any starting torque4 ,it is therefore necessary to provide a rotating field for a single phase induction motor to start on1 which1 after the motor has come up to speed1 may be cut out and the motor will then operate with the oscillating field4 ,a rotating field may be obtained from single phase current by what is known as splitting the phase4 ,illustration3 ,,fig4 #a1heh4--,switch end view of ,general ,electric drawn shell type fractional horse power single phase motor4 ,ques4 ,describe one method of splitting the phase4 ,ans4 ,the field of the motor is provided1 in addition to the main single phase winding1 with an auxiliary single phase winding1 and the two windings are connected in parallel to the single phase supply mains with a resistance or a condenser placed in series with the single phase winding1 as shown in diagram fig4 #a1hcj1 the two windings being displaced from each other on the armature about #ij magnetic degrees1 just as in the ordinary two phase motor4 ,ques4 ,what is the construction of the two windings8 ,ans4 ,the main coils are of more turns than the auxiliary1 being spread over more surface1 and are heavier because they are for constant use2 whereas the auxiliary coils are used only while starting4 ,illustration3 ,,figs4 #a1hei to #a1hfb4--;,detail construction of clutch parts of ,general ,electric drawn shell type fractional horse power single phase motor4 ,the starting switch1 which is assembled within the motor frame1 consists essentially of three parts3 a rotating member mounted on the armature and provided with two spring controlled pivoted levers in contact with an insulated collector ring4 ,ques4 ,what are the auxiliary coils sometimes called8 ,ans4 ,starting coils4 ,ques4 ,what are shading coils8 ,ans4 ,auxiliary coils as placed on fan motors in the manner shown in fig4 #a1hfc4 ,ques4 ,how can single phase motors be started without the use of external phase splitting devices8 ,ans4 ,such apparatus may be avoided by having the auxiliary winding of larger self-inductance than the main winding4 ,ques4 ,what is the character of the starting torque produced by splitting the phase8 ,ans4 ,it does not give strong starting torque4 ,illustration3 ,,fig4 #a1hfc4--,single phase fan motor with shading coils for starting4 ,in addition to the main field coils1 one tip of each pole piece is surrounded by a short circuited coil of wire or frame of copper1 as indicated in the figure4 ,this coil1 or copper frame1 is called a shading coil and it causes a phase difference between the pulsating flux that emanates from the main portion of each polar projection and the pulsating flux which emanates from the pole tip1 thus introducing a two phase action on the armature which is sufficiently pronounced to start the motor4 ,ques4 ,how is the plain squirrel cage armature modified to enable the motor to start with a heavier load8 ,ans4 ,an automatic clutch is provided which allows the armature to turn free on the shaft until it accelerates almost to running speed4 ,this type motor is known as the clutch type of single phase induction motor4 ,in operation when the circuit is closed1 the armature starts to revolve upon the shaft2 when it reaches a premeditated speed1 a centrifugal clutch expands and engages the clutch disc1 which is fastened to the shaft4 ,illustration3 ,,fig4 #a1hfd4--;,diagram showing action of shading coil in alternating current motor4 ,the extremities of these pole pieces are divided into two branches1 one of which a copper ring called a shading coil is placed as shown1 while the other is left unshaded4 ,the action of the shading coils is as follows3 ,consider the field poles to be energized by single phase current1 and assume the current to be flowing in a direction to make a north pole at the top4 ,consider the poles to be just at the point of forming4 ,lines of force will tend to pass downward through the shading coil and the remainder of the pole4 ,any change of lines within the shading coil generates an e4m4f41 which causes to flow through the coil a current of a value depending on the e4m4f4 and always in a direction to oppose the change of lines4 ,the field flux is1 therefore1 partly shifted to the free portion of the pole1 while the accumulation of lines through the shading coil is retarded4 ,illustration3 ,,figs4 #a1hfe and #a1hff4--;,fort ,wayne split phase factional horse power induction motor with stationary armature4 ,the object of placing the squirrel cage armature winding on the stationary part or frame is to decrease the radial depth of the latter more than would be possible with the usual arrangement where the armature forms the rotating part4 ,the small radial depth of the stationary armature makes possible a revolving field of maximum diameter giving in turn an exceptionally large air gap area1 which reduces the magnetizing current1 hence improves the power factor of the motor4 ,ques4 ,explain in detail the action of the clutch type of motor in starting4 ,ans4 ,it can start a load which requires much more than full load torque at starting1 because the motor being nearly up to full speed1 has available not only its maximum overload capacity1 but also the momentum of the armature to overcome the inertia of the driven apparatus4 ,in this it is assisted by a certain amount of slippage in the clutch1 which is the case when the armature speed is pulled down to such a point as to reduce the grip of the centrifugal clutch4 ,illustration3 ,,figs4 #a1hfg and #a1hfh4--;,general ,electric disassembled clutch as used on clutch type1 single phase 7,,ks7 induction motor4 ,in starting1 the armature revolves freely on the shaft until approximately #ge per cent4 of normal rated speed is reached4 ,the load is then picked up by the automatic action of a centrifugal clutch1 which rigidly engages an outer shell1 keyed directly to the shaft4 ,the brass friction band of the clutch is permanently keyed to the pulley end of the armature4 ,commutator ,motors4--,machines of this class are similar in general construction to direct current motors4 ,they have a closed coil winding1 which is connected to a commutator4 ,there are several types of commutator motor1 namely3 #a4 ,series2 #b4 ,shunt2 #c4 ,compensated2 #d4 ,repulsion4 ,since1 as stated1 commutator motors are similar to direct current motors1 the question may be asked3 ,is it possible to run a direct current motor with alternating current8 ,if the mains leading to a direct current motor be reversed1 the direction or rotation remains the same1 because the currents through both the field magnets and armature are reversed4 ,it must follow then that an alternating current applied to a direct current motor would cause rotation of the armature4 ,illustration3 ,,fig4 #a1hfi4--,wagner single phase variable speed commutator motor4 ,the commutator is of the regular horizontal type and the brushes remain in contact all the time4 ,as the torque of alternating motors varies directly as the square of the applied pressure1 wide speed variation may be obtained by varying the voltage applied at the motor terminals4 ,action of ,closed ,coil ,rotating in ,alternating ,field4--,when a closed coil rotates in an alternating field1 there are several different pressures set up and in order to carefully distinguish between them1 they may be called3 ,illustration3 ,,figs4 #a1hgj and #a1hga4--;,diagrams illustrating construction and operation of ,wagner unity power factor single phase motor4 ,in the field construction1 fig4 #a1hgj1 two windings are used4 ,the main winding #a produces the initial field magnetization as heretofore2 the auxiliary winding #b controls the power factor or compensates the motor4 ,the main structural departure is in the armature1 the construction of which is more clearly indicated in fig4 #a1hga4 ,here again two windings are employed4 ,the main or principal winding #d is of the usual well known squirrel cage type and occupies the bottom of the armature slots4 ,the second or auxiliary winding #c is of the usual commuted type1 is connected to a standard form of horizontal commutator and occupies the upper portion of the armature slots4 ,between the two is placed a magnetic separator in the form of a rolled steel bar4 ,two sets of brushes are provided1 as indicated in the diagram of connections shown in fig4 #a1hgj4 ,the main pair of brushes #e-f is placed in the axis of the main field winding #a and is short circuited4 ,the auxiliary pair of brushes #g-h is placed at right angles to the axis of the main field winding and is connected in series with it at starting4 ,the auxiliary field winding #b is permanently connected to one auxiliary brush #g1 and is adapted to be connected to the other auxiliary brush #h by means of the switch #i4 ,the purpose of the peculiar armature construction illustrated in fig4 #a1hga and of the brush arrangement and connections shown in fig4 #a1hgj is to accentuate1 at starting1 the effect of the squirrel cage along the axis #e-f of the main field winding #a1 while suppressing it as far as possible along the axis #g-h at right angles to main winding4 ,the magnetic separator placed above the squirrel cage winding #d tends to suppress the effect of that winding along all axes1 by making it less responsive to outside inductive effects4 ,but the influence of the separator is nullified along the axis of the main field winding by the presence of the short circuited brushes #e-f1 while no means are provided for nullifying its effects along the axis at right angles to that of the main field winding4 ,thus the main field winding #a will be able to induce heavy currents in both armature windings because of the short circuited brushes in the axis #e-f1 and in spite of the magnetic separator2 while the armature winding #c1 connected in series with #a1 will not be able to produce heavy currents in the squirrel cage winding #d along the axis #g-h because of the magnetic separator between #c and #d1 which shunts or side tracks the inducing magnetic flux4 ,in operation1 at starting1 switch #i of fig4 #a1hgj is open1 the commuted winding #c along the axis #g-h being connected in series with the main field winding #a and across the mains4 ,the winding #a induces a large current in the armature windings #c and #d along the axis #e-f1 and the winding #c produces a large flux along the axis #g-h4 ,the armature currents in the main axis co-acting with the flux threading the armature along the auxiliary axis yield the greater part of the starting torque4 ,as the motor speeds up1 the squirrel cage gradually assumes those functions which it performs in the ordinary single phase1 squirrel cage motor1 developing a magnetic field of its own along the axis #g-h and a correspondingly powerful torque1 which increases very rapidly as synchronism is approached1 but falls suddenly to zero at or near actual synchronism4 ,it is known that the magnetizing currents circulating in the bars of the squirrel cage of a single phase motor have1 at synchronism1 double the frequency of the stator currents2 the fluxes they produce must therefore also be of double frequency4 ,now1 the magnetic separator is made of solid steel1 and1 while this separator forms a sufficiently effective shunt for the fluxes of line frequency induced from the field1 it is quite ineffective as a shunt for the double frequency fluxes produced by the armature4 ,with respect to the squirrel cage1 the effect of this magnetic separator diminishes with increasing speed1 and at synchronism the machine operates practically in the same manner as if the magnetic separator did not exist4 #a4 ,the transformer pressure2 #b4 ,the generated pressure2 #c4 ,the self-induction pressure4 ,these pressures may be defined as follows3 ,the transformer pressure is that pressure induced in the armature by the alternating flux from the field magnets4 ,illustration3 ,,fig4 #a1hgb4--;,diagram of ring armature in alternating field illustrating the principles of commutator motors4 ,for instance1 assuming in fig4 #a1hgb the armature to be at rest1 as the alternating current which energizes the magnets rises and falls in value1 the variations of flux which threads through the coils of the ring winding1 induce pressure in them in just the same way that pressure is induced in the secondary of a transformer4 ,a ring winding is used for simplicity2 the same conditions obtain in a drum winding4 ,the generated pressure is that pressure induced in the armature by the cutting of the flux when the armature rotates4 ,the self-induction pressure is that pressure induced in both the field and armature by self-induction4 ,nature of the ,generated ,pressure4--,in fig4 #a1hgb1 the generated pressure induced by the rotation of the armature is minimum at the neutral plane ,c ,d and maximum at ,a ,b4 ,it tends to cause current to flow up each half of the armature from ,d to ,c1 producing poles at these points4 ,illustration3 ,,fig4 #a1hgc4--,wagner single phase repulsion induction commutator motor4 ,its working principle is repulsion start and induction operation4 ,starting with the machine at rest1 brushes in pairs cross connected through a low resistance conductor1 bear upon the commutator1 temporarily short circuiting the armature winding then developing a strong starting torque on the repulsion principle4 ,on attaining full load speed the individual segments of the commutator are all positively connected together by the operation of an automatic centrifugal governor1 thereby transforming the armature winding to the squirrel cage form1 the motor then continuing as an induction motor4 ,the governor at the same time removes the brushes from contact with the commutator to save wear4 ,if the power service should fail for any reason1 the motor returns to the starting condition1 and picks up its load when the power comes on again without attention of the operator4 ,nature of the ,transformer ,pressure4--,this is caused by variations of the flux passing through each coil of the armature winding4 ,evidently this variation is least at the plane ,a ,b because at this point the coils are inclined very acutely to the flux1 and greatest at the plane ,c ,d where the coils are perpendicular to the flux4 ,accordingly1 the transformer pressure induced in the armature winding is least at ,a ,b and greatest at ,c ,d4 ,the transformer pressure acts in the same direction as the generated pressure as indicated by the long arrows and gives rise to what may be called local armature currents4 ,illustration3 ,,figs4 #a1hgd and #a1hge4--;,armature of ,wagner single phase repulsion-induction commutator motor as seen from the commutator and rear ends1 showing the vertical commutator and type of governor employed on the smaller sizes4 ,the operation is explained in fig4 #a1hgc4 ,nature of the ,self-induction ,pressure4--,the self-induction pressure1 being opposite in direction to the impressed pressure1 it must be evident that in the operation of an alternating current commutator motor1 the impressed pressure must overcome not only the generated ,illustration3 ,,fig4 #a1hgf4--;,general ,electric single phase compensated repulsion motor4 ,the frame is of the riveted form and the field winding consists of distributed concentric coils1 each being separately insulated and taped up to each core slot4 ,the compensating winding 7depending usually on the size of frame71 forms either the center portion of the main winding or a separate winding concentric therewith4 ,the polar groupings are arranged for a frequency of #be and #fj4 ,there are four terminal leads permitting interchangeability of operation on #aaj or #bbj volt circuits4 ,by connecting adjacent pairs of these terminals in multiple1 motors of this type are made adaptable for #aaj volt service2 for double this pressure the four leading in wires are connected in series4 ,the motor will operate satisfactorily where the arithmetical sum of voltage and frequency variation does not exceed #aj per cent42 that is1 the voltage may be #aj per cent4 high if the frequency remain at normal1 or the frequency may be #aj per cent4 high assuming no variation in voltage4 ,a decrease of #e per cent4 in frequency accompanied by a similar increase in voltage is permissible or1 as above stated1 any similar combination whose arithmetical sum is within #aj per cent4 of normal4 ,the armature winding is of the series drum type connected to a commutator carrying two sets of brushes1 each set being displaced electrically from the other by #ij degrees4 ,the first set1 known as the energy brushes1 is permanently short circuited and disposed at an angle to the lines of field or primary magnetization1 as in an ordinary repulsion motor4 ,the second set1 or compensating brushes1 is connected to a small portion of the primary winding included in the field circuit1 so as to impress upon the armature an electromotive force1 which serves both to raise the power factor and at the same time maintain approximately synchronous speed at all loads4 ,the armature laminations are built up on a cast iron sleeve having the same inside bore as the commutator4 ,in case the shaft become damaged or worn1 it can be readily pressed out and replaced without disturbing the commutator or windings4 ,the motor is connected to run counter clockwise4 ,clockwise rotation is obtained by interchanging the leads to the compensating brushes and slightly shifting the brush holder yoke4 ,this type motor may be thrown on the line without the use of a rheostat1 and is suitable for operating refrigerating machines1 air compressors1 house pumps or similar apparatus where a float switch or pressure regulator is used to close or open the supply circuit4 _/ pressure but also the self-induction pressure4 ,hence1 as compared to an equivalent direct current motor1 the applied voltage must be greater than in the direct current machine1 to produce an equal current4 ,illustration3 ,,fig4 #a1hgg4--;,armature of ,general ,electric single phase compensated repulsion motor1 assembled ready for dip and banding4 ,illustration3 ,,fig4 #a1hgh4--;,cast brush rigging of ,general ,electric single phase compensated repulsion motor as used for the #c and #e horse power motors4 ,the ,local ,armature ,currents4--,these currents produced by the transformer pressure occur in those coils undergoing commutation4 ,they are large1 because the maximum transformer action occurs in them1 that is1 in the coils short circuited by the brushes4 ,ques4 ,why do the local armature currents cause sparking8 ,ans4 ,because of the sudden interruption of the large volume of current1 and also because the flux set up by the local currents being in opposition to the field flux1 tends to weaken the field just when and where its greatest strength is required for commutation4 ,illustration3 ,,fig4 #a1hgi4--;,field of ,sprague single phase compensated repulsion motor4 ,the frame is of the skeleton form which exposes the core1 giving effective heat radiation4 ,the single phase field winding is of the distributed concentric type4 ,to facilitate connection to circuits of either #aaj or #bbj volts1 four plainly tagged leads are brought out to the back of the removable terminal board4 ,ques4 ,what is the strength of the local current8 ,ans4 ,they may be from #e to #ae times the strength of the normal armature current4 ,ques4 ,upon what does the local armature current depend8 ,ans4 ,upon the number of turns of the short circuited coils1 their resistance1 and the frequency4 ,ques4 ,how can the local currents be reduced to avoid heavy sparking8 ,ans4 #a4 ,by reducing the number of turns of the short circuited coils1 that is1 providing a greater number of commutator bars2 #b1 reducing the frequency2 and #c1 increasing the resistance of the short circuited coil circuit3 a1 by means of high resistance connectors2 or b1 by using brushes of higher resistance4 ,illustration3 ,figs4 #a1hhj to #a1hhd4--;,assembly and disassembled view of short circuiting device as used on ,bell single phase repulsion induction motor4 ,the armature1 which is wound in a similar manner to those used in direct current motors1 has a commutator1 and brushes1 which being short circuited on themselves1 allow great starting torque1 with small starting current4 ,the motor starts by the repulsion principle1 and on reaching nearly full speed1 a centrifugal governor pushes the copper ring against the commutator segments1 thereby short circuiting them1 and the motor then operates on the induction principle4 ,ques4 ,what are high resistance connectors8 ,ans4 ,the connectors between the armature winding and the commutator bars1 as shown in fig4 #a1hhe4 ,ques4 ,does the added resistance of preventive leads1 or high resistance brushes1 materially reduce the efficiency of the machine8 ,ans4 ,not to any great extent1 because it is very small in comparison with the resistance of the whole armature winding4 ,illustration3 ,,fig4 #a1hhe4--,section of ring armature of commutator motor showing local current set up by transformer action of the alternating flux4 ,ques4 ,what is the objection to reducing the number of turns of the short circuited coils to diminish the tendency to sparking8 ,ans4 ,the cost of the additional number of commutator bars and connectors as well as the added mechanism4 ,ques4 ,what effect has the inductance of the field and armature on the power factor8 ,ans4 ,it produces phase difference between the current and impressed pressure resulting in a low power factor4 ,ques4 ,what is the effect of this low power factor8 ,ans4 ,the regulation and efficiency of the system is impaired4 ,the frequency1 the field flux and the number of turns in the winding have influence on the power factor4 ,ques4 ,how does the frequency affect the power factor8 ,ans4 ,lowering the frequency tends to improve the power factor4 ,the use of very low frequencies has the disadvantage of departing from standard frequencies1 and the probability that the greater cost of transformers and alternators would offset the gain4 ,illustration3 ,,fig4 #a1hhf4--;,general ,electric #e ,h4,p41 #f pole adjustable speed single phase compensated repulsion motor4 ,this type is suitable for service requirements demanding the use of a motor whose speed can be adjusted over a considerable range1 this speed at a fixed controller setting remaining practically unaffected by any load within the motor's rated capacity4 ,with the controller on the high speed points1 the motor possesses an inherent speed regulation between no load and full load of approximately #f per cent4 ,at the low speed points1 under similar load conditions1 the speed variation will be approximately #bj per cent4 ,to secure adjustable speed control1 the armature circuits employ transformers1 whose primaries are excited by the line circuit4 ,the secondaries of these transformers are divided into two sections2 the first or regulating circuit is placed across the energy brushes2 the other section1 since it is connected in series with the compensating winding1 maintains the high power factor and speed regulation obtained in the constant speed type4 ,the speed range is #b3a1 approximately one-half of this range being below and one-half above synchronous speed4 ,series ,motors4--,this class of commutator motor is about the simplest of the several types belonging to this division4 ,in general design the series motor is identical with the series direct current motor1 but all the iron of the magnetic circuit must be laminated and a neutralizing winding is often employed4 ,it will be readily understood that the torque is produced in the same way as in the direct current machine1 when it is remembered that the direction of rotation of the direct current series motor is independent of the direction of the voltage applied4 ,at any moment the torque will be proportional to the product of the current and the flux which it is at that moment producing in the magnetic system1 and the average torque will be the product of the average current and the average flux it produces1 so that if the iron parts be unsaturated1 as they must be if the iron losses are not to be too high1 the torque will be proportional simply to the square of the current1 there being no question of power factor entering into the consideration4 ,illustration3 ,,fig4 #a1hhg4--;,diagram of single phase series commutator motor4 ,it is practically the same as the series direct current motor1 with the exception that all the metal of the magnetic circuit must be laminated4 ,ques4 ,what are the characteristics of the series motor8 ,ans4 ,they are similar to the direct current series motor1 the torque being a maximum at starting and decreasing as the speed increases4 ,ques4 ,for what service is the series motor especially suited8 ,ans4 ,on account of its powerful starting torque it is particularly desirable for traction service4 ,neutralized ,series ,motor4--,a chief defect of the series motor is the excessive self-induction of the armature1 hence in almost every modern single phase series motor a neutralizing coil is employed to diminish the armature self-induction4 ,the neutralizing coil is wound upon the frame #ij magnetic degrees or half a pole pitch from the field winding and arranged to carry a current equal in magnetic pressure and opposite in phase to the current in the armature4 ,illustration3 ,,fig4 #a1hhh4--;,diagram of neutralized series motor2 conductive method4 ,in the simple series motor1 there will be a distortion of the flux as in the direct current motor4 ,as the distorting magnetic pressure is in phase with that of the magnets1 the distortion of the flux will be a fixed effect4 ,if the poles be definite as in direct current machines1 this distortion may not seriously affect the running of the motor1 but with a magnetizing system like that universally adopted in induction motors the flux will be shifted as a whole in the direction of the distortion1 which will produce the same effect as if in the former case the brushes had been shifted forward1 whereas for good commutation they should have been shifted backward4 ,as in direct current machines1 this distortion is undesirable since it is not conducive to sparkless working1 and also reduces to a more or less extent the torque exerted by the motor4 ,the simplest remedy is to provide neutralizing coils displaced #ij magnetic degrees to the main field coils as shown4 ,the neutralizing current is obtained by the method of connecting the neutralizing coils in series in the main circuit4 ,the current through the neutralizing winding may be obtained1 either #a4 ,conductively2 or #b4 ,inductively4 ,in the conductive method1 fig4 #a1hhh1 the winding is connected in series as shown4 ,in the inductive method1 fig4 #a1hhi1 the winding is short circuited upon itself and the current obtained inductively1 the neutralizing winding being virtually the secondary of a transformer1 of which the armature is the primary4 ,ques4 ,when is the conductive method to be preferred8 ,ans4 ,when the motor is to be used on mixed circuits4 ,illustration3 ,,fig4 #a1hhi4--;,diagram of neutralized series motor2 inductive method4 ,although the conductive method of neutralization is employed in nearly all machines1 it is possible merely to short circuit the neutralizing winding upon itself1 instead of connecting it in series with the armature circuit4 ,in this case the flux due to the armature circuit cannot be eliminated altogether1 as sufficient flux must always remain to produce enough pressure to balance that due to the residual impedance of the neutralizing coil4 ,it would be a mistake to infer1 however1 that on this account this method of neutralization is less effective than the conductive one1 since the residual flux simply serves to transfer to the armature circuit a drop in pressure precisely equivalent to that due to the resistance and local self-induction of the neutralizing coil in the conductive method4 ,shunt ,motors4--,the simple shunt motor has inherently many properties which render it unsuitable for practical use1 and accordingly is of little importance4 ,owing to the many turns of the field winding there is large inductance in the shunt field circuit4 ,illustration3 ,,fig4 #a1hij4--;,diagram of simple shunt commutator motor4 ,owing to its many inherent defects it is of little importance4 ,illustration3 ,,fig4 #a1hia4--;,compensated shunt induction single phase motor4 ,the transformer shown in the arrangement is capable of being replaced by a coil placed on the frame having the same axis as the field winding1 so that the flux produced by the field winding induces in the coil a pressure in phase with the supply pressure4 ,such a coil will now be at right angles to the circuit to which it is connected4 ,in a similar manner a coil at right angles to the armature circuit1 that is1 the circuit parallel to the stator axis1 if connected in series with that circuit1 will also serve to compensate the motor4 ,the inductance of the armature is small as compared with that of the field2 accordingly1 the two currents differ considerably in phase4 ,the phase difference between the field and armature currents and the corresponding relation between the respective fluxes results in a weak torque4 ,illustration3 ,,fig4 #a1hib4--;,fynn's shunt conductive single phase motor4 ,in order to supply along the stator axis a constant field1 suitable for producing the cross flux to which the torque is due by its action on the circuit perpendicular to the stator axis1 the armature circuit1 as it may be called1 has a neutralizing coil in series with it1 so that the armature circuit and neutralizing coil together produce no flux4 ,in addition to this1 there is a magnetizing coil along the same axis1 which is connected across the mains and so produces the same flux as the primary coil in a shunt induction machine4 ,fynn has proposed a number of methods of varying the speed and compensating this machine4 ,it is1 however1 complicated in itself1 and is only suited for very low voltages1 so that on ordinary circuits it would need a separate transformer4 ,it is necessary to use laminated construction in the field circuit to avoid eddy currents1 which otherwise would be excessive4 ,fig4 #a1hij is a diagram of a simple shunt commutator motor4 ,repulsion ,motors4--,in the course of his observations on the effects of alternating currents1 in #ahhf-g1 ,elihu ,thomson observed that a copper ring placed in an alternating magnetic field tends either to move out of the field1 that is1 it is repelled by the field 7hence the name repulsion motor71 or to return so as to set itself edgeways to the magnetic lines4 ,the explanation of the repulsion phenomenon is as follows3 ,when a closed coil is suspended in an alternating field so that lines of force pass through it1 as in fig4 #a1hic1 an alternating pressure will be induced in the coil which will be #ij later in phase than the inducing flux1 and since every coil contains some inductance the resulting current will lag more or less with respect to the pressure induced in the coil4 ,illustration3 ,,fig4 #a1hic4--;,effect of alternating field on copper ring4 ,if a copper ring be suspended in an alternating field so that the plane of the ring is oblique to the lines of force1 it will turn until its plane is parallel to the lines of force1 that is1 to the position in which it does not encircle any lines of force4 ,the turning moment acting upon the ring is proportional to the current in it1 to the strength of the field1 and to the cosine of the angle 4 ,hence it is proportional to the product sin cos 4 ,the tendency to turn is zero both at #j and at #ij2 in the former case because there is no current1 in the latter because the current has no leverage4 ,it is a maximum when #de4 ,even in this position there would be no torque if there were no lag of the currents in the ring1 for the phase of the induced pressure is in quadrature with the phase state of the field4 ,when the field is of maximum strength there is no pressure1 and when the pressure reaches its maximum there is no field4 ,if there be self-induction in the ring causing the current to lag1 there will be a net turning moment tending to diminish 4 ,the largest torque will be obtained when the lag of the current in the ring is #de4 ,the cosine of this phase relation becomes a negative quantity which means that the coil is repelled by the field4 ,it is only when the ring is in an oblique position that it tends to turn4 ,if it be placed with its plane directly at right angles to the direction of the magnetic lines1 it will not turn2 if ever so little displaced to the right or left1 it will turn until its plane is parallel to the lines4 ,illustration3 ,,figs4 #a1hid to #a1ijh4--,parts of ,general ,electric single phase compensated repulsion motor4 ,the field frame employs the riveted form of construction1 so that the ends of the laminations are exposed directly to the air1 insuring low operating temperatures and high overload capacity4 ,the field winding consists of a main winding of the distributed concentric type and a compensating winding4 ,the series type of winding is employed1 and the completed rotor is treated with a special insulating compound1 which renders the coils moisture proof under ordinary conditions4 ,on motors of more than #b horse power capacity a ventilating fan is attached to the rotor which provides a continuous supply of cool air while the motor is in operation4 ,two types of brush holder yoke are used4 ,the smaller motors use a moulded yoke of insulting compound1 reinforced by a cast iron ,l section ring embedded in the moulded structure4 ,cast iron yokes are used on larger motors4 ,the brushes are of carbon with copper pigtails1 which carry all the current4 ,the brushes in this machine remain permanently in contact with the commutator4 ,the parts are3 ,a1 field2 ,b1 field winding2 ,c1 line terminal2 ,d1 tube terminal2 ,e1 compensating terminal2 ,f1 terminal board2 ,g1 brush yoke2 ,h1 brush holder2 ,i1 carbon brush2 ,j1 brush stud2 ,k1 short circuit connection2 ,l1 armature2 ,m1 commutator1 ,n1 shaft2 ,o1 fan2 ,p1 commutator end shield2 ,q1 pulley end shield2 ,r1 oil well cover2 ,s1 oil plugs2 ,t1 oil gauge2 ,u1 bearing lining2 ,v1 oil ring2 ,w1 pulley2 ,x1 pulley set screw2 ,y1 commutator end shield holding bolts2 ,z1 pulley end shield holding bolts2 ,,aa1 base2 ,,bb1 float bolts2 ,,cc1 belt tightener screw4 ,the production of torque may be explained by saying that the current induced in the ring produces a cross field which being out of phase with1 and inclined to the field impressed by the primary alternating current1 causes a rotary field1 and this in turn1 reacting on the conductor1 a turning moment results4 ,illustration3 ,,fig4 #a1iji4--;,fynn's compensated shunt induction motor4 ,this is a combination of the compensated shunt induction motor with the ordinary squirrel cage form4 ,in one form1 in addition to the ordinary drum winding on the armature1 there is another three phase winding into the star1 of which the drum winding is connected4 ,this second winding is connected to three slip rings which are short circuited when the machine is up to speed4 ,upon the commutator are placed a pair of brushes connected to an auxiliary winding placed on the frame in such a position that the flux from the primary coil induces in it a pressure of suitable phase to produce compensation4 ,the same pair of brushes is also used for starting4 ,elihu ,thompson took an ordinary direct current armature1 placed it in an alternating field1 and having short circuited the brushes1 placed them in an oblique position with respect to the direction of the field4 ,the effect was to cause the armature to rotate with a considerable torque4 ,the inductors of the armature acted just as an obliquely placed ring1 but with this difference1 that the obliquity was continuously preserved by the brushes and commutator1 notwithstanding that the armature turned1 and thus the rotation was continuous4 ,this tendency of a conductor to turn from an oblique position was thus utilized by him to get over the difficulty of starting a single phase motor4 ,with this object in view he then constructed motors in which the use of commutator and brushes was restricted to the work of merely starting the armature1 which when so started was then entirely short circuited on itself1 though disconnected from the rest of the circuit1 the operation then being solely on the induction principle4 ,illustration3 ,fig4 #a1iaj4--;,diagram of connection of ,sprague single phase compensated repulsion motor4 ,to reverse direction of rotation interchange leads ,c and ,c and slightly shift the brush holder yoke4 ,brushes ,e and ,e are permanently short circuited4 ,this diagram of connections applies also to fig4 #a1iaa4 ,ques4 ,what difficulty was experienced with ,thomson's motor8 ,ans4 ,since an open coil armature was used1 the torque developed was due to only one coil at a time1 which involved a necessarily high current in the short circuited coil resulting in heavy sparking4 ,ques4 ,how was this remedied8 ,ans4 ,by the use of closed coil armatures in later construction4 ,ques4 ,did this effectually stop sparking8 ,ans4 ,no4 ,ques4 ,what other means is employed in modern designs to reduce sparking8 ,ans4 ,compensation and the use of a distributed field winding1 high resistance connectors1 high resistance brushes1 etc4 ,ques4 ,what are the names of the two classes of repulsion motor8 ,ans4 ,the simple and the compensated types4 ,ques4 ,describe a simple repulsion motor4 ,ans4 ,it consists essentially of an armature1 commutator and field magnets4 ,the armature is wound exactly like a direct current armature1 and the windings are connected to a commutator4 ,the carbon brushes which rest on this commutator are not connected to the outside line1 however1 but are all connected together through heavy short circuiting connectors4 ,the brushes are placed about #fj or #gj from the neutral axis4 ,the field is wound exactly like that of the usual induction motor4 ,ques4 ,what is the action of this type of motor8 ,ans4 ,if nothing be done to prevent1 the motor will increase in speed at no load until the armature bursts1 just as it will in a series direct current motor4 ,ques4 ,what provision is made to avoid this danger8 ,ans4 ,a governor is usually mounted on the armature which short circuits the windings1 after the motor has been started4 ,the motor then runs as a squirrel cage induction motor4 ,as a rule the brushes are lifted off the commutator when the armature is short circuited1 so as to prolong their life4 ,this is a very successful motor1 but it is of course more costly than the simple squirrel cage motor used on two and three-phase circuits4 ,illustration3 ,,fig4 #a1iaa4--;,diagram of connections of ,sprague variable speed single phase compensated repulsion motor and controller4 ,the controller is designed to give speed reduction and speed increase as resistance or reactance is inserted in the energy and compensating circuits4 ,with the exception of the leads brought out from these circuits1 the constant speed and variable speed motors are identical4 ,the standard controller gives approximately #b3a speed variation4 ,ques4 ,what name may appropriately be applied to the motor8 ,ans4 ,it may be called the repulsion induction motor1 because it is constructed for repulsion start and induction running4 ,ques4 ,describe a compensated repulsion motor4 ,ans4 ,in its simplest form it consists of a simple repulsion motor in which there are two independent sets of brushes1 one set being short circuited1 while the other set is in series with the field magnet winding1 as in the series alternating current motor4 ,ques4 ,what names are given to the two sets of brushes on a compensated repulsion motor8 ,ans4 ,the energy or main short circuiting brushes1 and the compensating brushes4 ,illustration3 ,fig4 #a1iab4--;,diagram of connections of ,sprague reversing type of single phase compensated repulsion motor4 ,as shown1 there is a special reverse field winding having terminals for connection to a four pole double throw switch4 ,ques4 ,what is the behavior of the armature of a compensated repulsion motor at starting8 ,ans4 ,it possesses at starting most of the apparent reactance of the motor1 and the effect of speed is to decrease such apparent reactance1 the latter becoming zero at either positive or negative synchronism1 and negative at higher speeds in either direction4 ,ques4 ,what is the nature of the field circuit of the compensated repulsion motor at starting8 ,ans4 ,at starting it is practically non-inductive1 the effect of speed being to introduce a spurious resistance which increases directly with the speed1 and becomes negative when the speed is reversed4 ,ques4 ,for what use is the compensated repulsion motor especially adapted8 ,ans4 ,for light railroad service4 ,ques4 ,when employed thus what is the method of control8 ,ans4 ,a series transformer is used in the field circuit4 ,ques4 ,what frequencies are employed with this motor8 ,ans4 #be to #fj1 the preferred frequency being #dj4 ,ques4 ,to what important use is the repulsion principle put8 ,ans4 ,it is sometimes employed for starting on single phase induction motors4 ,in this method1 after bringing the motor up to speed1 the winding is then short circuited upon itself1 and the motor then operates on the induction principle4 ,ques4 ,what name is given to this type of motor8 ,ans4 ,it is called the repulsion induction motor4 ,power ,factor of ,induction ,motors4--,in the case of a direct current motor1 the energy supplied is found by multiplying the current strength by the voltage1 but in all induction motors the effect of self-induction causes the current to lag behind the pressure1 thereby increasing the amount of current taken by the motor4 ,accordingly1 as the increased current is not utilized by the motor in developing power1 the value obtained by multiplying the current by the voltage represents an apparent energy which is greater than the real energy supplied to the motor4 ,illustration3 ,fig4 #a1iac4--;,fairbanks-,morse squirrel cage armature1 showing ball bearings4 ,it is evident1 that if it were possible to eliminate the lag entirely1 the real and apparent watts would be equal1 and the power factor would be unity4 ,the importance of power factor and its effect upon both alternator capacity and voltage regulation is deserving of the most careful consideration with all electrical apparatus1 in which an inherent phase difference exists between the pressure and the current1 as for instance in static transformers and induction motors4 ,while the belief is current that any decrease in power factor from unity value does not demand any increase of mechanical output1 this is not true1 since all internal alternator and line losses manifest themselves as heat1 the wasted energy to produce this heat being supplied by the prime mover4 ,apart from the poor voltage regulation of alternating current generators requiring abnormal field excitation to compensate for low power factor1 some of the station's rated output is rendered unavailable and consequently produces no revenue4 ,the poor steam economy of underloaded engines is also a serious source of fuel wastage4 ,illustration3 ,fig4 #a1iad4--;,fairbanks-,morse #bj horse power squirrel cage induction motor connected to a #bj inch self-feed rip and chamfering saw4 ,the absence of commutator and brushes on the squirrel cage armature eliminates sparking and therefore renders this type of motor particularly adapted for use in places where sparking would be dangerous1 such as in wood working plants1 textile mills1 etc4 ,careful investigations have shown that the power factor of industrial plants using induction motor drive with units of various sizes will average between #fj and #hj per cent4 ,with plants supplying current to underloaded motors having inherently high lagging current values1 a combined factor as low as #ej per cent4 may be expected4 ,since standard alternators are seldom designed to carry their rated kilowatt load at less than #hj per cent4 power factor1 the net available output is1 therefore1 considerably increased4 ,illustration3 ,fig4 #a1iae4--,method of casting end rings on squirrel cage armatures of ,fairbanks-,morse induction motors4 ,the metal being fused to the bars at a temperature in excess of #a1hcb degrees ,fahr41 it is readily seen that the destructive effect of any subsequent heating is eliminated4 ,while giving the most intimate contact at the joints1 a multiplicity of joints is avoided as well as solder4 ,speed and ,torque of ,motors4--,the speed of an induction motor depends chiefly on the frequency of the circuit and runs within #e per cent4 of its rated speed2 it will produce full torque if the line voltage do not vary more than #e to #aj per cent4 ,at low voltage the speed will not be greatly reduced as in a direct current motor1 but as the torque is low the motor is easily stopped when a light load is thrown on4 ,the current taken by an induction motor from a constant pressure line varies with the speed as in a direct current motor4 ,when a load is thrown on1 the speed is reduced correspondingly and as the self-induction or reactance is diminished1 more current circulates in the squirrel cage winding1 which in turn reacts on the field coils in a similar manner and more current flows in them from the line4 ,in this manner the motor automatically takes current from the line proportional to the load and maintains a nearly constant speed4 ,the so-called constant speed motors require slight variations in speed to automatically take current from the line when the load varies4 ,induction motors vary in speed from #e to #aj per cent41 while synchronous motors vary but a fraction of one per cent4 ,single phase motors to render efficient service must be able1 where requisite1 to develop sufficient turning moment or torque to accelerate1 from standstill1 loads possessing large inertia or excessive static friction2 for example1 meat choppers and grinders1 sugar or laundry centrifugals2 heavy punch presses2 group driven machines running from countershafts with possibly over taut belting1 poor alignment1 lubrication1 etc4 ,,chapter ,,lii ,,transformers ,the developments in the field of electrical engineering which have rendered feasible the transmission of high pressure currents over long distances1 together with the reliability and efficiency of modern generating units1 have resulted in notable economies in the generation and distribution of electric current4 ,this has been accomplished largely by the use of distant water power or the centralization of the generating plants of a large territory in a single power station4 ,the transformer is one of the essential factors in effecting the economical distribution of electric energy1 and may be defined as an apparatus used for changing the voltage and current of an alternating circuit4 ,a transformer consists essentially of3 #a4 ,a primary winding2 #b4 ,a secondary winding2 #c4 ,an iron core4 ,basic ,principles4--,if a current be passed through a coil of wire encircling a bar of soft iron the iron will become a magnet2 when the current is discontinued the bar loses its magnetization4 ,conversely3 ,if a bar of iron carrying a coil of wire be magnetized in a direction at right angles to the plane of the coil a momentary electric pressure will be induced in the wire2 if the current be reversed1 another momentary pressure will be induced in the opposite direction in the coil4 ,these actions are fully explained in chaps4 ,x and ,,xi1 and as they are perfectly familiar phenomena1 a detailed explanation of the principles upon which they depend is not necessary here4 ,from the first two statements given above it is evident that if a bar of iron be provided with two coils of wire1 one of which is supplied from a source of alternating current1 as shown diagrammatically by fig4 #a1iaf1 at each impulse of the exciting current a pressure will be induced in the secondary coil1 the direction of these impulses alternating like that of the exciting current4 ,ques4 ,what name is given to the coil through which current from the source flows8 ,ans4 ,the primary winding4 ,illustration3 ,fig4 #a1iaf4--;,diagram of elementary transformer with non-continuous core and connection with single phase alternator4 ,the three essential parts are3 primary winding1 secondary winding1 and an iron core4 ,ques4 ,what name is given to the coil in which a current is induced8 ,ans4 ,the secondary winding4 ,similarly1 the current from the source 7alternator7 is called the primary current and the induced current1 the secondary current4 ,ques4 ,what is the objection to the elementary transformer shown in fig4 #a1iaf8 ,ans4 ,the non-continuous core4 ,with this type core1 the flux emanating from the north pole of the bar has to return to the south pole through the surrounding air2 and as the reluctance of air is much greater than that of iron1 the magnetism will be weak4 ,ques4 ,how is this overcome8 ,ans4 ,by the use of a continuous core as shown in fig4 #a1iag4 ,ques4 ,is this the best arrangement1 and why8 ,ans4 ,no4 ,if the windings were put on as in fig4 #a1iag1 the leakage of magnetic lines of force would be excessive1 as indicated by the dotted lines4 ,in such a case the lines which leak through air have no effect upon the secondary winding1 and are therefore wasted4 ,illustration3 ,fig4 #a1iag4--;,diagram of elementary transformer with continuous core and connections with alternator4 ,the dotted lines show the leakage of magnetic lines4 ,to remedy this the arrangement shown in fig4 #a1iah is used4 ,ques4 ,how is the magnetic leakage reduced to a minimum in commercial transformers8 ,ans4 ,in these1 and even in ordinary induction coils 7the operating principle of which is the same as that of transformers7 the magnetic leakage is reduced to the lowest possible amount by arranging the coils one within the other1 as shown in cross section in fig4 #a1iah4 ,the ,induced ,voltage4--,the pressure induced in the secondary winding will depend on the ratio between the number of turns in the two windings4 ,for example1 a transformer with #ejj turns of wire in its primary winding and #ej turns in its secondary winding would have a transformation ratio of #aj to #a1 and if it were supplied with primary current at #a1jjj volts1 the secondary pressure at no load would be #ajj volts4 ,illustration3 ,fig4 #a1iah4--;,cross section showing commercial arrangement of primary and secondary windings on core4 ,one is superposed on the other4 ,this arrangement compels practically all of the magnetic lines created by the primary winding to pass through the secondary winding4 ,,example4--,if ten amperes flow in the primary winding and the transformation ratio be #aj1 then #aj #aj #ajj amperes will flow through the secondary winding4 ,thus1 a direct proportion exists between the pressures and turns in the two windings and an inverse proportion between the amperes and turns1 that is3 primary voltage3 secondary voltage primary turns3 secondary turns primary current3 secondary current secondary turns3 primary turns ,from the above equations it is seen that the watts of the primary circuit equal the watts of the secondary circuit4 ,ques4 ,are the above relations strictly true1 and why8 ,ans4 ,no1 they are only approximate1 because of transformer losses4 ,in the above example1 the total wattage in the primary circuit is #a1jjj #aj #aj kw41 and that in the secondary circuit is #ajj #ajj #aj kw4 ,hence1 while both volts and amperes are widely different in the two circuits1 the watts for each are the same in the ideal case1 that is1 assuming perfect transformer action or #ajj efficiency4 ,now1 the usual loss in commercial transformers is about #aj1 so that the actual watts delivered in the secondary circuit is 7#ajj #ajj7 #ij #i kw4 ,illustration3 ,,fig4 #a1iai4--,wagner transformer coil formed1 ready for taping4 ,these are known as pan cake coils4 ,they are wound with flat cotton covered copper strip4 ,in heavy coils1 several strips in parallel are used per turn in order to facilitate the winding and produce a more compact coil4 ,the ,no ,load ,current4--,when the secondary winding of a transformer is open or disconnected from the secondary circuit no current will flow in the winding1 but a very small current called the no load current will flow in the primary circuit4 ,the reason for this is as follows3 ,the current flowing in the primary winding causes repeated reversals of magnetic flux through the iron core4 ,these variations of flux induce pressures in both coils2 that induced in the primary called the reverse pressure is opposite in direction and very nearly equal to the impressed pressure1 that is1 to the pressure applied to the primary winding4 ,accordingly the only force available to cause current to flow through the primary winding is the difference between the impressed pressure and reverse pressure1 the effective pressure4 ,illustration3 ,,fig4 #a1ibj4--,wagner coils with insulation ready for core assembly4 ,the flat coils1 sometimes called pancake coils are wound of flat1 cotton covered1 copper strip with ample insulation between layers4 ,in heavy coils several flat strips in multiple are used per turn in order to facilitate the winding and produce a more compact coil4 ,in many cases normal current flow per high tension coil is very low and could be carried with a very small cross sectional area of copper2 however1 flat strip is almost always used on account of the increased mechanical stability thus obtained4 ,the ,magnetizing ,current4--,the magnetizing current of a transformer is sometimes spoken of as that current which the primary winding takes from the mains when working at normal pressure4 ,the true magnetizing current is only that component of this total no load current which is in quadrature with the supply pressure4 ,the remaining component has to overcome the various iron losses1 and is therefore an in phase component4 ,the relation between these two components determines the power factor of the so called magnetizing current4 ,illustration3 ,,figs4 #a1iba and #a1ibb4--;,assembled coils of ,westinghouse #aj and #ae kva4 transformers2 views showing ventilating ducts4 ,the true magnetizing component is small if the transformer be well designed1 and be worked at low flux density4 ,action of ,transformer with ,load4--,if the secondary winding of a transformer be connected to the secondary circuit by closing a switch so that current flows through the secondary winding1 the transformer is said to be loaded4 ,the action of this secondary current is to oppose the magnetizing action of the slight current already flowing in the primary winding1 thus decreasing the maximum value reached by the alternating magnetic flux in the core1 thereby decreasing the induced pressure in each winding4 ,the amount of this decrease1 however1 is very small1 inasmuch as a very small decrease of the induced pressure in the primary coil greatly increases the difference between the pressure applied to the primary coil and the opposing pressure induced in the primary coil1 so that the primary current is greatly increased4 ,in fact1 the increase of primary current due to the loading of the transformer is just great enough 7or very nearly7 to exactly balance the magnetizing action of the current in the secondary coil2 that is1 the flux in the core must be maintained approximately constant by the primary current whatever value the secondary current may have4 ,when the load on a transformer is increased1 the primary of the transformer automatically takes additional current and power from the supply mains in direct proportion to the load on the secondary4 ,when the load on the secondary is reduced1 for example by turning off lamps1 the power taken from the supply mains by the primary coil is automatically reduced in proportion to the decrease in the load4 ,this automatic action of the transformer is due to the balanced magnetizing action of the primary and secondary currents4 ,illustration3 ,,fig4 #a1ibc4--,rear view of ,fort ,wayne distributing transformer1 showing hanger irons for attaching to pole cross arm4 ,classification of ,transformers4--,as in the case of motors1 the great variety of transformer makes it necessary that a classification1 to be comprehensive1 must be made from several points of view1 as3 #a4 ,with respect to the transformation1 as a4 ,step up transformers2 b4 ,step down transformers4 #b4 ,with respect to the arrangement of the coils and magnetic circuit1 as a4 ,core transformers2 b4 ,shell transformers2 c4 ,combined core and shell transformers4 #c4 ,with respect to the kind of circuit they are to be used on1 as a4 ,single phase transformers2 b4 ,polyphase transformers4 #d4 ,with respect to the method employed in cooling1 as a4 ,dry transformers2 b4 ,air cooled transformers natural draught2 forced draught1 or air blast2 c4 ,oil cooled transformers2 d4 ,water cooled transformers4 #e4 ,with respect to the nature of their output1 as a4 ,constant pressure transformers2 b4 ,constant current transformers2 c4 ,current transformers2 d4 ,auto-transformers4 #f4 ,with respect to the kind of service1 as a4 ,distributing2 b4 ,power4 #g4 ,with respect to the circuit connection that the transformer is constructed for1 as a4 ,series transformers2 b4 ,shunt transformers4 ,step ,up ,transformers4--,this form of transformer is used to transform a low voltage current into a high voltage current4 ,such transformers are employed at the generating end of a transmission line to raise the voltage of the alternators to such value as will enable the electric power to be economically transmitted to a distant point4 ,illustration3 ,,fig4 #a1ibd4--;,diagram of elementary step up transformer4 ,as shown the primary winding has two turns and secondary #aj turns1 giving a ratio of voltage transformation of #aj #b #e4 ,since only as much current flows in the secondary winding as in the primary1 the latter requires heavier wire than the former4 ,copper ,economy with ,step ,up ,transformers4--,to comprehend fully the bearing of the matter1 it must be remembered that the energy supplied per second is the product of two factors1 the current and the pressure at which that current is supplied2 the magnitudes of the two factors may vary1 but the value of the power supplied depends only on the product of the two2 for example1 the energy furnished per second by a current of #aj amperes supplied at a pressure of #b1jjj volts is exactly the same in amount as that furnished per second by a current of #djj amperes supplied at a pressure of #ej volts2 in each case1 the product is #bj1jjj watts4 ,now the loss of energy that occurs in transmission through a well insulated wire depends also on two factors1 the current and the resistance of the wire1 and in a given wire is proportional to the square of the current4 ,in the above example the current of #djj amperes1 if transmitted through the same wire as the #aj amperes current1 would1 because it is forty times as great1 waste sixteen hundred times as much energy in heating the wire4 ,it follows that1 for the same loss of energy1 the #aj ampere current at #b1jjj volts may be carried by a wire having only _/th of the sectional area of the wire used for the #djj ampere current at #ej volts4 ,the cost of copper conductors for the distributing lines is therefore very greatly economized by employing high pressures for distribution of small currents4 ,illustration3 ,,fig4 #a1ibe4--;,diagram of elementary step down transformer4 ,as shown the primary winding has #aj turns and the secondary #b1 giving a ratio of voltage transformation of #b #aj 4#b4 ,the current in the secondary being #e times greater than in the primary will require a proportionately heavier wire4 ,step ,down ,transformers4--,when current is supplied to consumers for lighting purposes1 and for the operation of motors1 etc41 considerations of safety as well as those of suitability1 require the delivery of the current at comparatively low pressures ranging from #ajj to #bej volts for lamps1 and from #ajj to #fjj volts for motors4 ,this involves that the high pressure current in the transmission lines must be transformed to low pressure current at the receiving or distributing points by step down transformers1 an elementary transformer being shown in fig4 #a1ibe4 ,illustration3 ,,figs4 #a1ibf and #a1ibg4--;,core type transformer4 ,it consists of a central core of laminated iron1 around which the coils are wound4 ,a usual form of core type transformer consists of a rectangular core1 around the two long limbs of which the primary and secondary coils are wound1 the low tension coil being placed next the core4 ,transformers of this type have a large number of turns in the primary winding and a small number in the secondary1 in ratio depending on the amount of pressure reduction required4 ,core ,transformers4--,this type of transformer may be defined as one having an iron core1 upon which the wire is wound in such a manner that the iron is enveloped within the coils1 the outer surface of the coils being exposed to the air as shown in figs4 #a1ibf and #a1ibg4 ,shell ,transformers4--,in the shell type of transformer1 as shown in fig4 #a1ibh1 the core is in the form of a shell1 being built around and through the coils4 ,a shell transformer has1 as a rule1 fewer turns and a higher voltage per turn than the core type4 ,ques4 ,what is the comparison between core and shell transformers8 ,ans4 ,the relative advantages of the two types has been the subject of considerable discussion among manufacturers2 the companies who formerly built only shell type transformers1 now build core types1 while with other builders the opposite practice obtains4 ,illustration3 ,,fig4 #a1ibh4--,shell type transformer4 ,in construction the laminated core is built around and through the coils as shown4 ,for large ratings this type has some advantages with respect to insulation1 while for small ratings the core type is to be preferred in this respect4 ,the shell arrangement of the core gives better cooling2 with this arrangement minimum magnetic leakage is easily obtained4 ,ques4 ,upon what does the choice between the two types chiefly depend8 ,ans4 ,upon manufacturing convenience rather than operating characteristics4 ,the major insulation in a core type transformer consists of several large pieces of great mechanical strength1 while in the shell type1 there are required an extremely large number of relatively small pieces of insulating material1 which necessitates careful workmanship to prevent defects in the finished transformer1 when thin or fragile material is used4 ,both core and shell transformers are built for all ratings2 for small ratings the core type possesses certain advantages with reference to insulation1 while for large ratings1 the shell type possesses better cooling properties1 and has less magnetic leakage than the core type4 ,illustration3 ,,fig4 #a1ibi4--,view illustrating the construction of cores and coils of ,maloney transformers4 ,illustration3 ,,fig4 #a1icj4--,maloney mica shield between primary and secondary coils1 showing lapping feature which prevents the wrinkling and cracking of the mica4 ,combined ,core and ,shell ,transformers4--,an improved type of transformer has been introduced which can be considered either as two superposed shell transformers with coils in common1 or as a single core type transformer with divided magnetic circuit and having coils on only one leg4 ,it is best considered however1 as a combined core and shell transformer1 and for small sizes it possesses most of the advantages of both types4 ,it can be constructed at less cost than can either a core or a shell transformer having the same operating characteristics and temperature limits4 ,illustration3 ,,figs4 #a1ica and #a1icb4--,the ,berry combined core and shell transformer4 ,it consists of a number of inner and outer vertical and radial laminated iron blocks built up of the usual thin sheet iron1 with the coils between4 ,the magnetic circuit is completed at the top and bottom by other laminated blocks placed horizontally1 and the whole is held together between top and bottom cast iron frame plates by a bolt passing right down the center4 ,fig4 #a1ica gives a general view1 ,w being the winding1 and ,b1 ,b1 ,b1 etc41 the outer laminated blocks4 ,the construction will be better understood from fig4 #a1icb1 where it may be supposed that the top cap and laminated cross pieces have been removed4 ,here ,i1 ,i1 ,i and ,o1 ,o1 ,o are respectively the inner and outer radial vertical blocks1 ,p the primary1 and ,s1 ,s the secondary2 the latter being in two sections with the primary sandwiched between1 as an extra precaution against shock4 ,it will be evident that this form of transformer possesses excellent ventilation2 and this is still further enhanced by opening out the winding at intervals to leave ventilating apertures1 as at ,a1 ,a1 ,a4 ,fig4 #a1icb shows only #f sets of radial blocks1 but the usual plan is to provide #bd or #cf1 according to the size of the transformer4 ,fig4 #a1icb shows a cross section of the first transformer of this type to be developed commercially1 and known as an iron clad transformer2 this construction has been used in ,england for some time4 ,fig4 #a1icc shows the ,american practice4 ,illustration3 ,,fig4 #a1icc4--,plan of core of ,general ,electric combined core and shell transformer4 ,the core used contains four magnetic circuits of equal reluctance1 in multiple2 each circuit consisting of a separate core4 ,in this construction one leg of each circuit is built up of two different widths of punchings forming such a cross section that when the four circuits are assembled together they interlock to form a central leg1 upon which the winding is placed4 ,the four remaining legs consist of punchings of equal width4 ,these occupy a position surrounding the coil at equal distances from the center1 on the four sides2 forming a channel between each leg and coil1 thereby presenting large surfaces to the oil and allowing its free access to all parts of the winding4 ,the punchings of each size transformer are all of the same length1 assembled alternately1 and forming two lap joints equally distributed in the four corners of the core1 thereby giving a magnetic circuit of low reluctance4 ,ques4 ,how is economy of construction obtained in designing combined core and shell transformers8 ,ans4 ,the cross section of iron in the central leg of the core is made somewhat less than that external to the coils1 in order to reduce the amount of copper used in the coils4 ,single and ,polyphase ,transformers4--,a single phase transformer may be defined as one having only one set of primary and secondary terminals1 and in which the fluxes in the one or more magnetic circuits are all in phase1 as distinguished from a polyphase transformer1 or combination in one unit of several one phase transformers with separate electric circuits but having certain magnetic circuits in common4 ,in polyphase transformers there are two or more magnetic circuits through the core1 and the fluxes in the various circuits are displaced in phase4 ,ques4 ,is it necessary to use a polyphase transformer to transform a polyphase current8 ,ans4 ,no1 a separate single phase transformer may be used for each phase4 ,illustration3 ,,figs4 #a1icd and #a1ice4--,top view showing core and coils in place1 and view of coils of ,westinghouse distributing transformer4 ,the coils are wound from round wire in the smaller sizes of transformers and from strap copper in the larger sizes4 ,strap wound coils allow a greater current carrying conductor section than coils wound from large round wire1 as there is little waste space between the different turns of the conductor4 ,the coils are arranged concentrically with the high tension winding between the two low tension coils1 this arrangement giving the fine regulation found in these transformers4 ,the low tension coils are wound in layers which extend across the whole length of the coil opening in the iron1 while the high tension coils are wound in two parts and placed end to end4 ,this construction reduces the normal voltage strains to a value which will not give trouble under any condition of service4 ,the magnetic circuit is built up of laminated1 alloy steel punchings1 each layer of lamin being reversed with reference to the preceding layer and all joints butted4 ,this gives a continuous magnetic circuit of low reluctance1 low iron loss and low exciting current4 ,when assembled1 the magnetic circuit consists of four separate parallel circuits encircling the coils and protecting the windings from mechanical injury4 ,separate high and low tension terminal blocks of glazed porcelain are mounted upon extensions of the upper end frames4 ,all danger of confusing the leads or inadvertently making an electrical connection between the high and low tension sides of the transformer is thus averted4 ,the high tension winding has four leads brought to the studs in the terminal block4 ,adjustable brass connectors or links between the studs provide for series or multiple connections between two points of the high tension winding4 ,the position of the studs and the length of the links are so proportioned that wrong connections on the block are impossible4 ,barriers on the porcelain block separate the studs and prevent danger of arcing4 ,leads with means of preventing creeping of oil by capillary action are attached to these studs and brought out of the core through porcelain bushings4 ,ques4 ,is there any choice between a polyphase transformer and separate single phase transformers for transforming a polyphase current8 ,ans4 ,yes1 the polyphase transformer is preferable1 because less iron is required than would be with the several single phase transformers4 ,the polyphase transformer therefore is somewhat lighter and also more efficient4 ,illustration3 ,,figs4 #a1icf and #a1icg4--;,core and shell types of three phase transformer4 ,in the core type1 fig4 #a1icf1 there are three cores ,a1 ,b1 and ,c1 joined by the yokes ,d and ,d'4 ,this forms a three phase magnetic circuit1 since the instantaneous sum of the fluxes is zero4 ,each core is wound with a primary coil ,p1 and a secondary coil ,s4 ,as shown1 the primary winding of each phase is divided into three coils to ensure better insulation4 ,the primaries and secondaries may be connected star or mesh4 ,the core ,b has a shorter return path than ,a and ,c1 which causes the magnetizing current in that phase to be less than in the ,a and ,c phases4 ,this has sometimes been obviated by placing the three cores at the corners of an equilateral triangle 7as in figs4 #a1ici and #a1idj71 but the extra trouble involved is not justified1 as the unbalancing is a no load condition1 and practically disappears when the transformer is loaded4 ,the shell type1 fig4 #a1icg1 consists practically of three separate transformers in one unit4 ,the flux paths are here separate1 each pair of coils being threaded by its own flux1 which does not1 as in the core type1 return through the other coils4 ,this gives the shell type an advantage over the core type1 for should one phase burn out1 the other two may still be used1 especially if the faulty coils be short circuited4 ,the effect of such short circuiting is to prevent all but a very small flux from threading the faulty coil4 ,ques4 ,name two varieties of polyphase transformer8 ,ans4 ,the core1 and the shell types as shown in figs4 #a1icf and #a1icg4 ,ques4 ,how should a three phase transformer be operated with one phase damaged8 ,ans4 ,the damaged windings should be separated electrically from the other coils4 ,the pressure winding of the damaged phase should be short circuited upon itself and the corresponding low pressure winding should also be short circuited upon itself4 ,the winding thus short circuited will choke down the flux passing through the portion of the core surrounded by them without producing in any portion of the winding a current greater than a small fraction of the current which would normally exist in such portion at full load4 ,transformer ,losses4--,as previously mentioned1 the ratio between the applied primary voltage and the secondary terminal voltage of a transformer is not always equal to the ratio of primary to secondary turns of wire around the core4 ,the commercial transformer is not a perfect converter of energy1 that is1 the input1 or watts applied to the primary circuit is always more than the output or watts delivered from the secondary winding4 ,illustration3 ,,fig4 #a1ich4--;,interior of ,general ,electric oil cooled #ejj kva4 #cc1jjj volt outdoor transformer showing lifting arrangement4 ,this is due to the various losses which take place1 and the difference between the input and the output is equal to the sum of these losses4 ,they are divided into two classes3 #a4 ,the iron or core losses2 #b4 ,the copper losses4 ,the iron or core losses are due to #a4 ,hysteresis2 #b4 ,eddy currents2 #c4 ,magnetic leakage 7negligibly small74 ,illustration3 ,,figs4 #a1ici and #a1idj4--,triangular arrangements of cores of three phase transformer4 ,fig4 #a1ici1 form with three cornered yokes at bottom and top of cores2 fig4 #a1idj1 form with circular yokes4 ,while these designs give perfect symmetry for the three phases1 there is some trouble in the mechanical arrangement of the yokes4 ,if these be stamped out triangularly and inserted horizontally between the three cores1 it is necessary to interpose a layer of insulation1 otherwise there would be objectionable eddy currents formed in the stampings4 ,those which are classed as copper losses are due to #a4 ,heating the conductors 7the ,i,r loss72 #b4 ,eddy currents in conductors4 ,hysteresis4--,in the operation of a transformer the alternating current causes the core to undergo rapid reversals of magnetism4 ,this requires an expenditure of energy which is converted into heat4 ,illustration3 ,,fig4 #a1ida4--,view showing mechanical construction of coil and core of ,moloney pole type to #ej kw4 transformer4 ,moloney standard transformers of these sizes are regularly wound for #a1ajj to #b1bjj primary volts4 ,for #a1ajj volts the primary coils are connected in parallel by means of connecting links2 for #b1bjj volts1 they are connected in series4 ,the porcelain primary terminal board is provided with two connecting links so that connections can be made for either #a1ajj or #b1bjj volts4 ,this loss of energy as before explained is due to the work required to change the position of the molecules of the iron1 in reversing the magnetization4 ,extra power then must be taken from the line to make up for this loss1 thus reducing the efficiency of the transformer4 ,ques4 ,upon what does the hysteresis loss depend8 ,ans4 ,upon the quality of the iron in the core1 the magnetic density at which it is worked and the frequency4 ,ques4 ,with a given quality of iron how does the hysteresis loss vary8 ,ans4 ,it varies as the #a4f power of the voltage with constant frequency4 ,ques4 ,in construction1 what is done to obtain minimum hysteresis loss8 ,ans4 ,the softest iron obtainable is used for the core1 and a low degree of magnetization is employed4 ,illustration3 ,,fig4 #a1idb4--;,fort ,wayne transformer coils and core complete4 ,illustration3 ,,fig4 #a1idc4--,top view of ,fort ,wayne 7type ,a7 transformer cover removed1 showing assembly of coils and core and disposition of leads4 ,eddy ,currents4--,the iron core of a transformer acts as a closed conductor in which small pressures of different values are induced in different parts by the alternating field1 giving rise to eddy currents4 ,energy is thus consumed by these currents which is wasted in heating the iron1 thus reducing the efficiency of the transformer4 ,ques4 ,how is the loss reduced to a minimum8 ,ans4 ,by the usual method of laminating the core4 ,the iron core is built up of very thin sheet iron or steel stampings1 and these are insulated from each other by varnish and are laid face to face at right angles to the path that the eddy currents tend to follow1 so that the currents would have to pass from sheet to sheet1 through the insulation4 ,ques4 ,in practice1 upon what does the thickness of the lamin or stampings depend8 ,ans4 ,upon the frequency4 ,the lamin vary in thickness from about 4#jad to 4#jbe inch1 according as the frequency is respectively high or low4 ,illustration3 ,,fig4 #a1idd4--;,general ,electric #aj kva41 7type ,h7 transformer removed from tank4 ,that part of the steel core composing the magnetic circuit outside of the winding is divided into four equal sections4 ,each section is located a sufficient distance from the winding so that all portions of the winding and core are equally exposed to the cooling action of the oil4 ,on all except the very smallest sizes the winding is divided by channels and ducts through which a continual circulation of oil is maintained4 ,the result is uniform temperature throughout the transformer1 thus eliminating the detrimental effects of unequal expansion in the coils with consequent rubbing and abrasion of the insulation4 ,ques4 ,does a transformer take any current when the secondary circuit is open8 ,ans4 ,yes1 a no load current passes through the primary4 ,ques4 ,why8 ,ans4 ,the energy thus supplied balances the core losses4 ,illustration3 ,,fig4 #a1ide4--;,cover construction of ,wagner #cej kva41 oil filled #a1ajj-b1bjj volt transformer4 ,in transformers with corrugated cases1 the base and top ring are cast to the corrugated iron sheets4 ,ques4 ,are the iron or copper losses the more important1 and why8 ,ans4 ,the iron losses1 because these are going on as long as the primary pressure is maintained1 and the copper losses take place only while energy is being delivered from the secondary4 ,strictly speaking1 on no load 7that is when the secondary circuit is open7 a slight copper loss takes place in the primary coil but because of its smallness is not mentioned4 ,it is1 to be exact1 included in the expression iron losses1 as the precise meaning of this term signifies not only the hysteresis and eddy current losses but the copper loss in the primary coil when the secondary is open4 ,the importance of the iron losses is apparent in noting that in electric lighting the lights are in use only a small fraction of the #bd hours1 but the iron losses continue all the time1 thus the greater part of each day energy must be supplied to each transformer by the power company to meet the losses1 during which time no money is received from the customers4 ,some companies make a minimum charge per month whether any current is used or not to offset the no load transformer losses and rent of meter4 ,illustration3 ,,figs4 #a1idf to #a1idh4--,methods of connecting the low tension sides of ,westinghouse transformers using the connectors illustrated in figs4 #a1idi to #a1iec4 ,ques4 ,how may the iron losses be reduced to a minimum8 ,ans4 ,by having short magnetic paths of large area and using iron or steel of high permeability4 ,the design and construction must keep the eddy currents as low as possible4 ,as before stated the iron losses take place continually1 and since most transformers are loaded only a small fraction of a day it is very important that the iron losses should be reduced to a minimum4 ,with a large number of transformers on a line1 the magnetizing current that is wasted1 is considerable4 ,during ,may1 #aiaj1 the ,u4 ,s4 ,bureau of ,standards issued a circular showing that each watt saving in core losses was a saving of #hh cents1 which is evident economy in the use of high grade transformers4 ,copper ,losses4--,since the primary and secondary windings of a transformer have resistance1 some of the energy supplied will be lost by heating the copper4 ,the amount of this loss is proportional to square of the current1 and is usually spoken of as the ,i,r loss4 ,illustration3 ,,figs4 #a1idi to #a1iec4--,westinghouse low tension transformer connectors for connecting the low tension leads to the feeder wires4 ,the transformers of the smaller capacities have knuckle joint connectors and those of the larger sizes have interleaved connectors4 ,these connectors form a mechanically strong joint of high current carrying capacity4 ,since the high tension leads are connected directly to the cut out or fuse blocks1 connectors are not required on these leads4 ,the use of these connectors allows a transformer to be removed and another of the same or a different capacity substituted usually without soldering or unsoldering a joint4 ,the connectors also facilitate changes in the low tension connections4 ,ques4 ,define the copper losses4 ,ans4 ,the copper losses are the sum of the ,i,r losses of both the primary and secondary windings1 and the eddy current loss in the conductors4 ,ques4 ,is the eddy current loss in the conductors large8 ,ans4 ,no1 it is very small and may be disregarded1 so that the sum of the ,i,r losses of primary and secondary can be taken as the total copper loss for practical purposes4 ,ques4 ,what effect has the power factor on the copper losses8 ,ans4 ,since the copper loss depends upon the current in the primary and secondary windings1 it requires a larger current when the power factor is low than when high1 hence the copper losses increase with a lowering of the power factor4 ,illustration3 ,,fig4 #a1ied4--,method of bringing out the secondary leads in ,wagner central station transformers4 ,each primary lead is brought into the case through a similar bushing4 ,observe the elimination of all possibility of grounding the cable on the case or core4 ,ques4 ,what effect other than heating has resistance in the windings8 ,ans4 ,it causes poor regulation4 ,this is objectionable1 especially when incandescent lights are in use1 because the voltage fluctuates inversely with load changes1 that is1 it drops as lamps are turned on and rises as they are turned off1 producing disagreeable changes in the brilliancy of the lamps4 ,cooling of ,transformers4--,owing to the fact that a transformer is a stationary piece of apparatus1 not receiving ventilation from moving parts1 its efficient cooling becomes a very strong feature of the design1 especially in the case of large high pressure transformers4 ,the effective cooling is rendered more difficult because transformers are invariably enclosed in more or less air tight cases1 except in very dry situations1 where a perforated metal covering may be permitted4 ,illustration3 ,,figs4 #a1iee and #a1ief4--,westinghouse transformer terminal blocks for high and low tension conductors4 ,the final degree to which the temperature rises after continuous working for some hours1 depends on the total losses in iron and copper1 on the total radiating surface1 and on the facilities afforded for cooling4 ,there are various methods of cooling transformers1 the cooling mediums employed being #a4 ,air2 #b4 ,oil2 #c4 ,water4 ,the means adopted for getting rid of the heat which is inevitably developed in a transformer by the waste energy is one of the important considerations with respect to its design4 ,ques4 ,what is the behaviour of a transformer with respect to heating when operated continuously at full load8 ,ans4 ,the temperature gradually rises until at the end of some hours it becomes constant4 ,the difference between the constant temperature and that of the secondary atmosphere is called the temperature rise at full load4 ,its amount constitutes a most important feature in the commercial value of the transformer4 ,illustration3 ,,figs4 #a1ieg to #a1ifj4--,porcelain bushing for ,westinghouse transformers4 ,ques4 ,why is a high rise of temperature objectionable8 ,ans4 ,it causes rapid deterioration of the insulation1 increased hysteresis losses1 and greater fire risk4 ,dry ,transformers4--,this classification is used to distinguish transformers using air as a cooling medium from those which employ a liquid such as water or oil to effect the cooling4 ,air ,cooled ,transformers4--,this name is given to all transformers which are cooled by currents of air without regard to the manner in which the air is circulated4 ,there are two methods of circulating the air1 as by #a4 ,natural draught2 #b4 ,forced draught1 or blast4 ,ques4 ,describe a natural draught air cooled transformer4 ,ans4 ,in this type1 the case containing the windings is open at the top and bottom4 ,the column of air in the case expands as its temperature rises1 becoming lighter than the cold air on the outside and is consequently displaced by the latter1 resulting in a circulation of air through the case4 ,the process is identical with furnace draught4 ,illustration3 ,,figs4 #a1ifa to #a1ifc4--;,fuse blocks for ,westinghouse transformers4 ,the fuses furnished with the transformers are mounted in a weather proof porcelain fuse box of special design4 ,the stationary contacts are deeply recessed in the porcelain and are well separated from each other4 ,the contacts are so constructed that the plug is held securely in place by giving it a partial turn after inserting it4 ,when the plug is in position1 the fuse is in sight and its condition can be noted which eliminates all danger of pulling the fuse while same is still intact and the transformer is under load4 ,ques4 ,describe a forced draught or air blast transformer4 ,ans4 ,the case is closed at the bottom and open at the top4 ,a current of air is forced through from bottom to top as shown in fig4 #a1ifd by a fan4 ,ques4 ,how are the coils best adapted to air cooling8 ,ans4 ,they are built up high and thin1 and assembled with spaces between them1 for the circulation of the air4 ,ques4 ,what are the requirements with respect to the air supply in forced draught transformers8 ,ans4 ,air blast transformers require a large volume of air at a comparatively low pressure4 ,this varies from one-half to one ounce per square inch4 ,the larger transformers require greater pressure to overcome the resistance of longer air ducts4 ,illustration3 ,,fig4 #a1ifd4--;,forced draught or air blast transformer4 ,as is indicated by the classification1 this type of transformer is cooled by forcing a current of air through ducts1 provided between the coils and between sectionalized portions of the core4 ,the cold air is forced through the interior of the core containing the coils by a blower1 the air passing vertically through the coils and out through the top4 ,part of the air is sometimes diverted horizontally through the ventilating ducts provided in the core1 passing off at one side of the transformer4 ,the amount of air going through the coils1 or through the core1 may be controlled independently by providing dampers in the passages4 ,ques4 ,how much air is used ordinarily for cooling per kw4 of load8 ,ans4 ,about #aej cu4 ft4 of air per minute4 ,in forced draught transformers1 the air pressure maintained by the blower varies from to #a oz4 per square inch4 ,forced draught or air blast transformers are seldom built in small sizes or for voltages higher than about #ce1jjj volts4 ,oil ,cooled ,transformers4--,in this type of transformer the coils and core are immersed in oil and provided with ducts to allow the oil to circulate by convection and thus serve as a medium to transmit the heat to the case1 from which it passes by radiation4 ,illustration3 ,,fig4 #aife4--,looking down into a ,wagner central station transformer1 showing the connection board1 which provides facility for varying the ratio of transformation and also for interchanging the primaries4 ,ques4 ,explain in detail the circulation of the oil4 ,ans4 ,the oil1 heated by contact with the exposed surfaces of the core and coils1 rises to the surface1 flows outward and descends along the sides of the transformer case1 from the outer surface of which the heat is radiated into the air4 ,ques4 ,how may the efficiency of this method of cooling be increased8 ,ans4 ,by providing the case with external ribs or fins1 or by fluting so as to increase the external cooling surface4 ,illustration3 ,,fig4 #aiff4--,section through ,westinghouse kilovolt ampere type ,s transformer4 ,,fig4 #aifg4--,section through ,westinghouse #ej kilovolt ampere type ,s transformer showing large oil ducts4 ,ques4 ,in what types of transformer is this mode of oil cooling used8 ,ans4 ,lighting transformers4 ,in such transformers1 the large volume of oil absorbs considerable heat1 so that the rise of temperature is retarded4 ,hence1 for moderate periods of operation1 say #c or #d hours1 the average lighting period1 the maximum temperature would not be reached4 ,ques4 ,in what other capacities except that of cooling agent1 does the oil act8 ,ans4 ,it is a good insulator1 preserves the insulation from oxidation1 increasing the breakdown resistance of the insulation1 and generally restores the insulation in case of puncture4 ,illustration3 ,,fig4 #a1ifh4--,wagner #cjj kva1 #d1djj volt three phase oil cooled transformer4 ,in this type of transformer the case is filled with oil and fluted so as to increase the cooling surface1 an oil drain valve is screwed to a wrought iron nipple cast into the base1 the duct to which is in such a position as to make it possible not only to drain all of the oil from the transformer1 but when desirable1 to draw off a small quantity from the bottom4 ,should any moisture be in the oil it is therefore drawn off first4 ,ques4 ,what is the special objection to oil8 ,ans4 ,danger of fire4 ,ques4 ,what kind of oil is used in transformers8 ,ans4 ,mineral oil4 ,ques4 ,what are the requirements of a good grade of transformer oil8 ,ans4 ,it should show very little evaporation at #bab ,fahr41 and should not give off gases at such a rate as to produce an explosive mixture with the air at a temperate below #cef4 ,it should not contain moisture1 acid1 alkali or sulphur compounds4 ,illustration3 ,,fig4 #a1ifi4--,section through ,fort ,wayne 7type ,a7 transformer showing interior of case1 core conductors1 and insulation1 also division of lamin4 ,the presence of moisture can be detected by thrusting a red hot nail in the oil2 if the oil crackle1 water is present4 ,moisture may be removed by raising the temperature slightly above the boiling point1 #bab ,fahr41 but the time consumed 7several days7 is excessive4 ,water ,cooled ,transformers4--,a water cooled transformer is one in which water is the cooling agent1 and1 in most cases1 oil is the medium by which heat is transferred from the coils to the water4 ,in construction1 pipes or a jacketed casing is provided through which the cooling water is passed by forced circulation1 as shown in figs4 #a1igj and #a1iga4 ,illustration3 ,,fig4 #a1igj4--,water cooled transformer with internal cooling coil1 that is1 with cooling coil within the transformer case4 ,in this type1 the cooling coil1 through which the circulating water passes1 is placed in the top of the case or tank1 the latter is filled with oil so that the coil is submerged4 ,the oil acts simply as a medium to transfer the heat generated by the transformer to the water circulating through the cooling coil4 ,in operation a continual circulation of the oil takes place1 as indicated by the arrows1 due to the alternate heating and cooling it receives as it flows past the transformer coils and cooling coil respectively4 ,in some cases tubular conductors are provided for the circulation of the water4 ,water cooled transformers may be divided into two classes1 as those having3 #a4 ,internal cooling coils2 #b4 ,external cooling coils4 ,ques4 ,describe the first named type4 ,ans4 ,inside the transformer case near the top is placed a coil of wrought iron pipe1 through which the cooling water is pumped4 ,the case is filled with oil1 which by thermo-circulation flows upward through the coils1 transferring the heat absorbed from the coils to the water2 on cooling it becomes more dense 7heavier7 and descends along the inside surface of the casing4 ,illustration3 ,,fig4 #a1iga4--,water cooled transformer with external cooling coil4 ,in this arrangement the cooling coil is placed in a separate tank as shown4 ,here forced circulation is employed for both the heat transfer medium 7oil7 and the cooling agent 7water71 two pumps being necessary4 ,the cool oil enters the transformer case at the lowest point and absorbing heat from the transformer coils it passes off through the top connection leading to the cooling coil and expansion tank4 ,since the transformer tank is closed1 an expansion tank is provided to allow for expansion of the oil due to heating4 ,the water circulation is arranged as illustrated4 ,ques4 ,how much circulating water is required8 ,ans4 ,it depends upon the difference between the initial and discharge temperatures of the circulating water4 ,illustration3 ,,fig4 #a1igb4--;,interior of ,general ,electric water cooled #adj1jjj volt transformer showing cooling coil4 ,ques4 ,in water cooled transformers how much cooling surface is required for an internal cooling coil8 ,ans4 ,the surface of the cooling coil should be from 4#e to #a4c sq4 in4 per watt of total transformer loss1 depending upon the amount of heat which the external surface of the transformer case will dissipate4 ,for a water temperature rise of #dc ,fahr41 #a4cb lbs4 of water per minute is required per kw4 of load4 ,transformer ,insulation4--,this subject has not1 until the last few years1 been given the same special attention that many other electrical problems have received1 although the development of the transformer from its original form1 consisting of an iron core enclosed by coils of wire1 to its present degree of refinement and economy of material1 has been comparatively rapid4 ,in transformer construction it is obviously very important that the insulation be of the best quality to prevent burn outs and interruptions of service4 ,ques4 ,what is the major insulation8 ,ans4 ,the insulation placed between the core and secondary 7low pressure7 coils1 and between the primary and secondary coils4 ,illustration3 ,,fig4 #a1igc4--;,assembled coils of ,general ,electric water cooled #ejj kva41 #ff1jjj volt transformer4 ,it consists usually of mica tubes1 sometimes applied as sheets held in place by the windings1 when no ventilating ducts are provided1 or moulded to correct form and held between sheets of tough insulating material where ducts are provided for air or oil circulation4 ,ques4 ,describe the minor insulation4 ,ans4 ,it is the insulation placed between adjacent turns of the coils4 ,since the difference of pressure is small between the adjacent turns the insulation need not be very thick4 ,it usually consists of a double thickness of cotton wrapped around each conductor4 ,for round conductors1 the ordinary double covered magnet wire is satisfactory4 ,ques4 ,what is the most efficient insulating material for transformers8 ,ans4 ,mica4 ,it has a high dielective strength1 is fireproof1 and is the most desirable insulator where there are no sharp corners4 ,illustration3 ,,fig4 #a1igd4--,three ,westinghouse #bj kva1 outdoor transformers1 for irrigation service4 ,these are mounted on a drag so that they may be readily transported from place to place4 #cc1jjj volts high tension2 #b1bjj and #ddj volts low tension1 #ej cycles4 ,these outdoor transformers are of the oil immersed1 self-cooling type and have been developed to meet the requirements for transformers of capacities greater or of voltages higher than are usually found in distribution work4 ,they are in reality distributing transformers for high voltage1 outdoor installations1 single or three phase service1 for voltages up to #aaj1jjj4 ,where the magnitude of the load does not warrant an expensive installation1 transformers of the outdoor type are particularly applicable4 ,the cost of a building and outlet bushings which is often the item of greatest expense is eliminated where outdoor type transformers are installed4 ,oil ,insulated ,transformers4--,high voltage transformers are insulated with oil1 as it is very important to maintain careful insulation not only between the coils1 but also between the coils and the core4 ,in the case of high voltage transformers1 any accidental static discharge1 such as that due to lightning1 which might destroy one of the air insulated type1 might be successfully withstood by one insulated with oil1 for if the oil insulation be damaged it will mend itself at once4 ,by providing good circulation for the oil1 the transformer can get rid of the heat produced in it readily and operate at a low temperature1 which not only increases its life but cuts down the electric resistance of the copper conductors and therefore the ,i,r loss4 ,efficiency of ,transformers4--,the efficiency of transformers is the ratio of the electric power delivered at the secondary terminals to the electric power absorbed at the primary terminals4 ,accordingly1 the output must equal the input minus the losses4 ,if the iron and copper losses at a given load be known1 their values and consequently the efficiency at other loads may be readily calculated4 ,,example4--,if a #aj kilowatt constant pressure transformer at full load and temperature have a copper loss of 4#af kilowatt1 or #a4f per cent41 and the iron loss be the same1 then its output #aj efficiency ------ -------------- #ajj #if4i per cent4 input #aj 4#af 4#af ,at three-quarters load the output will be #g4e kilowatts2 and as the iron loss is practically constant at all loads and the copper loss is proportional to the square of the load1 the output #g4e efficiency ------ --------------- #ajj #if4h per cent4 input #g4e 4#af 4#ji ,the matter of efficiency is important1 especially in the case of large transformers1 as a low efficiency not only means a large waste of power in the form of heat1 but also a great increase in the difficulties encountered in keeping the apparatus cool4 ,the efficiency curve shown in fig4 #a1ige1 serves to indicate1 however1 how slight a margin actually remains for improvement in this particular in the design and construction of large transformers4 ,illustration3 ,,fig4 #a1ige4--;,efficiency curve of ,westinghouse #cge kw41 transformer4 ,pressure #ejj to #ae1jjj volts2 frequency #fj4 ,efficiencies at different loads3 full load efficiency1 #ih2 full load efficiency1 #ih2 full load efficiency1 #ig4f2 full load efficiency1 #if4a2 regulation non-inductive load1 #a4d2 load having 4#i power factor1 #c4c4 ,the efficiency of transformers is1 in general1 higher than that of other electrical machines2 even in quite small sizes it reaches over #ij per cent41 and in the largest1 is frequently as high as #ih4e per cent4 ,to measure the efficiency of a transformer directly1 by measuring input and output1 does not constitute a satisfactory method when the efficiency is so high4 ,a very accurate result can be obtained1 however1 by measuring separately1 by wattmeter1 the core and copper losses4 ,the core loss is measured by placing a wattmeter in circuit when the transformer is on circuit at no load and normal frequency4 ,the copper loss is measured by placing a wattmeter in circuit with the primary when the secondary is short circuited1 and when enough pressure is applied to cause full load current to flow4 ,if it be desired to separate the load losses from the true ,i,r loss1 the resistances can be measured1 and the ,i,r loss calculated and subtracted from the wattmeter reading4 ,the losses being known1 the efficiency at any load is readily found by taking the core loss as constant and the copper loss as varying proportionally to the square of the load4 ,thus1 output efficiency --------------- #ajj output losses ,all ,day ,efficiency of ,transformers4--,this denotes the ratio of the total watt hour output of a transformer to the total watt hour input taken over a working day4 ,to compute this efficiency it is necessary to know the load curve of the transformer over a day4 ,suppose that this is equivalent to #e hours at full load1 and #ai hours at no load4 ,then1 if ,w be the core loss in watts1 ,w the copper loss at rated load1 and ,w the rated output1 output #e ,w1 losses #e 7,w ,w7 #ai ,w1 input #e 7,w ,w ,w7 #ai ,w1 and the all day efficiency is equal to #e,w #ajj --------------------- per cent4 #e7,w ,w ,w7 #ai,w ,commercial or all day efficiency is a most important point in a good transformer4 ,the principal factor in securing a high all day efficiency is to keep the core loss as low as possible4 ,the core loss is constant--it continues while current is supplied to the primary1 while copper loss takes place only when the secondary is delivering energy4 ,in general1 if a transformer is to be operated at light loads the greater part of the day1 it is much more economical to use one designed for a small iron loss than for a small full load copper loss4 ,illustration3 ,,figs4 #a1igf and #a1igg4--,westinghouse double pole fuse box2 views showing box open with tubes in place1 and with tubes removed4 ,transformer ,fuse ,blocks4--,these may be of either the single pole or double pole type4 ,fig4 #a1igf shows a double pole fuse box opened1 and fig4 #a1igg1 the fuse box opened and the tubes removed4 ,of the four wires1 ,w1 ,w1 ,w1 ,w1 entering the box from beneath1 two are from the primary mains1 and two lead to the primary coil of the transformer4 ,these wires terminate in metallic receptacles ,r1 ,r1 ,r1 ,r1 in the porcelain plate ,p1 fig4 #a1igg1 which are bridged over in pairs by fuse wires placed inside porcelain tubes ,t1 ,t1 as shown in fig4 #a1igf4 ,these tubes are air tight except for a small outlet ,o in each1 which fit into the receptacles ,b1 ,b1 in the porcelain plate and open out at the back of the block1 as shown in fig4 #a1igg4 ,the fuse wires are connected between metallic spring tubes ,s1 ,s1 ,s1 ,s1 which fit into the receptacles ,r1 ,r1 ,r1 ,r4 ,if a sudden load or a short circuit occur in the transformer1 the intense heat1 accompanying the melting or blowing of the fuse1 causes a rapid expansion of the air inside the tube1 so that a strong blast of air rushes through the outlet ,o of the tube and immediately extinguishes the arc4 ,by this arrangement1 sustained arcing is prevented1 as the action of the tube causes the arc to extinguish itself automatically when the current is interrupted4 ,the porcelain tubes are held in position by the spring ,k1 and the primary of the transformer becomes entirely disconnected from the circuit when the tubes are lifted out4 ,this form of construction enables the lineman to detach the tubes from the fuse box1 and insert the fuse at his convenience4 ,furthermore1 when inserting a fuse in a short circuited line1 he does not run the risk of being hurt1 as the heated vapor of the exploding fuse can escape through the outlet provided for that purpose1 and in a predetermined direction4 ,the method of attaching the lid not only permits of quick access to the interior of the box1 but enables the lineman to tighten the joints by means of the thumb screws ,l1 ,l1 so as to keep the box waterproof4 ,illustration3 ,,fig4 #a1igh4--;,diagram illustrating connections and principles of auto-transformers as explained in the accompanying text4 ,auto-transformers4--,in this class of transformer1 there is only one winding which serves for both primary and secondary4 ,on account of its simplicity it is made cheaply4 ,auto-transformers are used where the ratio of transformation is small1 as a considerable saving in copper and iron can be effected1 and the whole transformer reduced in size as compared with one having separate windings4 ,fig4 #a1igh illustrates the electrical connections and the relations between the volts and number of turns4 ,by using the end wire and tapping in on turn ,no4 #bj a current at #bj volts pressure is readily obtained which may be used for starting up motors requiring a large starting current and yet not draw heavily on the line4 ,since the primary is connected directly to the secondary it would be dangerous to use an auto-transformer on high pressure circuits4 ,this type of transformer has only a limited use1 usually as compensator for motor starting boxes4 ,illustration3 ,,figs4 #a1igi and #a1ihj4--,two winding transformer and single winding or auto-transformer4 ,fig4 #a1igi shows a #bjj3ajj volt transformer having a #aj amp4 primary and a #bj amp4 secondary1 the currents being in opposite directions4 ,if these currents be superposed by using one winding only1 the auto-transformer shown in fig4 #a1ihj is obtained where the winding carries #aj amp4 only and requires only one-half the copper 7assuming the same mean length of turn74 ,if ,r be the ratio of an auto-transformer1 the relative size of it compared with a transformer of the same ratio and output is 77,r - #a7 _/ ,r73#a4 ,for example1 a #aj kw4 transformer of #djj volts primary and #cjj volts secondary could be replaced by an auto-transformer of #aj 7#a4cc - #a7 _/ #a4cc #b4e kw42 or1 in other words1 the amount of material used in a #b kw4 transformer could be used to wind an auto-transformer of #djj3cjj ratio and #aj kw4 output4 ,constant ,current ,transformers for ,series ,arc ,lighting4--,the principle of the constant current transformer as used for series arc lighting is readily understood by reference to the elementary diagram shown in fig4 #a1iha4 ,a constant alternating current is supplied to the stationary primary coil which induces a current in the movable secondary coil4 ,the pressure induced in the coil will depend on the number of lines of flux which pass through it and by changing its position in the magnetic field over the primary a variable e4m4f4 can be produced and a constant current maintained in the lighting circuit when the lamps are turned on or off1 or if the resistance of the circuit be lowered by the consumption of the carbons4 ,illustration3 ,,fig4 #a1iha4--;,elementary diagram illustrating the principles of constant current transformer as used for series arc lighting4 ,since the induced currents in the secondary are repelled by the primary there is a tendency for the secondary coil to jump out of the primary field1 and in case of a very large current due to a short circuit in the lamp circuit1 the secondary current is quickly reduced to normal by the rapid movement of the coil upward4 ,by adjusting the counterweight for a given number of amperes required by the arc1 the current will be maintained constant by the movement of the secondary coil4 ,the magnetic field produced by the primary must be kept the same by a constant current from the alternator1 therefore1 when the lamp load is increased the primary voltage increases similar to that of an ordinary series wound direct current dynamo4 ,in other words the alternator and regulating transformer supply a constant current and variable voltage4 ,illustration3 ,,fig4 #a1ihb4--,mechanism of ,general ,electric air cooled constant current transformer4 ,it operates on the principle explained in the accompanying text and is built to supply #be to #ajj arc lamps at #f4f to #g4e amperes4 ,the transformers are interchangeable and will operate on #fj or #abe cycles4 ,the relative positions of the two coils may be changed in order to regulate the strength of the current more closely1 by shifting the position of the arc carrying the counterbalance by means of the adjusting screw on it4 ,a dash pot filled with special oil prevents sudden movements of the secondary coil and keeps the current through the lamps nearly constant1 when they are being cut in or out of the circuit4 ,in starting up a constant current transformer1 it is necessary to separate the two coils as far as possible and then close the primary circuit switch and allow the two coils to come together4 ,if the primary circuit be thrown directly on the generator the heavy rush of current which will follow due to the two coils being too close together might injure the lamps4 ,constant current incandescent lighting systems for use in small towns also use this method for automatically regulating the current4 ,regulation4--,this term applies to the means adopted either to obtain constancy of pressure or current4 ,in the transformer1 regulation is inherent1 that is1 the apparatus automatically effects its own regulation4 ,the regulation of a transformer means1 the change of voltage due to change of load on the secondary2 it may be defined more precisely as3 the percentage increase in the secondary voltage as the load is decreased from its normal value to zero4 ,thus1 observation should be made of the secondary voltage1 at full load and at no load1 the primary pressure being held constant at the normal value4 ,illustration3 ,,fig4 #a1ihc4--;,general ,electric air cooled constant current transformer4 ,view showing external appearance with case on4 ,the regulation is said to be good or close1 when this change is small4 ,in the design of a transformer1 good regulation and low iron losses are in opposition to one another when the best results are desired in both4 ,a well designed transformer1 however1 should give good results1 both as to regulation and iron losses1 the relative value depending upon the class of work it has to do1 and size4 ,transformer ,connections4--,the alternating current has the advantage over direct current1 in the ease with which the pressure and current can be changed by different connections of transformers4 ,on single phase circuits the transformer connections can be varied to change current and pressure1 and in addition on polyphase circuits the phases can also be changed to almost any form4 ,single ,phase ,connections4--,the method of connecting ordinary distributing transformers to constant pressure mains is shown by the elementary diagram1 fig4 #a1ihd1 where a transformer of #aj to #a ratio is indicated with its primary winding connected to a #a1jjj volt main1 and a secondary winding to deliver #ajj volts4 ,illustration3 ,,fig4 #a1ihd4--,single phase transformer connection with constant pressure main4 ,illustration3 ,,fig4 #a1ihe4--,usual method of single phase transformer connections for residence lighting with three wire secondaries4 ,a balancing transformer is connected to the three wire circuit near the center of distribution as shown4 ,fig4 #a1ihf shows a transformer with each winding divided into two sections4 ,each primary section is wound for #a1jjj volts1 and each secondary section for #ej volts4 ,by connecting the entire primary winding in series1 the transformer may be supplied from a #b1jjj volt main1 as indicated1 and if the secondary winding be also connected all in series1 as shown1 the no load voltage will be #ajj between the secondary terminals4 ,illustration3 ,,fig4 #a1ihf4--;,diagram of single phase transformer having primary and secondary windings in two sections1 showing voltages per section with series connections4 ,the sections of the primary winding may be connected in parallel to a #a1jjj volt main1 and #ajj volts obtained from the secondary1 or the primary and secondary windings may be connected each with its two sections in parallel1 and transformations made from #a1jjj to #ej volts as represented in fig4 #a1ihg4 ,this is a very common method of construction for small transformers1 which are provided with convenient terminal blocks for combining the sections of each winding to suit the requirements of the case4 ,when the two sections of either winding are connected in parallel as shown in fig4 #a1ihg1 care must be taken to connect corresponding ends of the two sections together4 ,combining ,transformers4--,two or more transformers built to operate at the same pressure and frequency may be connected together in a variety of ways2 in fact1 the primary and secondary terminals may each be considered exactly as the terminals of direct current dynamos1 with certain restrictions4 ,illustration3 ,,fig4 #a1ihg4--;,diagram of single phase transformer with primary and secondary windings of two sections each1 showing voltages per section with parallel connection4 ,ques4 ,what are the two principal precautions which must be observed in combining transformer terminals8 ,ans4 ,the terminals must have the same polarity at a given instant1 and the transformers should have practically identical characteristics4 ,the latter condition is not absolutely essential1 but it is emphatically preferable4 ,for example1 if a transformer1 which has #b per cent4 regulation1 be connected in parallel1 as indicated in fig4 #a1ihh1 with one which has #c per cent4 regulation1 at no load the transformers will give exactly the same voltage at the secondary terminals1 but at full load one will have a secondary pressure of1 say1 #ih volts1 while the other has #ig volts4 ,the result is that the transformer giving only #ig volts will be subject to a reverse pressure of one volt from its mate4 ,this will not cause excessive current to flow backward through the secondary winding of the low voltage transformer1 but it will disturb the phase relations and lower the power factor and efficiency of the combination4 ,in such a case it is much better to work the secondary circuits of the two transformers separately4 ,in case the transformers have practically the same characteristics it is necessary1 as stated above1 to make sure that the secondary terminals connected together have the same polarity at a given instant2 it is not necessary to find out definitely what the polarity is1 merely that it is the same for both terminals4 ,this can be easily done as shown in fig4 #a1ihi4 ,illustration3 ,,fig4 #a1ihh4--;,diagram showing unlike single phase transformers in parallel4 ,ques4 ,what may be said with respect to operating transformer secondaries in parallel8 ,ans4 ,it is seldom advantageous4 ,occasionally it may be necessary as a temporary expedient1 but where the load is such as to require a greater capacity than that of a transformer already installed1 it is much better to replace it by a large transformer than to supplement it by an additional transformer of its own size4 ,ques4 ,how are the secondaries arranged in modern transformers and why8 ,ans4 ,the secondary windings are divided into at least two sections so that they may be connected either in series or parallel4 ,ques4 ,explain how secondary connections are made for different voltages4 ,ans4 ,if1 for instance1 the secondary pressure of a transformer having two sections be #ajj volts with the terminals in parallel1 as in fig4 #a1iij1 then connecting them in series will give #bjj volts at the free secondary terminals1 as indicated in fig4 #a1iia4 ,ques4 ,what precaution should be taken in connecting secondary sections in parallel in core type if the two sections be wound on different limbs of the cores8 ,illustration3 ,,fig4 #a1ihi4--,method of comparing instantaneous polarity4 ,two of the terminals are connected as shown by a small strip of fuse wire1 and then touching the other two terminals together4 ,if the fuse blows1 then the connections must be reversed2 if it does not1 then they may be made permanent4 ,ans4 ,it will be advisable to make the connections ample and permanent1 so that there will not be any liability to a difference between the current flowing in one secondary winding and that flowing through the other4 ,two ,phase ,connections4--,in the case of two phase distribution each circuit may be treated as entirely independent of the other so far as the transformers are concerned4 ,two transformers are used1 one being connected to one primary phase and supplying one secondary phase1 the other being connected to the other primary phase and supplying the other secondary phase as indicated in fig4 #a1iif1 exactly as though each primary and secondary phase were an ordinary single phase system1 independent of the other phase4 ,illustration3 ,,figs4 #a1iij and #a1iia4--,methods of altering the secondary connections of a transformer having two sections in the secondary to obtain a different voltage4 ,fig4 #a1iij shows the two sections in parallel giving say #ajj volts2 fig4 #a1iia shows the two sections in series giving #bjj volts4 ,ques4 ,is the above method usually employed8 ,ans4 ,no1 the method shown in fig4 #a1iig is generally used4 ,three ,phase ,connections4--,there is not so much freedom in making three phase transformer connections1 as with single or two phase1 because the three phases are inseparably interlinked4 ,however1 the system gives rise to several methods of transformer connection1 which are known as3 #a4 ,star2 #b4 ,delta2 #c4 ,star-delta2 #d4 ,delta-star4 ,illustration3 ,,figs4 #a1iib to #a1iie4--,three phase transformer connections4 ,fig4 #a1iib delta connection2 fig4 #a1iic star connection2 fig4 #a1iid delta star connection2 fig4 #a1iie star-delta connection4 _/ ,delta ,connection4--,in the delta connection both primaries and secondaries are connected in delta grouping1 as in fig4 #a1iib4 ,star ,connection4--,this method consists in connecting both the primaries and secondaries in star grouping1 as in fig4 #a1iic4 ,delta-star ,connection4--,in this method the primaries are connected in delta grouping and the secondaries in star grouping1 as in fig4 #a1iid4 ,star-delta ,connection4--,this consists in connecting the primaries in star grouping1 and the secondaries in delta grouping1 as in fig4 #a1iie4 ,illustration3 ,,fig4 #a1iif4--,two phase transformer connections4 ,two single phase transformers are used and connections made just as though each phase were an ordinary single phase system4 ,ques4 ,what advantage has the star connection over the delta connection8 ,ans4 ,each star transformer is wound for only #eh of the line voltage4 ,in high voltage transmission1 this admits of much smaller transformers being built for high pressure than possible with the delta connection4 ,ques4 ,what advantages are obtained with the delta connection8 ,ans4 ,when three transformers are delta connected1 one may be removed without interrupting the performance of the circuit1 the two remaining transformers in a manner acting in series to carry the load of the missing transformer4 ,illustration3 ,,fig4 #a1iig4--,two phase transformer connections1 with secondaries arranged for three wire distribution1 the primaries being independently connected to the two phases4 ,in the three wire circuit1 the middle or neutral wire is made about one-half larger than each of the two outer wires4 ,in fig4 #a1iif it makes no difference which secondary terminal of a transformer is connected to a given secondary wire1 so long as no transformers are used in parallel4 ,for example1 referring to the diagram1 the left hand secondary terminal of transformer1 ,a1 could just as well be connected to the lower wire of the secondary phase1 ,a1 and its right hand terminal connected to the upper wire1 the only requirement being that the two pairs of mains shall not be mixed2 that is1 transformer1 ,a1 must not be connected with one secondary terminal to phase1 ,a and the other to phase1 ,b4 ,in the case shown by fig4 #a1iig1 there is not quite so much freedom in making connections4 ,one secondary terminal of each transformer must be connected to one of the outer wires and the other two terminals must be both connected to the larger middle wire of the secondary system4 ,it makes no difference1 however1 which two secondary terminals are joined and connected to the middle wire so long as the other terminal of each transformer is connected to an outer wire of the secondary system4 ,the desire to guard against a shut down due to the disabling of one transformer has led to the extensive use of the delta connection1 especially for the secondaries or low pressure side4 ,it should be noted that if one transformer be disabled1 the efficiency of the other two will be greatly reduced4 ,to operate a damaged three phase transformer1 the damaged windings must be separated electrically from the other coils1 the damaged primary and secondary being respectively short circuited upon themselves4 ,ques4 ,what kinds of transformers are used for three phase current8 ,ans4 ,either a three phase transformer1 or a separate single phase transformer for each phase4 ,illustration3 ,,fig4 #a1iih4--,three wire connections for transformer having two secondary sections on different legs of the core4 ,if the secondary terminals be connected up to a three wire distribution1 as here shown diagrammatically1 it is advisable to make the fuse1 #b1 in the middle wire1 considerably smaller than necessary to pass the normal load in either side of the circuit1 because1 should the fuse1 #a1 be blown1 the secondary circuit through the section1 ,sa1 will be open1 and the corresponding half of the primary winding1 ,pa1 will have a much higher impedance than the half of the primary winding1 ,pb1 the inductance of which is so nearly neutralized by the load on the secondary winding1 ,sb4 ,the result will be that the voltage of the primary section1 ,pa1 will be very much greater than that of the section1 ,pb1 and as the sections are in series the current must be the same through both halves of the winding2 the drop or difference of pressure1 therefore1 between the terminals of ,pa will be much higher than that between the terminals of ,pb1 consequently1 the secondary voltage of ,sb will be greatly lowered and the service impaired4 ,as the primary winding1 ,pa1 is designed to take only one-half of the total voltage1 the unbalancing referred to will subject it to a considerably higher pressure than the normal value2 consequently1 the magnetic density in that leg of the transformer core will be much higher than normal1 and the transformer will heat disastrously4 ,if the fuse1 #b1 in the middle wire be made1 say1 one-half the capacity of each of the other fuses1 this condition will be relieved by the blowing of this fuse1 and as the lamps in the live circuit would not be anywhere near candle power if the circuit remained intact1 the blowing of the middle fuse will not be any disadvantage to the user of the lamps4 ,some makers avoid the contingency just described by dividing each secondary coil into two sections and connecting a section on one leg in series with a section on the other leg of the core1 so that current applied to either pair of the secondary terminals will circulate about both legs of the core4 ,illustration3 ,,figs4 #a1iii to #b1jjb4--,three phase delta1 and star connections using three transformers4 ,there are two ways of connecting up the primaries and secondaries1 one known as the delta connection1 and illustrated diagrammatically by fig4 #a1iii1 and the other known as the star connection1 and illustrated by fig4 #b1jja4 ,in both diagrams the line wires are lettered1 ,a1 ,b and ,c4 ,fig4 #b1jjj shows the primaries and secondaries connected up delta fashion1 corresponding to fig4 #a1iii1 and fig4 #b1jjb shows them connected up star fashion1 corresponding to fig4 #b1jja4 ,in both of the latter sketches the secondary wires are lettered to correspond with the respective primary wires4 ,when the primaries are connected up delta fashion1 the voltage between the terminals of each primary winding is the same as the voltage between the corresponding two wires of the primary circuit1 and the same is true of the secondary transformer terminals and circuit wires4 ,the current1 however1 flowing through the transformer winding is less than the current in the line wire1 for the reason that the current from any one line wire divides between the windings of two transformers4 ,for example1 in figs4 #a1iii and #b1jjj1 part of the current from the line wire1 ,a1 will flow from ,a to ,b through the left hand transformer1 and part from ,a to ,c through the right hand transformer2 if the current in the line wire1 ,a1 be #ajj amperes1 the current in each transformer winding will be #eg4gce amperes4 ,when transformers are connected up star fashion1 as in figs4 #b1jja and #b1jjb1 the current in each transformer winding is the same as that in the line wire to which it is connected1 but the voltage between the terminals of each transformer winding is #eg4gce per cent4 of the voltage from wire to wire on the circuit4 ,for example1 if the primary voltage from ,a to ,b is #a1jjj volts1 the voltage at the terminals of the left hand transformer 7from ,a to ,j7 will be only #egg4ce volts1 and the same is true of each of the other transformers if the system is balanced4 ,these statements apply1 of course1 to both primary and secondary windings1 from which it will become evident that if the three transformers of a three phase circuit be connected up star fashion at the primaries1 and delta fashion at the secondaries1 the secondary voltage will be lower than if both sides are connected up star fashion4 ,for example1 if the transformers be wound for a ratio of #aj to #a1 and are connected up with both primaries and secondaries alike1 no matter whether it be delta fashion or star fashion1 the secondary voltage will be one-tenth of the primary voltage2 but if the primaries be connected up star fashion on a #a1jjj volt circuit1 and the secondaries be connected up delta fashion1 the secondary voltage will be only #eg4gce volts1 instead of #ajj volts4 ,the explanation of the difference between the voltage per coil in a delta system and that in a star system is that in the former each winding is connected directly across from wire to wire2 whereas in the star system1 two windings are in series between each pair of line wires4 ,the voltage of each winding is not reduced to one-half1 however1 because the pressures are out of phase with each other1 being #abj1 or one-third of a cycle1 apart2 consequently1 instead of having #ejj volts at the terminals of each coil in fig4 #b1jja the voltage is #egg4ce4 ,the same explanation applies to the current values in a delta system4 ,the current phase between ,a and ,b1 in fig4 #a1iii1 is #abj removed from that in the winding between ,a and ,c2 consequently the sum of the two currents1 in the wire1 ,a1 is #a4gcb times the current in each wire2 or1 to state it the opposite way1 the current in each winding is #eg4gce of the current in the wire1 ,a4 ,it will be well for the reader to remember that in all cases pressures differing in phase when connected in series1 combine according to the well-known law of the parallelogram of forces2 currents differing in phase1 and connected in parallel1 combine according to the same law4 ,ques4 ,what points are to be considered in choosing between three phase and single phase transformers for the three phase current transformation8 ,ans4 ,no specific rule can be given regarding the selection of single phase or three phase transformers since both designs are equally reliable2 local conditions will generally determine which type is preferable4 ,the following general remarks may1 however1 be helpful3 ,single phase transformers are preferable where only one transformer group is installed and where the expense of a spare transformer would not be warranted4 ,in such installations the burn out of one phase of a three phase unit would cause considerable inconvenience for the reason that the whole transformer would have to be disconnected from the circuit before repairs could be made4 ,if single phase transformers be used and connected in delta on both primary and secondary1 the damaged transformer can be cut out with a minimum amount of trouble and the other two transformers can be operated at normal temperature open delta at #eh per cent4 of the normal capacity of the group of three transformers1 until the third unit can be replaced4 ,with a three phase shell type transformer1 if both the primary and secondary be delta connected1 trouble in one phase will not prevent the use of the other two phases in open delta4 ,by short circuiting both primary and secondary of the defective phase1 and cutting it out of circuit the magnetic flux in that section is entirely neutralized4 ,this cannot be done1 however1 with any but delta connected shell type transformers4 ,illustration3 ,,fig4 #b1jjc4--;,diagram showing three wire secondary connections ,general ,electric 7type ,h7 transformer4 ,as will be seen1 the method adopted consists of distributing equally1 on each side of the primary coil1 both halves of the secondary winding1 so that each secondary throughout its length is closely adjacent to the entire primary winding4 ,in order to insure the exact equality of resistance and reactance in the two secondary windings necessary to obtain perfect regulation of the two halves1 the inside portion of the secondary winding on one side of the primary coil is connected in series with the outside portion of that on the other side4 ,as a result1 the drop of voltage in either side of the secondary under any ordinary conditions of unbalanced load1 does not exceed the listed regulation drop4 ,this particular arrangement is used because it is the simplest and best method for this construction4 ,where a large number of three phase transformers can be used1 it is generally advisable to install three phase units1 the following advantages being in their favor as compared with single phase units3 #a4 ,require less floor space than three single phase units2 #b4 ,weigh less than the single phase units2 #c4 ,simpler connections1 as only three primary and three secondary leads are generally brought out2 #d4 ,transformer presents a symmetrical and compact appearance4 ,ques4 ,what is the character of the construction of three phase transformers8 ,ans4 ,the three phase transformer is practically similar to that of the single phase1 except that somewhat heavier and larger parts are required for the core structure4 ,,comparison ,,of ,,air ,,blast1 ,,water ,,cooled1 ,,and ,,oil ,,cooled ,,transformers ,air blast type ,water cooled type ,oil cooled type ---------------------------------------- ---------------------------- #a4 ,,cost ---------------------------------------- ---------------------------- ,a4 ,first cost ,necessarily more ,least expensive of all ,necessarily more expensive than the types4 expensive than the air water cooled type of blast and water cooled similar rating4 type of similar rating4 ,b4 ,the installation is ,being heavier than the ,being heavier than the extremely simple4 air blast type1 these air blast and water transformers1 as a cooled type1 these ,moisture that may rule1 require heavier transformers require have collected on the apparatus for heavier apparatus for surfaces during installing4 ,both installing4 ,both transportation transformer and tank transformer and tank or storage should be should be thoroughly should be thoroughly thoroughly dried out4 dried out before being dried out before being filled with oil4 filled with oil4 ,the oil is usually supplied in #ej gal4 hermetically sealed steel barrels to minimize possibility of moisture during transportation4 ---------------------------------------- ---------------------------- ,c4 ,auxiliary apparatus ,a duct1 or chamber1 ,in most cases1 cooling ,do not require cooling of considerable size water may be obtained water or blower4 is required under the with sufficient natural transformers in order head4 ,however1 there to conduct the coolingare frequent cases in air to them4 which it can be obtained only by the ,a blower outfit for use of pumps4 supplying air is required4 ,a system of piping for the cooling water and oil drainage is required1 the cost of which depends1 of course1 on the station layout4 ,d4 ,maintenance ,an occasional ,a water pumping outfit ,no air or water cleaning1 for which a would possibly require circulation to demand supply of compressed a trifle more attentionattention4 air at about #bj lb4 than a blower outfit in pressure is which there are no recommended4 valves or piping4 ,the blower outfit requires no more care than any other similar apparatus4 #b4 ,,floor ,,space ---------------------------------------- ---------------------------- ,always requires space ,extra space only ,only require space for for cooling apparatus4required when auxiliarythe transformer as no pumping apparatus is extra apparatus is necessary4 necessary #c4 ,,location ---------------------------------------- ---------------------------- ,as the transformers ,transformers are ,transformers are are open at the top completely enclosed butcompletely enclosed but they should not be location should be suchlocation should be such located where there isthat no water will fallthat no water will fall much dust or dirt nor on leads or bushings4 on leads or bushings4 where water from any source is liable to ,the building should be fall on them4 well ventilated4 ,there is no auxiliary apparatus4 ,the blower should be ,location of auxiliary so situated as to apparatus will depend obtain clean dry air on the station layout4 of a temperature not greater than #gg ,fahr4 #d4 ,,general ,,appearance ---------------------------------------- ---------------------------- ,terminal leads may ,leads are brought out ,leads are brought out be located in the of the top of the of the top of the base and the air transformers4 transformers4 chamber may be used for conducting and ,water cooling pipes distributing the are connected at the connecting wiring4 top in most cases4 ,the absence of overhead wiring aids in simplifying the appearance of the station4 #e4 ,,operation ---------------------------------------- ------------------------------ ,equal reliability in all three types4 ,while full load efficiencies are practically equal in the three designs1 it is necessary to change the proportion of iron and copper losses somewhat as the copper loss of the air blast transformer is a smaller part of its total loss than of the water cooled and oil cooled types4 ,as a result1 the regulation of the air blast transformer is a trifle better4 #f4 ,,general ---------------------------------------- ------------------------------ ,the above information regarding selection of type is not applicable to air blast transformers for circuits materially in excess of #cc1jjj volts4 ,on account of the great thickness of the solid insulation needed and the consequent difficulty in radiating heat from the copper1 it is impracticable to design the air blast type for more than this voltage4 ,the oil immersed designs are therefore recommended for transformers above #cc1jjj volts4 ,both oil cooled and water cooled types are available for all voltages1 being restricted in this respect only by the limitations of transmission facilities4#h #h ,,note4--,no special foundations are necessary for any type of transformer other than a good1 even floor1 having sufficient strength to support the weight4 ,illustration3 ,,figs4 #b1jjd to #b1jaa4--;,connections of standard transformers4 ,all stock transformers are wound for some standard transformation ratio1 such as #aj to #a1 but various leads are brought out by means of which ratios of #e1 #aj and #bj to #a may be obtained for one transformer4 ,the figures show the voltage combinations possible with a standard transformer4 ,ques4 ,how are transformers connected for four wire three phase distribution8 ,ans4 ,when the secondaries of three transformers are star connected1 a fourth wire may be run from the neutral point1 thus obtaining the four wire system4 ,illustration3 ,,fig4 #b1jab4--,method of determining core loss4 ,connect voltmeter and wattmeter as shown in the illustration to the low tension side of the transformer4 ,by means of a variable voltage transformer bring the applied voltage to the point for which the transformer is designed4 ,the wattmeter indicates directly the core loss1 which includes a very small loss due to the current in the copper4 ,cautions4--#a4 ,make sure of the voltage and frequency4 ,the manufacturers' tabulated statements refer to a definite voltage and frequency and these have a decided influence upon the core loss4 #b4 ,the high tension circuit must remain open during the test4 ,the voltage between any main wire and the neutral will be #eg per cent4 of the voltage between any two main wires4 ,for general distribution this system is desirable1 requiring less copper and greater flexibility than other systems4 ,three phase #bjj volt motors may be supplied from the main wires and #aae volt lamps connected between each of the three main wires and the neutral2 if the lamp load be very nearly balanced the current flowing in the neutral wire will be very small1 as in the case of the ordinary three wire direct current system4 ,how to ,test ,transformers4--,the troubles incident to gas or water service have their parallels in electric power distribution4 ,illustration3 ,,fig4 #b1jac4--,method of determining copper loss4 ,connect ammeter and wattmeter to high tension side of transformer short circuit secondary leads1 as shown in the illustration1 and by means of a variable voltage1 adjust current to the full load value for which the transformer is intended4 ,the wattmeter reading shows the copper loss at full load4 ,the full load primary current of any transformer is found from the following equation4 full load current full load watts primary volts ,,example3 ,to find proper full load current on a five kw4 #b1bjj volt transformer1 divide #e1jjj watts by #b1bjj volts1 the full load current will then be #b4bg amperes4 ,a slight variation in primary current greatly increases or decreases the copper loss4 ,remarks4--,copper loss increases with temperature because the resistance of the metal rises4 ,do not overload the current coil of the wattmeter4 ,for greater accuracy the ,i,r drop of potential method should be used4 ,companies engaged in the former1 credit a large percentage of their losses to leaky valves and defective mains4 ,the remedy may involve heavy expense and the loss is often tolerated as the lesser of two evils4 ,in electric power distribution the transformer takes in part the place of the valve and pipe system4 ,an inferior or defective transformer usually treats both the central station and its customers badly1 being in this respect more impartial than the gas or water pipe which may annoy but one of the interested parties at a time4 ,like a neglected or defective gas fixture a transformer can menace life1 failing1 however1 to give the warning the former gives1 and with a more hidden threat on account of its location4 ,illustration3 ,,fig4 #b1jad4--;,diagram of connections for regulation test4 ,connect transformer under test to high tension supply circuit4 ,a second transformer with same or other known change ratio is also to be connected up1 as illustrated4 ,by means of a double pole double throw switch1 the voltmeter can be made to read the pressure on the secondary of either transformer4 ,supposing the same change ratio it is evident that if both remain unloaded the voltmeter will indicate the same pressure4 ,a gradually increasing lamp load up to the limit of the transformer capacity1 will be attended by a drop in pressure at the terminals4 ,this drop can be read as the difference of the voltmeter indications1 and when expressed in per cent4 of secondary voltage stands for regulation4 ,remarks3 ,the auxiliary transformer is necessary in order to make sure of the high tension line voltage4 ,a large transformer under test may cause primary drop in taking power4 ,this must be set down against it in testing regulation4 ,the second transformer gives notice of such drop1 whatever be the cause4 ,figs4 #b1jab to #b1jad used by courtesy of the ,moloney ,electric ,co4 ,apart from this1 corresponding to an exasperated customer who complains at home and to his friends of dim lamps1 blackened lamps1 you will find in the power station the manager1 who1 also worried and in no better humor1 contemplates the difference in meter readings at the end of the line4 ,his business does not increase and would not increase even if he could lower the rates1 which he cannot do because of these meter readings4 ,he may be confident of his engines and generators1 and that his line is up and all right1 but he very seldom knows what the transformers are doing on top of the poles4 ,perhaps he feels that this waste is so slight that it makes no material difference4 ,this can be readily ascertained by means of a set of testing instruments4 ,perhaps the transformers were purchased because of their attractive prices and never tested4 ,water1 plumbing1 gas and steam fittings are subjected to test4 ,why not transformers8 ,even more so because transformers take constant toll from the company installing them1 while gas and water fittings1 once passed1 are off the contractor's hands4 ,the busy manager has little time for complicated treatises and monographs on electrical measurements and even handbooks confront him with forbidding formul4 ,accordingly the methods of transformer testing1 which are very simple1 are illustrated in the accompanying cuts4 ,managers of electric power and lighting companies should study them carefully4 ,illustration3 ,,fig4 #b1jae4--,wagner central station core type transformer repair unit consisting of one half set of primary and secondary windings together with the section of the iron core upon which the coils are wound4 ,an ammeter1 voltmeter and wattmeter are required to make the tests4 ,losses are small in good transformers and hence the instruments should be accurate4 ,for the same reason instruments should be chosen of the proper capacity to give their best readings4 ,if there be any doubt about the testing instruments being correct1 they should be calibrated before being used4 ,the testing circuits should be properly fused for the protection of the instruments4 ,it is hardly logical1 but a very common practice is to mistrust meters and to watch them closely1 while the transformers are guilty of theft unchallenged1 and keep busily at it on a large scale4 ,illustration3 ,,fig4 #b1jaf4--,moloney tubular air draft oil filled transformer4 ,the case is made of cast iron1 with large steel tubes passing from the bottom through the top4 ,in operation the air in the tubes becomes hot and expands2 a draft is thus produced which carries away considerable heat4 ,transformer ,operation with ,grounded ,secondary4--,the operation of a transformer with a grounded secondary has been approved by the ,american ,institute of ,electrical ,engineers1 and by the ,national ,board of ,fire ,underwriters4 ,this method of operation effectually prevents a high voltage occurring upon the low tension wires in case of a breakdown or other electrical connections occurring between the primary and secondary windings4 ,illustration3 ,,fig4 #b1jag4--,moloney pressure transformer adapted for switchboard work in connection with voltmeters1 wattmeters the1 etc41 in sizes from #be to #ejj watts4 ,in case of a breakdown without the secondary grounded1 any one touching a part of the low tension system1 such as a lamp socket1 might receive the full high pressure voltage4 ,with the low tension grounded1 the fuse in the high tension circuit will blow and the fault be discovered upon replacing it4 ,transformer ,capacity for ,motors4--,the voltage regulation of a well designed transformer is within #c per cent4 of its rated voltage on a non-inductive load such as incandescent lamps1 but when motors are connected to the circuit their self-induction causes a loss of #e per cent4 or more1 and if the load be fluctuating1 it is better to use independent transformers for the motor1 which will prevent considerable fluctuations in the incandescent lamps4 ,arc lamps do not show slight voltage changes as much as incandescent lamps4 ,the proper rating of transformers for two phase and three phase induction motors is given in table on the next page4 ,illustration3 ,,fig4 #b1jah4--,moloney current transformer switchboard or indoor type4 ,it is used ordinarily for insulating an ammeter1 a current relay1 the current coil of a watt meter or watt hour meter from a high tension circuit1 for reducing the line current to a value suitable for these instruments4 ,a three phase induction motor may be operated from three single phase transformers or one three phase transformer4 ,while the one three phase transformer greatly reduces the space and simplifies the wiring1 the use of three single phase transformers is more flexible and1 in case one transformer burns out1 the connection can be readily changed so that two transformers will operate the motor at reduced load until the burned out transformer is replaced or repaired4 ,illustration3 ,,fig4 #b1jai4--;,diagram showing a method of operating a three phase motor on a two phase circuit1 using a transformer having a tap made in the middle of the secondary winding1 so as to get the necessary additional phase4 ,while this does not give a true balanced three phase secondary1 it is close enough for motor work4 ,in the above arrangement1 the main transformer supplies #ed per cent4 of the current and the other with the split winding #df per cent4 ,it is well to allow one kilowatt per horse power of the motor in selecting the size for the transformers1 excepting in the small sizes when a little larger kilowatt rating is found to be the most desirable4 ,transformers for ,two and ,three ,phase ,motors ---------------------------------------- -------------------- ,single phase transformer voltages ,delivered ---------------------------------------- -------- voltage of #aaj volt motor #bbj volt motor circuit ---------------------------------------- ------ ,primary ,secondary ,primary ,secondary ---------------------------------------- ----------------- #a1ajj #a1ajj #abb #a1ajj #bdd #b1bjj #b1bjj #abb #b1bjj #bdd ---------------------------------------- ----------------- ,very small transformers should not be used1 even when the motor is large compared to the work it has to do1 as the heavy starting current may burn them out4 ,the following tables give the proper sizes of transformer for three types of induction motor and the approximate current taken by three phase induction motors at #bbj volts4 ---------------------------------------- ------------------- ,capacities of ,transformers for ,induction ,motors ---------------------------------------- ------------------ ,kilowatts per transformer ,size of motor ---------------------------------------- - horse power ,two single ,three single ,one three phase phase phase transformers transformers transformer ---------------------------------------- ---------------- #a #j4f #j4f #b #a4e #a4j #b4j #c #b4j #a4e #c4j #e #c4j #b4j #e4j #g #d4j #c4j #g4e #aj #e4j #d4j #aj4j #ae #g4e #e4j #ae4j #bj #aj4j #g4e #bj4j #cj #ae4j #aj4j #cj4j #ej #be4j #ae4j #ej4j #ge #dj4j #be4j #ge4j #ajj #ej4j #cj4j #ajj4j ---------------------------------------- ---------------- ,current taken by ,three ,phase ,induction ,motors at #bbj ,volts ---------------------------------------- ------------------ ,horse power ,approximate full ,horse power ,approximate full of motor load current of motor load current ---------------------------------------- ------------------ #a #c4b #bj #ej4 #b #f4j #cj #ge4 #c #i4j #ej #abe4 #e #ad4j #ge #ahe4 #aj #bg4j #ajj #bej4 #ae #dj4j #aej #cgj4 ---------------------------------------- ------------------ ,transformer ,connections for ,motors4--,fig4 #b1jbj shows the connection of a three phase so called delta connected transformer with the three primaries connected to the lines leading from the alternator and the three secondaries leading to the motor4 ,the connections for a three phase motor using two transformers is shown in fig4 #b1jba and is identical with the previous arrangement1 except that one transformer is left out and the other two made correspondingly larger4 ,the copper required in any three wire three phase circuit for a given power and loss is #ge per cent4 that necessary with the two wire single phase or four wire two phase system having the same voltage between lines4 ,illustration3 ,,fig4 #b1jbj4--,three phase motor transformer connections2 the so-called ,delta connected transformers4 ,the connections of three transformers for a low tension system of distribution by the four wire three phase system are shown in fig4 #b1jbb4 ,the three transformers have their primaries joined in delta connection and the secondaries in ,y connection4 ,the three upper lines of the secondary are the three main three phase lines1 and the lowest line is the common neutral4 ,illustration3 ,,fig4 #b1jba4--,three phase motor connections using two transformers4 ,the voltage across the main conductors is #bjj volts1 while that between either of them and the neutral is #aae volts2 #bjj volt motors should be joined to the mains while #aae volt lamps are connected between the mains and neutral4 ,the arrangement is similar to the ,edison three wire system and the neutral carries current only when the lamp load is unbalanced4 ,the voltage between the mains should be used in calculating the size of wires1 and the size of the neutral wire should be made in proportion to each of the main conductors that the lighting load is to the total load4 ,illustration3 ,,fig4 #b1jbb4--;,delta-star connection of three transformers for low pressure1 three phase1 four wire system4 ,when lights only are used the neutral should be the same as the main conductors4 ,the copper required in such a system for a given power and loss is about #cc4c per cent4 as compared with a two wire single phase system or a four wire two phase system using the same voltage4 ,illustration3 ,,fig4 #b1jbc4--;,diagram of transformer connections for motors on the monocyclic system4 ,monocyclic ,motor ,system4--,motors on the monocyclic system are operated from two transformers connected as shown in fig4 #b1jbc4 ,in the monocyclic system the single phase current is used to supply the lighting load and two wires only are necessary1 but if a self-starting induction motor be required1 a third or teaser wire is brought to the motor and two transformers used4 ,the teaser wire supplies the quarter phase current required to start the motor1 which afterwards runs as a single phase synchronous motor and little or no current flows through the teaser circuit as long as the motor keeps in synchronism2 in case it fall behind1 the teaser current tends to bring it up to speed instead of the motor stopping1 as would be the case of a single phase motor4 ,illustration3 ,,fig4 #b1jbd4--,moloney flaming auto type arc lamp transformer for #aaj volts primary to #ee volts secondary4 ,a hook in bottom of case provides means for suspension of lamp4 ,the transformer may be operated on circuits from #ajj to #abj volts primary1 #ej to #fj volts secondary4 ,the secondary capacity is #h to #ab amperes4 ,the voltage of the transformers should be tested by means of a voltmeter or two incandescent lamps joined in series1 before starting up the motor1 to see if the proper transformer connections have been made and prevent an excessive flow of current4 ,if one of the transformers be reversed the voltage will be almost doubled2 in fact1 it is a good plan to check up all the transformer connections with the voltmeter or lamps which will often save a burn out4 ---------------------------------------- ----------------------- ,ratio of transformation at no load ---------------------- ,arrangement of ,primary ,for with with links on the coils circuit secondary secondary connecting board will be voltage coils in coils in connected normal at multiple series4 in ---------------------------------------- ---------------------- ,,illustration ,multiple #a1ajj #aj3a #e3a ---------------------------------------- ---------------------- ,,illustration ,multiple #a1ajj #i4je3a #d4eb3a ---------------------------------------- ---------------------- ,,illustration ,series #b1bjj #bj3a #aj3a ---------------------------------------- ---------------------- ,,illustration ,series #b1bjj #ai4je3a #i4e3a ---------------------------------------- ---------------------- ,,illustration ,series #b1bjj #ah4a3a #i4je3a ---------------------------------------- ---------------------- ,,figs4 #b1jbe to #b1jbi4--;,diagrams of ,wagner transformer connection board1 and table showing various arrangements of the terminal links1 corresponding transformation ratios1 and suitable primary voltages4 99_/ ,illustration3 ,,fig4 #b1jcj4--;,installation of a transformer on pole2 view showing method of attachment and disposition of the primary and secondary leads1 cutouts1 etc4 ,illustration3 ,,fig4 #b1jca4--;,diagram of static booster or regulating transformer4 ,it is used for regulating the pressure on feeders4 ,in the figure1 ,b are the station bus bars1 ,r the regulable transformer1 ,f the two wire feeders1 and ,t a distant transformer feeding into the low pressure three wire distributing network ,n4 ,the two ends of the primary1 and one end of the secondary of ,r1 are connected to the bus bars as shown4 ,the other end of the secondary1 as well as a number of intermediate points1 are joined up to a multiple way switch ,s1 to which one of the feeder conductors is attached1 the other feeder main being connected to the opposite bus bar4 ,as will be evident from the figure1 by manipulating ,s extra volts may be added to the bus bar pressure at will1 and the drop along ,f compensated for4 ,r is a step transformer1 the total secondary difference of pressure being comparatively small4 ,the above device possesses rather serious drawbacks1 in that the switch ,s has to carry the main current1 and that the supply would be stopped if the switch got out of order4 ,kapp improved on the arrangement by putting the switch in the primary circuit4 ,,chapter ,,liii ,,converters ,the alternating current must change to a direct current in many cases as in railroad work because the induction motor is not so satisfactory as the direct current series motor and the alternating current series motor is slow in coming into general use4 ,in all kinds of electrolytic work1 transformation must be made1 and in many cities where the direct current system was started1 it is still continued for local distribution1 but the large main stations generating alternating currents and frequently located some distance away from the center of distribution have replaced a number of small central stations4 ,transformation may be made by any of the following methods3 #a4 ,rotary converters2 #b4 ,motor generator sets2 #c4 ,a,mercury vapor rectifiers2 #d4 #i;,electrolytic rectifiers4 #i ,,note4--,rectifiers are explained in detail in ,chapter ,,liv4 ,strictly speaking1 a converter is a revolving apparatus for converting alternating current into direct current or vice versa2 it is usually called a rotary converter and is to be distinguished from the other methods mentioned above4 ,broadly1 however1 a converter may be considered as any species of apparatus for changing electrical energy from one form into another4 ,according to the standardization rules of the ,a4 ,i4 ,e4 ,e4 converters may be classified as3 #a4 ,direct current converters2 #b4 ,synchronous converters2 #c4 ,motor converters2 #d4 ,frequency converters2 #e4 ,rotary phase converters4 ,illustration3 ,,figs4 #b1jcb and #b1jcc4--;,gramme ring dynamo and alternator armatures illustrating converter operation4 ,the current generated by the dynamo is assumed to be #ajj amperes4 ,now1 suppose1 an armature similar to fig4 #b1jcb to be revolving in a similar field1 but let its windings be connected at two diametrically opposite points to two slip rings on the axis1 as in fig4 #b1jcb4 ,if driven by power1 it will generate an alternating current4 ,as the maximum voltage between the points that are connected to the slip rings will be #ajj volts1 and the virtual volts 7as measured by a voltmeter7 between the rings will be #gj4g 7 #ajj #b71 if the power applied in turning this armature is to be #aj kilowatts1 and if the circuit be non-inductive1 the output in virtual amperes will be #aj1jjj #gj4g #ada4d4 ,if the resistances of each of the armatures be negligibly small1 and if there be no frictional or other losses1 the power given out by the armature which serves as motor will just suffice to drive the armature which serves as generator4 ,if both armatures be mounted on the same shaft and placed in equal fields1 the combination is a motor dynamo4 ,in actual machines the various losses are met by an increase of current to the motor4 ,since the armatures are identical1 and as the similarly placed windings are passed through identical magnetic fields1 one winding with proper connections to the slip rings and commutator will do for both4 ,in this case only one field is needed2 such a machine is called a converter4 ,a direct current converter converts from a direct current to a direct current4 ,a synchronous converter 7commonly called a rotary converter7 converts from an alternating current to a direct current4 ,a motor converter is a combination of an induction motor with a synchronous converter1 the secondary of the former feeding the armature of the latter with current at some frequency other than the impressed frequency2 that is1 it is a synchronous converter in combination with an induction motor4 ,a ,frequency ,converter 7preferably called a frequency changer7 converts alternating current at one frequency into alternating current of another frequency with or without a change in the number of phases or voltages4 ,a ,rotary ,phase ,converter changes alternating current of one or more phases into alternating current of a different number of phases1 but of the same frequency4 ,illustration3 ,,fig4 #b1jcd4--;,diagram of ring wound single phase rotary converter4 ,it is a combination of a synchronous motor and a dynamo4 ,the winding is connected to the commutators in the usual way1 and divided into two halves by leads connecting segments #ahj apart to collector rings4 ,a bipolar field is shown for simplicity2 in practice the field is multipolar and energized by direct current4 ,rotary ,converters4--,the synchronous or rotary converter consists of a synchronous motor and a direct current generator combined in one machine4 ,it resembles a direct current generator with an unusually large commutator and an auxiliary set of collector rings4 ,ques4 ,in general1 how does a rotary converter operate8 ,ans4 ,on the collector ring side it operates as a synchronous motor1 while on the commutator side1 as a dynamo4 ,its design in certain respects is a compromise between alternating current and direct current practice most noticeably with respect to the number of poles and speed4 ,ques4 ,upon what does the speed depend8 ,ans4 ,since the input side consists of a synchronous motor1 the speed is governed by the frequency of the alternating current supplied1 and the number of poles4 ,illustration3 ,,fig4 #b1jce4--;,diagram of two phase rotary converter4 ,this is identical with the single phase machine with the exception that another pair of collector rings are added1 and connected to points on the winding at right angles to the first1 giving four brushes on the alternating side for the two phase current4 ,the pressure will be the same for each phase as in the single phase rotary4 ,neglecting losses the current for each phase will be equal to the direct current #a _/ #b direct current 4#gjg4 ,fig4 #b1jcd is a diagram of a ring wound rotary converter4 ,this style winding is shown to simplify the explanation4 ,in practice drum wound armatures are used1 the operation1 however1 is the same4 ,with this simple machine the following principles can be demonstrated3 #a4 ,if the coil be rotated1 alternating currents can be taken from the collector rings and it is called an alternator4 #b4 ,by connecting up the wires from the commutator segments1 a direct current will flow in the external circuit making a dynamo4 #c4 ,two separate currents can be taken from the armature1 one supplying alternating current and the other direct current2 such a machine is called a double current generator4 #d4 ,if a direct current be sent in the armature coil through the commutator1 the coil will begin to rotate as in a motor and an alternating current can be taken out of the collector rings4 ,such an arrangement is called an inverted rotary converter4 #e4 ,if the machine be brought up to synchronous speed by external means and then supplied with alternating current at the collector rings1 then if the direction of the current through the armature coil and the pole piece have the proper magnetic relation1 the coil will continue to rotate in synchronism with the current4 ,a direct current can be taken from the commutator1 and when used thus1 the machine is called simply a rotary converter4 ,illustration3 ,,fig4 #b1jcf4--;,diagram of three phase rotary converter4 ,in this type1 the winding is tapped at three points #abj distant from each other1 and leads connected with the corresponding commutator segments4 ,illustration3 ,,figs4 #b1jcg to #b1jda4--,various rotary converter and transformer connections4 ,fig4 #b1jcg two phase connections2 fig4 #b1jch three phase delta connections2 fig4 #b1jci three phase ,y or star connections2 fig4 #b1jdj six phase delta connections2 fig4 #b1jda six phase ,y connections4 ,ques4 ,what is the relation between the impressed alternating pressure and the direct pressure at the commutator8 ,ans4 ,the ratio between the impressed alternating pressure and the direct current pressure given out is theoretically constant1 therefore1 the direct pressure will always be as #a to 4#gjg for single phase converters or if the pressure of the machine used above indicate #ajj volts at the direct current end1 it will indicate #gj4g volts at the alternating current side of the circuit4 ,ques4 ,name two different classes of converter4 ,ans4 ,single phase and polyphase4 ,ques4 ,what is the advantage of polyphase converters8 ,ans4 ,in the majority of cases two or three phase converters are used on account of economy of copper in the transmission line4 ,ques4 ,how is the armature of a polyphase converter connected8 ,ans4 ,similar to that of an alternator with either delta or ,y connections4 ,figs4 #b1jcg to #b1jda show various converter connections between the collector rings and commutator4 ,fig4 #b1jcg indicates how the armature is tapped for two phase connections4 ,fig4 #b1jch shows three phase delta connections1 and fig4 #b1jci the three phase ,y or star connections4 ,six phase delta and ,y connections are frequently used as shown in fig4 #b1jdj and fig4 #b1jda1 both of which require two secondary coils in the transformer1 one set of which is reversed1 so as to supply the current in the proper direction4 ,ques4 ,with respect to the wave1 what is the relation between the direct and alternating pressures8 ,ans4 ,the direct current voltage will be equal to the crest of the pressure wave while the alternating voltage will depend on the virtual value of the maximum voltage of the wave according to the connections employed4 ,table of ,alternating ,current and ,voltage in ,terms of ,direct ,current 7,according to ,steinmetz7 ---------------------------------------- -------------------------------- ,,direct ,,single ,,two ,,three ,,current ,,phase ,,phase ,,phase ---------------------------------------- -------------------------- ,,volts ,,between ,,collector ,,ring #a ,,and ,,neutral ,,point #a_/7bb74ceda_/7bb74ced #a_/7bb74ced ---------------------------------------- -------------------------- ,,volts ,,between ,,adjacent #a #a_/b4gjg 4#e #c_/7bb74fab ,,collector ,,rings ---------------------------------------- -------------------------- ,,amperes #a #ba4dad #a_/b4gjg 7#bb7_/c4idc ,,per ,,line ---------------------------------------- -------------------------- ,,amperes ,,between #a #ba4dad 4#e 7#bb7_/7cc74ede ,,adjacent ,,lines ---------------------------------------- -------------------------------- ---------------------------------------- --------------------- ,,six ,,twelve n ,,phase ,,phase ,,phase ---------------------------------------- ---------------- ,,volts ,,between ,,collector ,,ring ,,and ,,neutral ,,point#a_/7bb74ceda_/7bb74ceda_/7bb74ce d ---------------------------------------- ---------------- ,,volts ,,between ,,adjacent #a_/7bb74ced 4#ahc 7,,sin7_/n77_/#b ,,collector ,,rings ---------------------------------------- ---------------- ,,amperes #b_/c4dgb 4#bcf 7#bb7_/n ,,per ,,line ---------------------------------------- ---------------- ,,amperes ,,between #b_/c4dgb 4#dee 7#b,,sin7_/n77_/n ,,adjacent ,,lines ---------------------------------------- --------------------- ,in a single phase rotary1 the value of the direct pressure is #a to 4#gjg1 therefore a rotary which must supply #fjj volts direct current must be supplied by #fjj 4#gjg #dbd volts alternating current4 ,for three phase rotaries the ratio is #a to 4#fab1 or in order to produce #fjj volts direct current1 #fjj 4#fab #cfg volts on the alternating current side of the rotary is required4 ,illustration3 ,,fig4 #b1jdb4--,westinghouse rotary converter armature coils4 ,these are wound from bar copper and are interchangeable4 ,the armature coils are heavily insulated to withstand the tests specified in the standardization rules of the ,american ,institute of ,electrical ,engineers4 ,fig4 #b1jcd shows a complete diagram of the electrical connections4 ,a single phase rotary is illustrated so as to simplify the wiring4 ,the table of ,steinmetz on page #a1dfd gives the values of the alternating volts and amperes in units of direct current4 ,ques4 ,how is the voltage of a rotary varied on the direct current side8 ,ans4 ,pressure or potential regulators are put in the high tension alternating current circuit and may be regulated by small motors operated from the main switchboard or operated by hand4 ,ques4 ,what is the advantage of unity power factor for rotary converters8 ,ans4 ,it prevents overheating when the rotary is delivering its full load in watts4 ,ques4 ,what greatly influences the power factor of the high tension line8 ,ans4 ,the strength of the magnetic field4 ,illustration3 ,,fig4 #b1jdc4--,westinghouse rotary converter armature spider4 ,it is made of cast iron or cast steel4 ,the dovetail grooves are machined in the feet or ends of the arms and in these slots the laminations forming the armature coil engage4 ,ques4 ,does variation of the field strength materially affect the voltage8 ,ans4 ,no4 ,since variation of the field strength does not materially affect the voltage1 by adjusting the resistance in series with the magnetic circuit1 the strength of the field can be changed and the power factor kept #a or nearly #a as different loads are thrown on and off the rotary4 ,ques4 ,what is the effect of a field too strong or too weak8 ,ans4 ,if too strong1 a leading current is produced1 and if too weak1 the current lags1 both of which reduce the power factor and are objectionable4 ,usually there is a power factor meter connected up in the main generating station and one also in the rotary substation1 and it is the duty of the attendant at the substation to maintain the proper power factor4 ,ques4 ,what is the ordinary range of sizes of rotaries8 ,ans4 ,from #c kw4 to #c1jjj kw4 ,illustration3 ,,fig4 #b1jdd4--;,equalizer connections of ,westinghouse rotary converter4 ,the armature coils are cross connected at points of equal voltage and taps are led out from the winding at suitable points to the slip rings4 ,this construction insures a uniform armature saturation below each pole piece and eliminates one cause of sparking at the commutator4 ,ques4 ,what is the general construction of a rotary converter8 ,ans4 ,it is built similar to a dynamo with the addition of suitable collector rings connected to the armature windings at points having the proper phase relations4 ,standard rotary converters have been developed for #be and #fj cycles4 ,the standard railway machines are compound wound1 the series field being designed for a compounding of #fjj volts at no load and full load when supplied from a source of constant pressure with not more than #aj per cent4 resistance drop and with #bj to #cj per cent4 reactance in the circuit4 ,the large size machines are usually wound for six phase operation4 ,illustration3 ,,figs4 #b1jde and #b1jdf4--,westinghouse pole construction for converters4 ,fig4 #b1jde1 pole without windings2 fig #b1jdf1 pole with windings4 ,poles are built up of steel laminations held together with rivets4 ,projections on the inner ends of the poles form seats for the field coils and hold them in position4 ,copper dampers set in slots in the pole faces insure stable operation4 ,rotary converters for railway service are almost invariably compound wound4 ,the series windings are formed of bare copper strap4 ,the shunt windings are of insulated copper strap or wire4 ,spaces between coil turns and sections are provided for ventilation4 ,compounding of ,rotary ,converters4--,compounding is desirable where the load is variable1 such as is the case with interurban railway systems4 ,the purpose of the compounding is to compensate automatically for the drop due to line1 transformer1 and converter impedance4 ,on account of the low power factor caused by over compounding1 and the fact that substations are customarily connected to the trolley at its nearest point without feeder resistance1 over compounding is not recommended4 ,an adjustable shunt to the series field is provided with each machine4 ,shunt wound converters are satisfactory for substations in large cities and similar installations where due to the larger number of car units demanding power1 the load is more nearly constant4 ,ratio of ,conversion4--,the relation between the alternating and direct current voltages varies slightly in different machines1 due to differences in design4 ,the best operating conditions exist when the desired direct current voltage is obtained with unity power factor at the converter terminals when loaded4 ,illustration3 ,,fig4 #b1jdg4--,westinghouse rotary converter brush rigging showing method of bracing the brushes4 ,the brushes are supported by a rigid cast iron rocker ring which fits accurately in the frame4 ,a handwheel worm and screw arrangement for shifting the brushes is provided4 ,cast iron arms bolted to1 but insulated from the rings1 carry the rods on which the brush holders are mounted4 ,brush holders are of brass cast in one piece1 of the sliding type and have braided copper shunts4 ,brush tension is adjustable4 ,illustration3 ,,fig4 #b1jdh4--,westinghouse commutating pole rotary converter4 ,the construction details are substantially the same as for the railway converter1 with exception of the commutating poles4 ,the application of commutating pole converters is particularly desirable where special requirements such as great overload capacity or large capacity and low voltage enable them to show to the greatest advantage4 ,commutating poles as applied to rotary converters fulfill the same functions as in the more familiar applications to dynamos and motors4 ,that is1 the commutating pole insures sparkless commutation from no load to heavy overloads with a fixed brush position4 ,brush shifting devices are not furnished on commutating pole converters4 ,commutating pole rotary converters for railway service are normally arranged for automatic compounding which is effected by the proper combination of series excitation and inductance between the generator and the rotary converter4 ,this inductance is normally included in the transformer but in special cases may be partly in a transformer and partly in a separate reactance4 ,it is possible to produce by this means a slight increase in the direct current voltage provided the voltage drop in the alternating current line be not excessive4 ,usually it is so arranged that the compounding that can be obtained is just sufficient to overcome the alternating current line voltage drop4 ,the standard ,westinghouse method of starting is alternating current self-starting4 ,with this method of self-starting1 the brushes of a commutating pole rotary converter must be lifted from the commutator during the starting operation to prevent sparking4 ,a mechanical device1 as shown in fig4 #b1jej1 is provided which accomplishes this4 ,with direct current or motor starting a brush lifting device is not necessary4 ,ques4 ,upon what does the ratio of conversion depend8 ,ans4 ,upon the number of phases and method of connecting the windings4 ,for single phase or two phase machines it is #a to 4#g2 for three phase1 #a to 4#fab1 or six phase1 #a to 4#g or #a to 4#fac depending upon the kind of connection used for the transformer4 ,for example1 a two phase rotary receiving alternating current at #dbf volts will deliver direct current at #fjj volts1 while a three phase rotary receiving alternating current at #cfg volts will deliver direct current at #fjj volts4 ,illustration3 ,,fig4 #b1jdi4--;,commutating pole of ,westinghouse commutating pole rotary converter4 ,the commutating poles are similar in general construction to the main poles4 ,the coils are of bare copper strap wound on edge4 ,ventilating spaces are provided between the pole and coil and between turns4 ,the copper winding is bare except for a few turns at each end4 ,insulating bolts retain the turns in their proper position4 ,ques4 ,what difficulty would be encountered if other ratios of conversion than those given above were required8 ,ans4 ,an armature with a single winding could not be used4 ,it would be necessary to use a machine with two distinct armature windings or else a motor generator set4 ,ques4 ,what change in voltage is necessary between a converter and the alternator which furnishes the current8 ,ans4 ,the voltage must be reduced to the proper value by a step down transformer4 ,voltage ,regulation4--,as the ratio of the alternating to the direct current voltage of a converter is practically constant1 means must be provided to compensate for voltage variation due to changes of load in order to maintain the direct current pressure constant4 ,illustration3 ,,fig4 #b1jej4--,westinghouse brush lifting device for commutating pole rotary converter4 ,a rack is attached to each brush as shown4 ,into this rack the spring hinged lifting hook of the raising device engages only when the lifting lever is shifted toward the raised position4 ,the lifting arrangement is independent of the brushes during normal running1 so it can in no way affect the operation of the machine4 ,each brush is merely raised and lowered within its own holder so the brush position or commutation is not altered4 ,there are several methods of doing this1 as by3 #a4 ,shifting the brushes 7objectionable72 #b4 ,split pole method2 #c4 ,regulating pole method2 #d4 ,reactance method2 #e4 ,multi-tap transformer method2 #f4 ,synchronous regulator4 ,shifting the ,brushes4--,were it not for the difficulties encountered1 this would be a most convenient method of voltage regulation1 since by this procedure the direct current voltage may be varied from maximum to zero4 ,it is1 however1 not practical because of the excessive sparking produced when the brushes are shifted out of the neutral plane4 ,illustration3 ,,figs4 #b1jea to #b1jec4--,woodbridge split pole rotary converter4 ,each pole is split into three sections and provided with windings as indicated in fig4 #b1jea4 ,when excited as in fig4 #b1jeb1 the commutator voltage is at its highest value2 when excited as in fig4 #b1jec1 the commutator voltage is low4 ,the change in commutator voltage for constant collector ring voltage is in virtue of the property of rotary converters that the ratio of these two voltages is a function of the width of the pole arc4 ,split ,pole ,method4--,in order to overcome the difficulty encountered in shifting the brushes the split pole method was devised by ,woodbridge in which each field pole is split into two or three parts4 ,the effect of this is the same as shifting the brushes except that no sparking results4 ,the other part is arranged so that its excitation may be varied1 thus shifting the resultant plane of the field with respect to the direct current brushes4 ,one of these parts is permanently excited and it produces near its edge the fringe of field necessary for sparkless commutation4 ,regulating ,pole ,method4--,as applied to the rotary converter regulating poles fulfill the same functions as commutating or interpoles 7see page #che7 on motors and dynamos1 that is1 they insure sparkless commutation from no load to heavy overloads with a fixed brush position4 ,illustration3 ,,fig4 #b1jed4--;,general ,electric regulating pole rotary converter4 ,the field structure is divided into two parts1 a main pole and a regulating pole4 ,the ratio between the voltages on the direct current and alternating current sides may be readily varied by varying the excitation of the regulating poles1 the only auxiliary apparatus required being a field rheostat for controlling the exciting current4 ,where automatic regulation is required1 machines may be provided with compound windings1 or automatic field regulators may be used responsive to either voltage or current4 ,these converters are adapted for a variety of purposes where a variable conversion ratio is required1 either to maintain constant ,d4 ,c4 voltage with varying ,a4 ,c4 voltage or to vary the ,d4 ,c4 voltage as required4 ,converters may be operated inverted where it is required to furnish constant or variable ,a4 ,c4 voltage from a ,d4 ,c4 source4 ,where converter and inverted converter operation are desired1 an opposite direction of rotation is required for the inverted operation4 ,converters of this type are built in capacities from #cjj kw4 up to #c1jjj kw41 and constructed to give a voltage range between #bdj and #cjj volts1 to cover the usual lighting circuit requirements4 ,in design1 they are similar to standard rotary converters1 with the exception that the regulating poles are located next to the main pole pieces and a slightly different form of pole piece bridge is used for the main poles1 in order to allow the auxiliary poles to be readily removed or assembled4 ,the regulating poles are used in order to vary the ratio between the alternating current collector rings and the direct current side without the use of auxiliary apparatus such as induction regulators or dial switches which involve complicated connections and many additional wires4 ,the regulating poles are arranged with suitable connection so that the current through them can be raised1 lowered or reversed4 ,illustration3 ,,fig4 #b1jee4--;,detail of ,westinghouse commutating pole rotary converter brush1 showing rack4 ,the brush lifting mechanism and its operation is explained in fig4 #b1jej4 ,the characteristics of the regulating pole converter being novel1 a detailed explanation of the principles involved is given to facilitate a clear understanding of its operation4 ,consider a machine with a field structure as shown in fig4 #b1jef resembling in appearance a machine with commutating poles1 but with the brushes so set that one of the regulating poles adds its flux to that of one main pole1 cutting the inductors between two direct current brushes4 ,the regulating pole is shown with a width equal to #bj per cent4 of that of the main pole4 ,to obtain definite figures1 it will be assumed that the machine at normal speed1 with the main poles excited to normal density1 but with no excitation on the regulating poles1 gives #bej volts direct current pressure4 ,then with each regulating pole excited to the same density as the main poles1 and with a polarity corresponding to that of the main pole in the same section between brushes1 the direct current pressure will rise to #cjj volts at the same speed1 since the total flux cutting the inductors in one direction between brushes has been increased #bj per cent4 ,if1 on the other hand1 the excitation of the regulating poles be reversed and increased to the same density as that of the main poles1 the direct current pressure will fall to #bjj volts1 since in this case the regulating poles give a reverse pressure1 that is1 a pressure opposing that generated by the main poles4 ,illustration3 ,,fig4 #b1jef4--;,diagram of field of regulating pole converter illustrating principles explained in the accompanying text4 ,now1 if the machine be equipped with collector rings1 that is1 if it be a converter1 this method of varying the direct current voltage from #bjj to #cjj volts does not give nearly as great a variation of the alternating current voltage2 in fact1 the latter voltage will be the same when delivering #bjj volts as when delivering #cjj volts direct current pressure1 if the field excitation be the same4 ,this may be seen by reference to fig4 #b1jeg1 which is a diagram of the alternating current voltage developed in the armature windings by the two sets of poles4#aj #aj ,,note4--,in the ,burnham split pole rotary converter1 each pole is divided into only two sections1 one larger than the other4 ,a main shunt winding is arranged on the large sections1 and a winding for providing the voltage regulation is placed on the other section4 ,when the current is sent through this latter winding in one direction the voltage is raised1 when in the other direction the voltage is lowered4 ,the horizontal line ,,oa represents the alternating current voltage generated by the main poles1 alone1 with the regulating poles unexcited1 that is1 when delivering #bej volts direct current pressure4 ,for a six phase converter ,,oa measures about #ahj volts diametrically1 that is1 between electrically opposite collector rings4 ,if now the regulating poles be excited to full strength1 to bring the direct current pressure up to #cjj volts1 the alternating current voltage generated by the regulating poles will be #ij degrees out of phase with that generated by the main poles 7since they are placed midway between the main poles71 and will be about #dj volts as shown by the line ,,ab4 ,the resultant alternating current volts across the collector rings will be represented by the line ,,ob with a value equal to #ahd4 ,illustration3 ,,fig4 #b1jeg4--,voltage diagram for regulating pole converter illustrating principles explained in the accompanying text4 ,again1 if the regulating poles be reversed at full strength1 to cut the direct current pressure down to #bjj volts1 the alternating current voltage of the main and regulating poles will be ,,oa and ,,ac respectively1 giving the resultant ,,oc equal to ,,ob with a value of #ahd volts4 ,accordingly1 the direct current pressure may be either #bjj or #cjj volts with the same alternating current pressure1 and if the main field be kept constant1 the direct current pressure may range between #bjj or #cjj volts1 while the alternating current pressure varies only between #ahj and #ahd volts4 ,the alternating current pressure can be kept constant through the full range of direct current voltage by changing the main field so as always to give an equal and opposite flux change to that of the regulating field4 ,a constant total flux may thus be obtained equal to the radius of the arc ,,bc1 fig4 #b1jeg4 ,in this case the line ,,oa1 representing the main field strength1 will equal ,,ob when the regulating field is not excited1 and #bej volts can only be obtained at this adjustment4 ,this method of operation gives unity power factor with a constant impressed pressure of #ahd volts alternating current with a range of direct current voltage from #bjj to #cjj volts4 ,illustration3 ,,figs4 #b1jeh and #b1jei4--;,diagrams illustrating the effect on the alternating current voltage due to varying the regulating field strength 7of a machine proportioned according to fig4 #b1jfj71 from a density equal to that in the main poles to the same density reversed1 the main field strength remaining constant4 ,the ,d4 ,c4 voltage in this case varies from #cj per cent4 above that produced by the main field alone to #cj per cent4 below1 or from #cbe to #age volts1 while the ,a4 ,c4 voltage varies only from #bjj to #age volts4 ,to keep the ,a4 ,c4 voltage constant with such a machine the main field must be strengthened as the regulating field is weakened or reversed to reduce the ,d4 ,c4 voltage4 ,this strengthening increases the core loss particularly on low direct current voltages1 which however1 are rarely required1 hence a machine proportioned as in fig4 #b1jfj1 would not be operated through so wide a range as #age to #cbe volts4 ,assume that the range is #bdj to #cjj volts1 and that at the highest voltage1 both main and regulating fields have the same density1 presenting to the armature practically one continuous pole face of uniform flux intensity4 ,the diagram of ,a4 ,c4 component voltages to give constant ,a4 ,c4 resultant voltage across the rings for the case1 is shown in fig4 #b1jei4 ,at #cjj volts ,d4 ,c41 the main field produces an ,a4 ,c4 voltage ,,oa1 and the regulating field1 a voltage ,,ab1 with a resultant ,,ob1 equal to about #bjj volts ,a4 ,c4 ,at #bgj volts ,d4 ,c41 the main field produces an ,a4 ,c4 voltage ,,oa1 and a regulating field voltage ,,ab1 giving a resultant ,a4 ,c4 voltage ,,ob1 equal to #bjj volts4 ,similarly1 at #bdj volts ,d4 ,c41 the main field produces an ,a4 ,c4 voltage ,,oa1 and the regulating field 7now reversed7 produces the reverse voltage ,,ab1 giving the resultant ,,ob again equal to #bjj volts4 ,it will be noted that1 theoretically the main field strength must be increased about #ae per cent4 above its value at #cjj volts ,d4 ,c4 in order to keep the ,d4 ,c4 voltage at #bej volts4 ,ques4 ,where should the regulating poles be located for best results8 ,ans4 ,a better construction is obtained by placing them closer to the corresponding main pole1 as in fig4 #b1jfj1 than when spaced midway between the main poles as in fig4 #b1jef4 ,ques4 ,when the regulating poles are spaced as in fig4 #b1jfj1 what is the effect on the direct current voltage8 ,ans4 ,the effect is the same as for the midway position 7fig4 #b1jef7 except for magnetic leakage from the main poles to the regulating poles when the latter is opposed to the former1 that is1 when the direct current voltage is being depressed4 ,illustration3 ,,fig4 #b1jfj4--;,diagram illustrating placement of regulating poles4 ,in practice machines are not built as indicated diagrammatically in fig4 #b1jef1 that is1 with regulating poles spaced midway between the main poles1 because a better construction is obtained by placing the regulating pole closer to the corresponding main pole1 as shown above4 ,ques4 ,what is the effect on the alternating current voltage8 ,ans4 ,it is somewhat altered as explained in figs4 #b1jeh and #b1jei4 ,reactance ,method4--,this consists in inserting inductance in the supply circuit and running the load current through a few turns around the field cores4 ,this method is sometimes called compounding1 and as it is automatic it is generally used where there is a rapidly fluctuating load4 ,illustration3 ,,fig4 #b1jfa4--,westinghouse #cjj kw41 #a1ejj volt1 three phase1 #be cycle1 commutating pole rotary converter4 ,the illustration shows clearly the commutating1 and main poles and the relative sizes1 also arrangement of the terminal connections4 ,if a lagging current be passed through an inductance1 the collector ring voltage will be lowered1 but will be raised in case of a leading current4 ,the degree of excitation governs the change in the phase of the current to the converter1 the excitation1 in turn1 being regulated by the load current4 ,accordingly with series inductance1 the effect of the series coils on the field of the converter is quite similar to that of the compounding of the ordinary railway dynamo4 ,multi-tap ,transformer ,method4--,the employment of a variable ratio step down transformer for voltage regulation is a non-automatic method of control and1 accordingly1 is not desirable except in cases where the load is fairly constant over considerable periods of time4 ,it requires no special explanation4 ,illustration3 ,,fig4 #b1jfb4--,mechanical oscillator and speed limit device of ,westinghouse commutating pole rotary converter4 ,it automatically prevents the armature of the converter remaining in one position and thus not allowing brushes to wear grooves in both commutator and collector rings4 ,the oscillator is a self-contained device carried at one end of the shaft4 ,the operating parts consist of a hardened steel ball and a steel plate with a circular ball race1 backed by a spring4 ,the machine is so installed with a slight inclination toward the end carrying the oscillator4 ,as the armature revolves the ball is carried upward and owing to the convergence of the steel race and shaft face1 the spring is compressed4 ,the reaction of the spring forces the armature away from its natural position and allows the ball to drop back to the lowest point of the race4 ,synchronous ,booster ,method4--,this consists of combining with the converter a revolving armature alternator having the same number of poles4 ,ques4 ,how is the winding of the booster alternator armature connected8 ,ans4 ,it is connected in series with the input circuits on the converter4 ,illustration3 ,,fig4 #b1jfc4--,westinghouse #b1jjj kw41 #bgj volt1 direct current1 #f phase1 #afg ,r4,p4,m41 synchronous booster rotary converter1 having a voltage range from #bcj to #caj volts4 ,it consists of a standard rotary converter in combination with a revolving armature alternator mounted on the same shaft with the rotary converter and having the same number of poles4 ,by varying the field excitation of the alternator1 the alternating current voltage impressed on the rotary converter can be increased or decreased as desired4 ,the direct current voltage delivered by the converter is thereby varied accordingly4 ,the principle of operation of the booster converter is therefore very simple and easily understood4 ,it is simply a combination of two standard pieces of electrical apparatus1 accordingly there are incorporated in it no details of construction essentially different from those encountered in standard rotary converters and alternators4 ,the only novelty is in their combination4 ,the frames may be supported either from the rotary converter frame1 as in the small units1 or from the bed plate1 as in the larger ones4 ,a synchronous booster converter can be built1 if necessary1 with a vertical shaft to satisfy special floor space and head room requirements4 ,ques4 ,how are the field windings connected8 ,ans4 ,they are either fed with current regulated by means of a motor operated field circuit rheostat1 or joined in series with the commutator leads of the converter4 ,illustration3 ,,fig4 #b1jfd4--;,armature of ,westinghouse synchronous booster converter4 ,heavy cast yokes form the frames4 ,they are proportioned to rigidly support the laminated steel field poles4 ,the poles are fastened to the frame with through bolts4 ,a lifting hook is provided on all frames4 ,the bed plates are in one piece for the smaller machines but two piece bed plates are used for the larger ones4 ,the bearings are ring oiling and have babbitt wearing surfaces that are renewable4 ,illustration3 ,,fig4 #b1jfe4--,westinghouse field frames and bearings for synchronous booster rotary converter4 ,the frames consist of heavy cast yokes4 ,the poles are fastened to the frames with through bolts4 ,the bed plate is in one piece for the small machines and in two pieces for the large ones4 ,ques4 ,for what service is the synchronous booster method desirable8 ,ans4 ,for any application where a relatively wide variation in direct current voltage is necessary4 ,it is particularly desirable for serving incandescent lighting systems where considerable voltage variation is required for the compensation of drop in long feeders1 for operation in parallel with storage batteries and for electrolytic work where extreme variations in voltage are required by changes in the resistance of the electrolytic cells4 ,illustration3 ,,fig4 #b1jff4--;,general ,electric motor generator set consisting of #b1cjj volt synchronous motor and #eej volt dynamo4 ,motor ,generator ,sets4--,the ordinary rotary converter is the most economical machine for converting alternating currents into direct currents1 and where slight variations in the direct current voltage is necessary1 they are mostly used on account of their high efficiency1 and because they are compact4 ,illustration3 ,,fig4 #b1jfg4--;,general ,electric motor generator set consisting of #bcj volt induction motor and #abe volt dynamo4 ,in many central stations where they supply a great variety of apparatus1 the motor generator sets are employed as the generator is independent of the alternating current line voltage and any degree of voltage regulation can be performed4 ,motor ,generator ,combinations4--,the following combinations of motor generators are made and used to suit local conditions3 ,synchronous motor dynamo ,induction motor dynamo ,direct current motor dynamo ,direct current motor alternator ,synchronous motor alternator ,induction motor alternator ,illustration3 ,,fig4 #b1jfh4--;,general ,electric generator set1 as installed for the ,cleveland ,electric ,illuminating ,company1 ,cleveland1 ,ohio4 ,it consists of #aa1dca volt motor and #bge volt generator4 ,speed1 #cfj revolutions per minute4 ,standard practice has adopted high tension alternating current for transmission systems1 but direct current distribution is very frequently used4 ,this is particularly true where alternating current apparatus has been introduced in old direct current lighting systems4 ,the synchronous motor or the induction motor connected to a generator stands next in importance to the rotary converter because it is easy to operate and the pressure may be changed by a rheostat placed in the field circuit of the generator4 ,the line wires carrying full voltage can usually be connected direct to the motor and thus do away with the necessary step-down transformer required by the rotary4 ,ques4 ,what is the behavior of a rotary converter when hunting8 ,ans4 ,it is liable to flash over at the direct current brushes1 which is common in high frequency converters where there are a great number of poles and the brushes are necessarily spaced close together around the commutator4 ,ques4 ,is this fault so pronounced with motor generator sets8 ,ans4 ,the motor generator operating on a high frequency circuit1 the generator can be designed with a few poles and the brushes set far apart which will greatly reduce the chance of flashing over4 ,a synchronous motor will drive a generator at a constant speed during changes in load on it1 and by having a field regulating resistance it can be used to improve the power factor of the system4 ,when an induction motor is used its speed drops off slowly as the load comes on the generator1 and it is necessary to regulate the voltage of the generator by means of a field rheostat1 or compound wound machines may be used4 ,while an induction motor requires no separate excitation of the field magnets like the synchronous motor1 its effect on the power factor of the system is undesirable4 ,although it is seldom necessary to convert direct current to alternating1 such an arrangement of a direct current motor driving an alternator is often justified in place of an inverted rotary converter1 as in this case the alternating current voltage can be changed independent of the direct current voltage4 ,the racing of an inverted rotary under a heavy inductive load or short circuit does not take place in motor generator set mentioned above4 ,illustration3 ,,fig4 #b1jfi4--;,general ,electric frequency changer set1 consisting of a #aa1jjj volt synchronous motor with direct connected exciter and a #b1cjj volt alternator4 ,frequency ,changing ,sets4--,a frequency of #be cycles is generally used on railway work and in large cities using the ,edison three wire system1 and as a #be cycle current is not desirable for electric lighting it is necessary to change it to #fj cycles by means of a frequency changer shown in fig4 #b1jfi for distribution in the outlying districts4 ,the two machines in this combination are of the same construction1 only the synchronous motor would have eight poles and have the #be cycle current passing through it1 while the generator would have #bj poles and produce #fb cycles per second at #cjj revolutions per minute4 ,by supplying the motor with #bd cycles1 the generator would produce #fj cycles4 ,illustration3 ,,fig4 #b1jgj4--;,general ,electric four unit frequency changer set consisting of a #aa1jjj volt synchronous motor1 #ac1bjj volt alternator1 #bej volt exciter1 and #ddj volt starting induction motor4 ,where parallel operation is required between synchronous motor driven frequency changers1 a mechanical adjustment is necessary between the fields or armatures of the alternator and motor to obtain equal division of the load4 ,the adjustment can be obtained by shifting the keyway1 or by special cradle construction4 ,in the latter method1 one machine is bolted to a cradle fastened to the base4 ,by taking out the bolts1 the frame can be turned around through a small angle relative to the cradle and therefore to the armature frame of the other machine1 when the bolts can be replaced4 ,it will be seen from the figure that the separate exciter is fastened on the base plate and has its armature directly connected to the shaft4 ,parallel ,operation of ,frequency ,changers4--,it is very difficult to construct two or more frequency changers and join them to synchronous motors so that the current wave of one machine will be in phase with the other1 since the speed of the motor will depend on the frequency of the line and be independent of the load thrown on it4 ,when alternators are run in parallel1 if one machine lag behind1 the other carries the load with the result that the lightly loaded machine will speed up and get in step with the other1 or in other words a synchronizing current will flow between the two alternators and tend to keep them in proper relation with respect to phase and load4 ,illustration3 ,,fig4 #b1jga4--;,diagram of ,cascade motor generator set or motor converter1 as it is called in ,england where it is used extensively for electric railway work4 ,in the diagram of motor armature winding1 some of the connections are omitted for simplicity4 ,the windings are ,y connected1 and as they are fed by wires joined to the slip rings at the right and center1 the rest of the power passes to the converter windings back to rotor winding and out to the slip rings so that part of the power enters the rotor and part through the converter4 ,cascade ,converter4--,this piece of apparatus was introduced by ,arnold and ,la ,cour4 ,briefly1 it consists of a combination of an induction motor having a wound armature and a dynamo1 the armatures being placed on the same shaft4 ,the windings are joined in cascade1 that is1 in series with those of the armature of the induction motor4 ,the line supplies three phase currents at high voltage direct to the field of the induction motor and drives it1 generating in it currents at a lower voltage depending on the ratio of the windings4 ,illustration3 ,,fig4 #b1jgb4--;,general ,electric shunt wound booster set4 ,sets of this class are used in railway stations to raise the pressure of the feeders extending to distant points of the system1 for storage battery charging and regulation1 and in connection with the ,edison three wire lighting system4 ,the design of the various sets is closely dependent upon their application4 ,booster sets are constructed in either series or shunt wound types and they may be arranged for either automatic or hand regulation1 depending on the nature of the service required4 ,where there are a number of lighting feeders connected and run at full load for only a short time each day it will generally be economical to install boosters rather than to invest in additional feeder copper4 ,it is important1 however1 to consider each case where the question of installing a booster arises1 as a separate problem1 and to determine if the value of the power lost represents an amount lower than the interest charge on the extra copper necessary to deliver the same voltage without the use of a booster4 ,part of the current thus generated in the armature passes into the armature of the dynamo and is converted by the commutator into direct current as in a rotary converter1 but is also increased by the current induced in the winding of the dynamo armature4 ,illustration3 ,,fig4 #b1jgc4--;,general ,electric #i4e kw4 #a1jjj ,r4,p4,m4 balancer set4 ,this is a form of direct current compensator1 the units of which it is composed may be alternately motor or generator1 and the secondary circuit is interconnected with the primary4 ,balancer sets are widely used to provide the neutral of ,edison three wire lighting systems4 ,they are also installed for power service in connection with the use of #bej volt motors on a #ejj volt service or #abe volt motors on a #bej volt service4 ,although the balancer set may be manufactured for more than one intermediate voltage1 the ,general ,electric ,company recommends the three wire system as it is simpler4 ,the motors may be more equally divided and the reserve margin of the motor is more conservative for that class of work requiring increase in torque with decrease in speed4 ,the voltage of the system being given1 the capacity of a three wire balancer set is fixed by the maximum current the neutral wire is required to carry4 ,this figure is a more definite specification of capacity than a statement in per cent4 of unbalanced load4 ,as designed for power work and generally for lighting service1 the brushes of each machine are set at the neutral point in order to get the best results for operating alternately either as a generator or motor4 ,where the changes of balance are so gradual as to permit of hand adjustment if desired1 a considerable increase in output is obtainable4 ,ques4 ,at what speed does the machine run8 ,ans4 ,assuming equal numbers of pole1 the armatures rotate at a speed corresponding to one half the circuit frequency4 ,illustration3 ,,figs4 #b1jgd and #b1jge4--;,general ,electric charging sets4 ,fig4 #b1jgd set consists of dynamo and direct current motor2 fig4 #b1jge set consists of a dynamo and alternating current motor4 ,fig4 #b1jgd set is equipped with #abe volt1 shunt wound dynamo and #bcj or #eej volt motor and range in capacity from 4#abe kw4 to #ac kw41 the speed varying from #b1bej ,r4,p4,m4 in the 4#abe kw4 set to #ibe ,r4,p4,m4 in the #ac kw4 set with #bcj volt motor4 ,both motor and dynamo have the same type and size of frame2 these are bolted together and form a compact and symmetrical outfit1 no base being necessary4 ,sets of the type shown in fig4 #b1jge range in capacity from 4#b kw4 to #aj kw4 and are equipped with #abe volt shunt wound dynamo and #aaj1 #bbj1 #ddj or #eej volt two or three phase motors4 ,if desired they can be furnished with single phase motors wound for #aaj or #bbj volts4 ,the speed of this type is #a1hjj ,r4,p4,m4 ,when a motor generator set is used to charge only one battery1 the insertion of a resistance between the charging dynamo and the battery is not necessary1 inasmuch as all adjustments of voltage can be made by varying the field strength of the dynamo1 and1 therefore1 there are no large losses due to resistance since the loss in the dynamo field rheostat is very small4 ,when a motor generator set is used to charge two or more batteries of different capacities1 or voltages1 or which are in different conditions of charge1 it is necessary to insert a resistance in series with each battery1 in order that the current may be properly adjusted for each particular battery4 ,thus if the motor have six poles and the frequency be #ej1 the rotary field revolves at #ej #fj #c #a1jjj ,r4,p4,m4 and the motor will revolve at one-half that speed or #a1jjj #b #ejj ,r4,p4,m4 ,since the connections are so arranged that these currents tend to set up in the armature a revolving field1 rotating at half speed in a sense opposite to that in which the shaft is rotating at half speed1 it follows that by the super-position of this revolving field upon the revolutions of the machine1 the magnetic effect is equivalent to a rotation of the armature at whole speed1 so that it operates in synchronism1 as does the armature of a rotary converter4 ,half the electric input into the motor part is1 therefore1 turned into mechanical energy to drive the shaft1 the other half acts inductively on the armature winding1 generating currents therein4 ,as to the dynamo part it is half generator1 receiving mechanical power by transmission along the shaft to furnish half its output1 and it is half converter1 turning the currents received from the armature into direct current delivered at the brushes4 ,ques4 ,what action takes place in the motor armature winding8 ,ans4 ,since it runs at one-half synchronous speed1 it generates alternating current of half the supply current frequency1 delivering these to the armature of the dynamo4 ,ques4 ,what claim is made for this type of apparatus8 ,ans4 ,the cost is said to be less than a motor generator set1 and it is claimed to be self-synchronizing and to require no special starting gear1 also to be #b4e per cent4 more efficient than a motor generator4 ,ques4 ,how is the machine started from the high pressure side8 ,ans4 ,the field winding is connected directly to the high pressure leads4 ,the three slip ring brushes are connected with external resistances which are used while starting1 the external resistances being gradually cut out of the circuit as the machine comes up to speed 7the same as with an ordinary slip ring motor74 ,ques4 ,how does a cascade converter compare with a synchronous converter8 ,ans4 ,it is about equally expensive as the synchronous converter with its necessary bank of transformers1 but is about one per cent4 less efficient4 ,it is claimed to be more desirable for frequencies above #dj on account of the improved commutation at the low frequency used in the dynamo member4 ,for lower frequencies the synchronous converter is preferable4 ,,chapter ,,liv ,,rectifiers ,the purpose of a rectifier is to change alternating current into a uni-directional or pulsating current4 ,there are several classes of apparatus to which the term rectifier may be applied1 as #a4 ,mechanical rectifiers2 #b4 ,electrolytic rectifiers2 #c4 ,mercury vapor rectifiers1 or1 mercury arc rectifiers2 #d4 ,electro-magnetic rectifiers4 ,mechanical ,rectifiers4--,by definition1 a mechanical rectifier is a form of commutator operating in synchronism with the generator and commutating or rectifying the negative waves of the alternating current as shown graphically in figs4 #b1jgf and #b1jgh4 ,the essential features of construction are shown in fig4 #b1jgi4 ,ques4 ,mention some application of a mechanical rectifier4 ,ans4 ,it is used on a compositely excited alternator as illustrated on page #a1aib4 ,electrolytic ,rectifiers4--,if two metals be placed in an electrolyte and then subjected to a definite difference of pressure1 they will 7under certain conditions7 offer greater resistance to the passage of a current in one direction1 than in the other direction4 ,on account of this so called valve effect1 electrolytic rectifiers are sometimes called valves4 ,ques4 ,what metal is generally used for the cathode8 ,ans4 ,aluminum4 ,illustration3 ,,figs4 #b1jgf to #b1jgh4--;,diagrams showing alternating currents1 and partial and complete rectification4 ,ques4 ,what is generally used for the other electrode8 ,ans4 ,lead or polished steel4 ,metals of low atomic weight exhibit the valve effect at high differences of pressure1 and heavier metals at low differences of pressure4 ,ques4 ,describe the ,nodon valve4 ,ans4 ,the cathode is of aluminum or aluminum alloy1 and the other electrode1 which has considerably more surface1 is the containing vessel4 ,the electrolyte is a neutral solution of ammonia phosphate4 ,ques4 ,describe its action4 ,ans4 ,it is due to the formation of a film of normal hydroxide of aluminum1 over the surface of the aluminum electrode4 ,this film presents a very high resistance to the current when flowing in one direction but very little resistance1 when flowing in the reverse direction4 ,illustration3 ,,fig4 #b1jgi4--,mechanical rectifier4 ,the rectifier consists of two castings ,m and ,s with teeth which fit together as shown1 being insulated so they do not come in contact with each other4 ,every alternate tooth1 being of the same casting1 is connected together1 the same as though joined by a conducting wire4 ,there are as many teeth as there are poles4 ,the part ,m of the rectifier is connected to one of the collector rings by ,f1 and the part ,s to the other ring by ,g4 ,ques4 ,what is the effect when a ,nodon cell is supplied with alternating current8 ,ans4 ,half of the wave will be suppressed and an intermittently pulsating current will result as shown in fig4 #b1jgg4 ,illustration3 ,,figs4 #b1jhj and #b1jha4--,two views of ,nodon valve4 ,this is an electrolytic rectifier in which the cathode is a rod of aluminum alloy held centrally in a leaden vessel which forms the anode and contains the electrolyte1 a concentrated solution of ammonium phosphate4 ,only a short portion at the lower end of the cathode is utilized1 the rest1 which is rather smaller in diameter1 being protected from action by an enclosing glass sleeve4 ,the current density at the cathode ranges from #e to #aj amp4 per sq4 dm4 ,in the larger sizes1 the cells are made double1 and a current of air is kept circulating between the walls by means of a motor driven fan4 ,in order to utilize both halves of the supply wave1 the ,gratz method of connection is adopted4 ,the maximum efficiency is obtained at about #adj volts1 and the efficiency lies between #fe and #ge per cent41 and is practically independent of the frequency between the limits of #be and #bjj 4 ,above a pressure of #adj volts1 the efficiency falls off very rapidly1 owing to breakdown of the film4 ,the pressure difference is high1 being over #ij per cent4 at full load4 ,temperature largely influences the action of the valve1 and should never exceed #abb ,fahr4 ,illustration3 ,,fig4 #b1jhb4--,oscillograph record from ,nodon valve showing original supply voltage and the corresponding pulsating current at the terminals of such a valve4 ,ques4 ,how may both halves of the alternating waves be utilized8 ,ans4 ,by coupling a series of cells in opposed pairs as in fig4 #b1jhj4 ,illustration3 ,,fig4 #b1jhc4--,performance curves of five ampere ,nodon valve4 ,constant secondary voltage test4 ,loaded on non-inductive resistances4 ,frequency #ej4 ,maximum power factor on valve 4#g4 ,ques4 ,upon what does the efficiency of the film depend8 ,ans4 ,upon the temperature4 ,it should not for maximum efficiency exceed #hf degrees ,fahr4 ,there is also a certain critical voltage above which the film breaks down locally1 giving rise to a luminous and somewhat disruptive discharge accompanied by a rapid rise of temperature and fall in efficiency4 ,illustration3 ,,fig4 #b1jhd4--,mohawk electrolytic rectifier and switchboard2 diagram showing connections for charging storage battery4 ,operating instructions3 ,after assembling battery as in fig4 #b1jhe1 the film must be formed on the aluminum alloy electrodes so that the rectifier will pass current only in the right direction4 ,open switch ,b1 close switch ,t to the right2 discharge lever can be in any position2 charging regulator lever must be to the extreme left1 the zero position2 now close main switch ,m4 ,moving regulator lever ,r from the zero position to the first button or contact1 let it remain there for a time1 not less than five minutes2 this is important1 as the proper rectification of the current depends on the film formed on the aluminum rods4 ,the ammeter after the first rush of current may not show any current as passing1 or it may show a reverse current4 ,in the latter case1 leave the contact finger on the first button until the needle comes back to zero4 ,this may take some time1 but the needle will eventually come back2 it also indicates that the film is properly formed when the needle returns to zero4 ,move regulator ,r to the extreme right step by step and note that the ammeter continues to return to zero1 which indicates that the film on rectifier electrodes is formed properly4 ,move regulator ,r to zero1 close switch ,t to the left in normal charging position4 ,close charging switch ,b4 ,to regulate the flow of current through the battery move charging lever ,r to the right slowly until ammeter indicates the correct charging current4 ,after the batteries are charged and ready for use1 discharge lever can be moved to connect either set of storage batteries to the load terminal4 ,the voltage of the batteries can be read at any time1 by pressing the strap key4 ,the discharge lever connects the batteries to the volt meter and it is possible by moving it to measure the voltage of either set of battery1 charging or discharging4 ,trouble in the rectifier demonstrates itself by the solution becoming heated4 ,the condition of the rectifier can be tested any time in a few seconds by opening switch ,b and closing switch ,t to the right4 ,if the rectifier be in proper condition the ammeter will read zero4 ,and if it be not rectifying and permitting ,a4 ,c4 current to flow through the rectifier1 the ammeter will read negative or to the left of the zero4 ,an old solution that is heating and not rectifying properly will turn a reddish brown color4 ,ques4 ,when an electrolytic rectifier is not in use for some time what happens8 ,ans4 ,the electrodes will loose the film4 ,illustration3 ,,figs4 #b1jhe and #b1jhf4--,mohawk electrolytic rectifier4 ,to put in commission1 clean out the jar4 ,fill with distilled or rain water4 ,add six pounds of electro salts1 stir and after all salts are dissolved place the cover in position4 ,the specific gravity of the solution should be #a4abe4 ,the middle iron electrode must hang straight down in the solution and not touch either of the other aluminum alloy electrodes4 ,the aluminum alloy electrodes are mounted on an insulated bracket that slides up and down on a rod4 ,this rod screws in the hole taped in the middle of the cover4 ,the electrodes give the best results only when perfectly smooth4 ,should they get rough1 covered with a deposit or a white coating remove from the solution1 and clean with fine sand paper4 ,finish with fine sand paper4 ,form the film again and the electrodes will be as good as new4 ,clean iron electrode occasionally4 ,ques4 ,what must be done in such case8 ,ans4 ,the electrodes must be reformed4 ,ques4 ,how is the loss of film prevented8 ,ans4 ,by ,removing the electrodes from the electrolyte and drying them4 ,illustration3 ,,fig4 #b1jhg4--,the ,fleming oscillation valve4 ,it depends for its action on the well-known ,edison effect in glow lamps4 ,the valve consists of a carbon filament glow lamp with a simple central horseshoe filament4 ,around this filament inside the exhausted bulb is fixed a small cylinder of nickel1 which is connected by means of a platinum wire sealed through the bulb to a third terminal4 ,the valve is used as follows3 ,the carbon loop is made incandescent by a suitable battery4 ,the circuits in which the oscillations are to be detected is joined in series with a sensitive mirror galvanometer1 the nickel cylinder terminal and the negative terminal of the filament of the valve being used4 ,the galvanometer will then be traversed by a series of rapid discharges all in the same direction1 those in the opposite direction being entirely suppressed4 ,ques4 ,what attention must be given to the electrolyte8 ,ans4 ,water must be added from time to time to make up for evaporation4 ,this is necessary to keep the solution at the proper density4 ,ques4 ,what is the indication that the rectifier needs recharging8 ,ans4 ,excessive heating of the solution with normal load4 ,ques4 ,what is the indication that a rectifier is passing alternating current8 ,ans4 ,it will heat1 and if the solution be very weak1 it will cause a buzzing sound4 ,illustration3 ,,fig4 #b1jhh4--,the ,churcher valve4 ,this is of the modified ,nodon type4 ,it differs from the latter in that it has two cathodes of aluminum and an anode of lead or platinum1 suspended in the one cell4 ,this permits the complete utilization of both halves of the supply wave with one cell instead of the four required in the ,gratz method4 ,the connections of such a cell are shown in the figure4 ,the secondary of the transformer carries a central tapping1 and is connected through the direct current load to the central anode1 while each of the cathodes is connected to the ordinary terminals of the transformer itself4 ,the practical limits of the cell are #ej volts direct current1 or #acj volts at the transformer terminals ,,ab4 ,f1 is the anode2 ,c1 cathode ,i2 ,d1 cathode ,,ii4 ,ques4 ,what harm is caused by operating a rectifier with a weak electrolyte8 ,ans4 ,the electrodes will eat away4 ,a few of the so called electrolytic valves are here briefly described3 ,the ,audion ,valve4--,this valve was invented by ,de ,forest in #aijj and is practically identical with the ,fleming oscillation valve1 the latter being illustrated in fig4 #b1jhf4 ,grisson ,valve4--,in this valve the cathode is a sheet of aluminum1 and the anode1 a sheet of lead1 supported1 in the original form1 horizontally in a vessel containing the electrolyte1 consisting of a solution of sodium carbonate4 ,cooling is effected by circulating water through metal tubes in the electrolyte itself4 ,illustration3 ,,figs4 #b1jhi and #b1jij4--,the ,de ,faria valve4 ,this is an aluminum lead rectifier4 ,the cathode is a hollow cylinder of aluminum placed concentrically in a larger cylinder of lead1 and the whole immersed in electrolyte of sodium phosphate in an ebonite containing vessel4 ,cooling is effected by promoting automatic circulation of the electrolyte by providing the lead cylinder with holes near its extremities2 the heated electrolyte then rises in the lead cylinder1 passes out at the upper holes1 is cooled by contact with the walls of the containing vessel1 and descends outside the lead cylinder4 ,it is claimed that this cooling action is sufficient to allow of a current density of #h amp4 per sq4 dm4 of aluminum4 ,pawlowski ,valve4--,this is an electrolytic valve employing a solid electrolyte4 ,it consists of a copper plate which has been coated with a crystalline layer of carefully prepared copper hemisulphide1 prepared by melting sulphur and copper together out of contact with air4 ,the prepared plate is placed in contact with an aluminum sheet and the combination is then formed by submitting it to an alternating pressure until sparking1 which at first occurs1 ceases4 ,giles ,electric ,valve4--,this consists of a combination of spark gaps and capacity used to protect electrical apparatus against damage due to atmospheric discharges and resonance surges4 ,the spark gaps are formed between the edges of sharp rimmed discs of non-arcing metal4 ,these discs are insulated from each other1 and from the central tube1 which provides a support for the apparatus and also an earth4 ,the condenser effect is obtained by means of the annular discs and the tube2 an adjustable spark gap1 a high resistance1 and a fuse all connected in series1 complete the valve4 ,illustration3 ,,fig4 #b1jia4--ge light ,westinghouse-,cooper ,hewitt mercury vapor rectifier constant current regulating transformer4 ,view showing assembly in case4 ,buttner ,valve4--,it is of the ,nodon type employing a cathode of magnesium-aluminum alloy1 and probably iron or lead as anode1 with an electrolyte of ammonium borate4 ,buttner claims that the borate is superior to the phosphate in that it does not attack iron1 and will keep in good working condition for longer periods4 ,illustration3 ,,fig4 #b1jib4--ge light ,westinghouse-,cooper ,hewitt mercury vapor rectifier constant current regulating transformer with case removed4 ,the transformer is of the repulsion coil type1 oil cooled and oil insulated4 ,it is so arranged as to give a constant secondary current and to insulate the arc lines from the primary circuit4 ,the regulating transformer contains two stationary secondary coils and two moving primary coils balanced against each other4 ,each secondary coil of the #ge light regulator is wound in two parts1 owing to the use of two rectifier bulbs in series in outfits of this capacity4 ,the repulsion between the primary and secondary coils changes the distance between them according to the variation of load1 and the induced current in the secondary is thus kept constant4 ,an increase in current causes the primary and secondary coils to separate1 and a decrease in current permits them to approach each other1 until the normal balance is restored4 ,the moving coils are hung from sheave wheels having roller bearings and are balanced so that they are sensitive to the slightest impulse tending to separate them or draw them closer together4 7,see figs4 #a1iha-b1 and #b1aaa47 ,the windings are insulated for a voltage considerably in excess of that existing in normal service4 ,several taps are provided to take care of different voltages and wave forms4 ,a combination of taps will be found which will be suitable for any wave form coming within the ,american ,institute of ,electrical ,engineer's limits for a sine wave4 ,the secondary coils are also provided with taps for #he per cent4 of normal load1 so that less than normal load can be taken care of at a good power factor4 ,any part of the full load can be carried temporarily with the full load connections of the transformer1 but at permanent light loads the power factor and efficiency will be improved by using the #he per cent4 connections4 ,standard regulating transformers are wound for #f4f and #d amperes1 and for primary circuits of #bbj1 #ddj1 #a1ajj #b1bjj1 #f1fjj and #ac1bjj volts4 ,regulators can be specially wound for #e4e amperes4 ,for three phase circuits three regulators can be used1 one on each phase1 or they can be furnished in pairs with an auxiliary auto-transformer to give a balanced load4 ,the regulators can be connected1 in cases where the unbalancing is not objectionable1 to separate phases4 ,mercury ,vapor ,rectifiers4--,the ,cooper ,hewitt mercury vapor rectifier1 as shown in fig4 #b1jic consists essentially of a hermetically sealed glass bulb filled with mercury vapor and provided with four electrodes4 ,the two upper electrodes are of solid material and the two lower of mercury4 ,the solid electrodes are the positive electrodes2 the mercury electrodes are the negative electrodes4 ,the mercury pools of the two lower electrodes are not in contact when the bulb is vertical1 but the bulb is so mounted that it can be tilted to bring these two pools temporarily in contact for starting4 ,illustration3 ,,fig4 #b1jic4--;,diagram of connections of ,westinghouse-,cooper ,hewitt mercury vapor rectifier arc light circuit4 ,the bulb contains highly attenuated vapor of mercury1 which1 like other metal vapors1 is an electrical conductor under some conditions4 ,the positive electrodes are surrounded by this vapor4 ,current can readily pass from either of the solid electrodes to the mercury vapor and from it to the mercury electrode1 but when the direction of flow tends to reverse1 so that current would pass from the vapor to the solid electrode1 there is a resistance at the surface of the electrode1 which entirely prevents the flow of current4 ,illustration3 ,,fig4 #b1jid4--;,cooper ,hewitt mercury vapor rectifier4 ,the mercury vapor rectifier as developed by ,peter ,cooper ,hewitt for changing alternating current into direct current is the result of a series of careful experiments and investigations of the action going on in his mercury vapor lamp for electric lighting used on direct current circuits only4 ,while many attempts have been made to produce an alternating current lamp1 up to the present time1 they have been unsuccessful4 ,the difficulty of operating a lamp on the alternating current circuit lies in the fact that while a current will flow freely through it in one direction1 when the current reverses the negative electrode or cathode acts as an electric valve and stops the current1 thus breaking the circuit and putting out the light4 ,by following up this new electrical action1 ,hewitt applied the principle in the construction of a vacuum tube with suitable electrodes1 and by using two electrodes of iron or graphite for the positive or incoming current and one of mercury for the negative or where the current leaves the tube1 the circuits could be arranged so that a direct current would flow from the mercury electrode and be used for charging storage batteries1 electro-chemical work or operating direct current flame arc lamps4 ,as shown in the figure1 the rectifier consists essentially of a glass bulb into which are sealed two iron or graphite anodes and one mercury cathode1 and a small starting electrode4 ,the bulb is filled with mercury vapor under low pressure4 ,the action of this device depends on the property of ionized mercury vapor of conducting electricity in one direction only4 ,in operation no current will flow until the starting or negative electrode resistance has been overcome by the ionization of the vapor in its neighborhood4 ,to accomplish this1 the voltage is raised sufficiently to cause the current to jump the gap between the mercury cathode and the starting cathode1 or by bringing the cathode and starting electrode together in the vapor by tilting and then separating them1 thus drawing out the arc4 ,when this has been done1 current will only flow from the anode to the mercury cathode1 and not in the reverse direction4 ,in order to maintain the action1 a lag is produced in each half wave by the use of a reactive or sustaining coil2 hence the current never reaches its zero value1 otherwise the arc would have to be restarted4 ,there are two kinds of losses in the tube3 #a1 arcing1 or leakage from one anode to the other1 and #b1 the mercury arc voltage drop4 ,this drop does not depend on the load1 the energy represented by the drop being converted into heat1 which is dissipated at the surface of the containing vessel4 ,according to ,steinmetz1 the limit of voltage must be very high1 as #cf1jjj volts has been rectified4 ,the current output is limited principally by the leading-in wires to the electrodes1 it being a difficult problem to seal into the glass container the large masses of metal required for the conduction of large currents4 ,frequency has but little influence4 ,the direct current voltage ranges from #bj to #ej per cent4 that of the arc supply4 ,the life of the valve depends somewhat upon its size1 being longer in the small sizes and never1 with fair usage1 less than #a1jjj hours4 ,illustration3 ,,fig4 #b1jie4--,three phase mercury arc rectifier4 ,the rectifier bulb is provided with three positive electrodes or anodes1 a negative electrode or cathode1 and a starting anode1 as shown4 ,the three phase leads are connected to the anodes at the top of the bulb1 a branch from one phase being brought down to the starting anode1 a resistance being placed in the circuit to prevent excessive current on account of the proximity of the two lower electrodes4 ,since there is always a pressure on one of the three anodes in the right direction1 a reactance coil is not necessary4 ,the apparatus is started in the usual way by tilting4 ,the alternating current supply circuit is connected to the two positive electrodes as shown in the diagram1 and as the electrodes will allow current to flow in only one direction and oppose any current flow in the opposite direction1 the pulsations of the current pass alternately from one or the other of the positive electrodes into the mercury4 ,as these currents cannot pass from the vapor into either positive electrode1 they are constrained to pass out all in one direction through the mercury electrode1 from which they emerge as a uni-directional current4 ,the positive electrodes of the rectifier thus act as check valves1 permitting current to pass into the mercury vapor but not allowing it to pass from the vapor to the solid electrodes4 ,illustration3 ,,figs4 #b1jif to #b1jih4--,westinghouse diagrams showing comparative efficiencies of different systems of series arc lighting4 ,ques4 ,what condition prevails before the bulb starts to rectify8 ,ans4 ,there appears to be a high resistance at the surface ,illustration3 ,,fig4 #b1jii4--,westinghouse-,cooper ,hewitt mercury vapor rectifier bulb4 ,it consists essentially of a hermetically sealed glass bulb filled with highly attenuated vapor of mercury1 and provided with electrodes4 ,its operation is fully explained in the accompanying text4 ,illustration3 ,,fig4 #b1ajj4--,westinghouse-,cooper ,hewitt mercury vapor rectifier bulb and box4 ,the life of the bulb is materially increased by operating it at certain temperatures and for this reason the bulbs in the arc light rectifier outfits are immersed in oil and mounted in the same tank with the regulating transformer4 ,two bulbs in series are used with the #ge light outfit4 ,the bulb is mounted on tilting trunnions in a box which can be lifted out through a door in the top of the tank without disconnecting any leads4 ,the containing box has contacts on the bottom so arranged that when it is lowered into place1 the bulb is automatically connected in circuit4 ,to replace a defective bulb it is only necessary to lift out the containing box by its handle through the door in the top of the case and mount a new bulb in it1 after which the box can be lowered into place4 ,it is desirable to have at each installation1 a spare bulb box in which a bulb can be kept1 connected ready for use4 ,if this be done1 it is only necessary1 in case of trouble with the bulb1 to withdraw the old bulb and box and replace them with the spare set4 ,this avoids having the lamps out of service4 of the mercury1 which must be broken down so that the current can pass4 ,ques4 ,what is this apparent surface resistance called8 ,ans4 ,the negative electrode resistance4 ,ques4 ,what must be done before any current can pass8 ,ans4 ,the negative electrode resistance must be overcome4 ,when once started the current will continue to flow1 meeting with practically no resistance as long as the current is uninterrupted4 ,ques4 ,what will happen if the current be interrupted even for the smallest instant of time8 ,ans4 ,the negative electrode resistance will re-establish itself1 and stop the operation of the bulb4 ,ques4 ,how is the negative electrode resistance overcome8 ,ans4 ,the bulb is tilted or shaken so that the space between the mercury electrodes is bridged by the mercury4 ,ques4 ,what happens when the bulb is tilted8 ,ans4 ,current then passes between the two mercury electrodes from the starting transformer and the little stream of mercury which bridges the space between the electrodes breaks with a spark as the bulb is returned to its vertical position4 ,ques4 ,what duty is performed by the spark8 ,ans4 ,it breaks down the negative electrode resistance4 ,ques4 ,what conditions are now necessary for continuous operation of the rectifier8 ,ans4 ,the rectifier will now operate indefinitely as long as the current supply is uninterrupted and the direct current load does not fall below the minimum required for the arc4 ,ques4 ,is the rectifier self-starting8 ,ans4 ,after the bulb has been started a few times1 as described above1 it becomes self-starting1 so that under all ordinary operating conditions it will commence to operate when the switches connecting it with the load and the alternating current supply are closed4 ,ques4 ,what provision is made in the ,westinghouse-,cooper ,hewitt rectifier to render it self-starting8 ,ans4 ,it is rendered self-starting by means of a condenser4 ,illustration3 ,,fig4 #b1aja4--;,general ,electric #ej light double tube combined unit series mercury arc rectifier outfit2 front view4 ,this unit consists of a constant current transformer1 reactance1 tube tank and exciting transformer mounted on a common base2 also a static discharger and pilot lamp mounted on top of the transformer4 ,this arrangement makes the rectifier outfit1 with the exception of the switchboard panel1 complete in itself4 ,ques4 ,describe the arrangement and operation of the condenser4 ,ans4 ,the condenser is connected between one of the positive electrodes and a coating of tinfoil outside ,illustration3 ,,fig4 #b1ajb4--;,general ,electric #b1bjj volts1 #fj cycle1 primary1 #f4f ampere1 secondary1 #ge light1 double tube mercury arc rectifier outfit with automatic shaking device1 the case being removed to show parts4 ,the constant current transformer is air cooled4 ,the winding which supplies energy for the exciter transformer is located at the top of1 and around the core of the constant current transformer4 ,the exciting transformer is mounted on the base of the constant current transformer inside of the casing4 ,it supplies low pressure currents to the starting anodes of the rectifier tube4 ,this current establishes an auxiliary arc when the tube is shaken1 which is necessary in order to start the rectifier4 ,the exciting transformer is wound for #aaj volts and it consumes about #bjj volt-amperes4 ,the direct current reactance is mounted on the base of the transformer and enclosed in the same casing4 ,it is connected in series with the lamp load and its function is to reduce the pulsations of the circuit to a value most satisfactory for operation4 ,the tube tank for holding the oil is mounted on the same base as the transformer4 ,it is provided with a cooling coil2 a tube carrier is provided for raising or lowering the tube in the tank4 ,a thermometer is provided to gauge the temperature of the oil in the tank4 ,the static dischargers consisting of horn gaps in series with resistance1 are connected between the anodes and the cathode in order to protect the tubes and other apparatus from excessive electrical strains4 ,the horn gaps open the circuit after discharge1 and in case the resistance becomes damaged the discharge passes across the spark gap provided1 thereby shunting the resistance4 the part of the bulb containing the mercury1 and induces static sparks on the surface of the mercury which break down the negative electrode resistance4 ,the action of the rectifier will be better understood by reference to the diagram of current waves and impressed pressure as shown in figs4 #b1ajc to #b1ajf4 ,illustration3 ,,figs4 #b1ajc to #b1ajf4--;,diagram of current waves and impressed pressure of ,westinghouse-,cooper ,hewitt mercury vapor rectifier4 ,the whole of the alternating current wave on both sides of the zero line is used4 ,the two upper curves in the diagram show the current waves in each of the two positive electrodes1 and the resultant curve ,,iii represents the rectified current flowing from the negative electrode4 ,curve ,,iv shows the impressed alternating current pressure4 ,it is evident that if the part of the wave below the zero line were reversed1 the resulting current would be a pulsating direct current with each pulsation varying from zero to a positive maximum4 ,such a current could not be maintained by the rectifier1 because as soon as the zero value was reached the negative electrode resistance of the rectifier would be re-established and the circuit would be broken4 ,to avoid this condition1 reactance is introduced into the circuit1 which causes an elongation of current waves so that they overlap before reaching the zero value4 ,the overlapping of the rectifier current waves reduces the amplitude of the pulsations and produces a comparatively smooth direct current as shown in curve ,,iii4 ,in this way the whole of the alternating current is transformed to direct current because each of the alternations in both directions is alternately rectified4 ,ques4 ,describe a mercury vapor rectifier outfit for series arc lighting4 ,ans4 ,it consists of a constant current regulating transformer1 a rectifier bulb1 and a control panel containing the necessary switches1 meters1 etc4 ,the transformer and rectifier bulb are mounted in the same tank4 ,illustration3 ,,fig4 #b1ajg4--;,general ,electrical rectifier tube as used on series mercury arc rectifier4 ,the tube rectifies the alternating current into direct current4 ,it consists of an exhausted glass vessel containing one anode or positive terminal in each of the two upper arms1 two mercury starting anodes and a cathode or negative terminal of mercury at the bottom of the tube4 ,it is submerged in oil and supported in a removable carrier4 ,the tube is put into operation by slightly shaking it4 ,in the combined unit set1 this shaking is accomplished by an electromagnet mounted above the tube tank and operated from a pull button switch on the panel4 ,an automatic shaker is sometimes installed which will automatically start the tube when the set is started1 or if its operation should become interrupted while in service4 ,the energy for the operation of this magnet 7#aaj volts alternating current7 is obtained from the small auxiliary winding on the main transformer which also supplies energy to the exciting transformer4 ,the oil in which the tube is placed is cooled by a circulation of water through the cooling coils on the inside of the tube tank4 ,the amount of water necessary for cooling the rectifier tubes varies according to local conditions1 depending upon the temperature of the water and that of the air in the station but under the most favorable conditions no water is required4 ,as rectifiers are commonly installed in steam driven stations1 the drip from the tube tanks is usually piped to the boiler supply thereby eliminating any loss for cooling water4 ,illustration3 ,,fig4 #b1ajh4--;,elementary diagram of mercury arc rectifier connections4 ,a4,a41 graphite anodes2 ,b1 mercury cathode2 ,c1 small starting electrode2 ,d1 battery connection2 ,e and ,f1 reactance coils2 ,g and ,h1 transformer terminals2 ,j1 battery4 ,ques4 ,describe the construction and operation of the mercury arc#aa rectifier shown in fig4 #b1ajh4 #aa ,,note4--,the terms vapor and arc as applied to rectifiers1 do not indicate a different principle2 the ,westinghouse ,co4 employ the former1 and the ,general ,electric ,co41 the latter4 ,ans4 ,fig4 #b1ajh is an elementary diagram of connections4 ,the rectifier tube is an exhausted glass vessel in which are two graphite anodes ,a1 ,a'1 and one mercury cathode ,b4 ,the small starting electrode ,c is connected to one side of the alternating circuit1 through resistance2 and by rocking the tube a slight arc is formed1 which starts the operation of the rectifier tube4 ,at the instant the terminal ,h of the supply transformer is positive1 the anode ,a is then positive1 and the arc is free to flow between ,a and ,b4 ,following the direction of the arrow still further1 the current passes through the battery ,j1 through one-half of the main reactance coil ,e1 and back to the negative terminal ,g of the transformer4 ,when the impressed voltage falls below a value sufficient to maintain the arc against the reverse pressure of the arc and load1 the reactance ,e1 which heretofore has been charging1 now discharges1 the discharge current being in the same direction as formerly4 ,this serves to maintain the arc in the rectifier tube until the pressure of the supply has passed through zero1 reversed1 and built up such a value as to cause the anode ,a to have a sufficiently positive value to start the arc between it and the cathode ,b4 ,the discharge circuit of the reactance coil ,e is now through the arc ,a',b instead of through its former circuit4 ,consequently the arc ,a',b is now supplied with current1 partly from the transformer1 and partly from the reactance coil ,e4 ,the new circuit from the transformer is indicated by the arrows enclosed in circles4 ,illustration3 ,,fig4 #b1aji4--;,diagram showing connections of ,general ,electric combined unit mercury arc single tube rectifier outfit with remote controlled non-automatic shaking device4 ,illustration3 ,,fig4 #b1aaj4--;,diagram showing connections of ,general ,electric combined unit mercury arc double tube rectifier outfit with remote controlled non-automatic shaking device4 ,illustration3 ,,fig4 #b1aaa4--;,diagram showing connections of ,general ,electric series mercury arc rectifier4 ,illustration3 ,,figs4 #b1aab and #b1aac4--;,diagram of current waves showing effect of reactance coil4 ,if the alternating current wave could be rectified without the use of the reactance coil1 the direct current produced would consist of a series of impulses which would rise and fall from the zero line as illustrated in fig4 #b1aab4 ,the action of the reactance coil not only maintains the current through the tube while the supply current is passing through zero1 but helps to smooth out the pulsations of the direct current which is passing out of the cathode terminal of the tube to the batteries1 or other direct current apparatus put in its circuit4 ,the smoothing out effect of the reactance is shown in fig4 #b1aac4 ,it will be seen from the diagram that the current does not drop down to zero and the pulsations of the direct current are greatly reduced4 ,the waves ,a1 ,a1 etc41 are from the positive waves of the alternating current supply1 while ,b1 ,b1 are from the negative waves1 and together they form the rectified current1 flowing in the same direction to the external direct current circuit shown at ,b in the diagram1 fig4 #b1ajh4 ,ques4 ,how is a mercury arc rectifier started8 ,ans4 ,a rectifier outfit with its starting devices1 etc41 is shown in figs4 #b1aad to #b1aaf4 ,to start the rectifier1 close in order named line switch and circuit breaker2 hold the starting switch in opposite position from normal2 rock the tube gently by rectifier shaker4 ,when the tube starts1 as shown by greenish blue light1 release starting switch and see that it goes back to normal position4 ,adjust the charging current by means of fine regulation switch on the left2 or1 if not sufficient1 by one button of coarse regulation switch on the right4 ,the regulating switch may have to be adjusted occasionally during charge1 if it be desired to maintain charging amperes approximately constant4 ,illustration3 ,,figs4 #b1aad to #b1aaf4--;,general ,electric mercury arc rectifier outfit1 or charging set4 ,the cut shows front1 rear1 and side views of the rectifier1 illustrating the arrangement on a panel1 of the rectifier tube with its connection and operating devices4 ,illustration3 ,,figs4 #b1aag and #b1aah4--;,general ,electric mercury arc bulbs or tubes for #bjj and #aj1jjj volt circuits4 ,ques4 ,in the manufacture of rectifiers1 could other metals be used for the cathode in place of mercury8 ,ans4 ,yes4 ,ques4 ,why are they not used8 ,ans4 ,because1 on account of the arc produced1 they would gradually wear away and could not be replaced conveniently4 ,in the case of mercury1 the excess vapor is condensed to liquid form in the large glass bulb or condensing chamber of the tube and gravitates back to the cathode1 where it is used over and over again4 ,illustration3 ,,fig4 #b1aai4--;,general ,electric series mercury arc rectifier outfit2 view showing method of replacing a tube4 ,the illustration also shows tube carrier and drip tray4 ,ques4 ,in the operation of rectifiers1 how is the heat generated in the bulb dissipated8 ,ans4 ,in small rectifier sets the heat generated is dissipated through the tube to the air1 and in large tubes such as used in supplying #dj to #fj kw4 for constant current flaming arc lights operating at #d or #f4f amperes1 the tubes are immersed in a tank of oil1 and cooled similar to the arrangement used for oil insulated water cooled transformers4 ,ques4 ,what results are obtained with oil cooled tubes8 ,ans4 ,in practice it is found that the life of oil cooled tubes is greatly increased and temperature changes do not affect the ability to start up as in the air cooled tubes4 ,illustration3 ,,fig4 #b1abj4--;,general ,electric #ajj volt mercury arc rectifier tube4 ,a1,a1 anodes2 ,b1 cathode2 ,c1 starting anode2 ,d1 tube or bulb4 ,ques4 ,in the operation of a rectifier1 name an inherent feature of the mercury arc4 ,ans4 ,a reverse pressure of approximately #ad volts is produced1 which remains nearly constant through changes of load1 frequency1 and voltage4 ,its effect is to decrease the commercial efficiency slightly on light loads4 ,ques4 ,what is the advantage of a rectifier set over a motor generator set8 ,ans4 ,higher efficiency and lower first cost4 ,ques4 ,what is the capacity of a rectifier tube8 ,ans4 #dj to #ej amperes4 ,ques4 ,how is greater capacity obtained8 ,ans4 ,when a greater ampere capacity is required1 two or more rectifier sets can be joined to one circuit4 ,the rectifier may be joined in series for producing an increased voltage or two tubes can be connected in series in a single set4 ,ques4 ,for what service is a rotary converter better adapted than a rectifier8 ,ans4 ,for power distribution and other cases where a great amount of alternating current is to be converted into direct current1 the rotary converter or large motor generator sets are more practical4 ,ques4 ,for what service is a rectifier especially adapted8 ,ans4 ,it is very desirable for charging storage batteries for automobiles from the local alternating current lighting circuit4 ,when the consumer installs and operates the apparatus for his own use and wear1 there is considerable saving over motor generator sets because a small one to two horse power motor generator outfit has an efficiency of only #dj to #ej per cent4 while mercury vapor rectifiers will have from #ge to #hj per cent4 ,ques4 ,what precautions should be taken in installing a rectifier8 ,ans4 ,it should be installed in a dry place and care should be taken to avoid dangling wires near the tube to prevent puncturing4 ,if the apparatus be installed in a room of uniform moderate temperature very little trouble will be experienced in starting1 while extreme cold will make starting more difficult4 #ab;,electro-magnetic ,rectifiers4--,devices of this class consist essentially of a double contact rocker which rocks on pivot 7midway between the contacts71 in synchronism with the frequency of the alternating current1 so changing the connections at the instants of reversals of the alternating current that a direct current is obtained4 #ab ,,note4--,the ,edison electromagnetic rectifier is described in detail in ,,guide ,no4 #d1 pages #idb to #ide4 ,fig4 #b1aba is a combined sketch and diagram of connections of a type of electromagnetic rectifier that has been introduced for changing alternating into direct current4 ,the actual apparatus consists of a box1 with perforated metal sides1 about ten inches square and six inches deep4 ,this box contains the step down transformer ,p1,s1,s'1 and the condensers ,k and ,k'1 the magnets and contact making device about to be described being fixed on the polished slate top of the box1 exactly as shown in the figure4 ,the transformer primary winding ,p may be connected through a switch s with a pair of ways on the nearest distribution box1 or to a plug connection or lamp-holder1 and the apparatus will give a rectified current of #f or #ab amperes at #bj volts1 according to the size4 ,s and ,s' is the secondary winding of the transformer1 with a tapping t midway1 joining it to a series circuit containing two alternating current electromagnets ,e and ,e'1 whose cores are connected by the long soft iron yoke ,y4 ,pivoted at ,p' is a steel bar ,,sb1 which is polarized by the two coils ,c and ,c' the current being supplied by a cell ,a4 ,fixed rigidly to ,,sb1 and moving with it1 is a double contact piece ,,cp with platinum contacts opposite similar ones on the fixed studs ,,cs1 ,,cs'4 ,,cp is flexibly connected through ,f to one of the direct current terminals ,t1 to which also are joined up one coating of each condenser ,k and ,k'4 ,illustration3 ,,fig4 #b1aba4--;,diagram showing essential features of ,premier ,ampero electromagnetic rectifier4 ,details of construction and principles of operation are given in the accompanying text4 ,the other direct current terminal ,t' is connected to the center of the transformer secondary at t2 and ,,cs and ,,cs' are respectively joined up to either end of the secondary winding and to the other coatings of the condensers4 ,illustration3 ,,fig4 #b1abb4--;,diagram of ,general ,electric 7,batten7 electromagnetic rectifier4 ,it is desirable for light and occasional service1 where direct current is required but only an alternating supply is available1 being used for charging storage batteries1 exciting spark coils1 performing electrolytic work1 etc4 ,the rectifier consists of a step down static transformer ,t1 by means of which the circuit pressure is reduced to about #ej volts2 also1 a polarized relay ,r1 the contact tongue ,c of which moves to one side or the other in sympathy with the alternations of the current in the primary winding ,p1 the secondary current induced in the winding ,s being thereby rendered direct in the outer circuit4 ,t'1 ,t' are the main terminals which are connected to the alternating current supply through the wires ,w4 ,lamps inserted at ,l are used as resistances in the primary circuit1 the reduction of the voltage already alluded to being effected by this means4 ,in charging storage batteries where a low pressure is required1 a lamp 7or lamps7 should be connected in the secondary circuit as shown1 ,s ,b being the storage battery1 and ,l' ,l' the lamp resistances in series therewith1 the battery has one end of the secondary ,s connected to its middle4 ,thus the alternating current leaving the transformer by the wire #a1 passes by flexible connection #b1 to the vibrating contact tongue ,c of the relay1 the latter causing the currents in either direction to flow through the two halves ,h1 ,h' of the battery1 whence the current re-enters the secondary of the transformer by the wire #c4 ,the soft iron core of the relay is in two halves ,s' ,s' and the armature ,a1 carrying ,c1 vibrates between their polar extremities4 ,m1 ,m' are two permanent magnets with their like poles together at the center ,c' where ,a is pivoted4 ,supposing these poles are north as indicated1 the extremities of ,a will be south4 ,the south ends of ,m1 ,m being in juxtaposition with the centers of the soft iron cores ,s'1 ,s' will render their extremities facing the ends of ,a of north polarity4 ,the windings on ,s'1 ,s' are connected in series with each other1 and in shunt with ,p across the main terminals ,t'1 ,t'4 ,then because of the polarization of ,a and ,s'1 ,s'1 the former will vibrate rapidly in sympathy with the alternations of the current4 ,k is a condenser shunted by a lamp resistance ,l1 this being found to improve the working of ,r4 ,when the alternating current circuit is broken1 the springs ,,sp1 ,,sp1 carried by ,,sb and bearing against the adjustable studs1 keep ,,sb1 ,,cs and ,,cs'4 ,the apparatus thus acts also as a no voltage circuit breaker1 for should the supply fail1 the storage battery ,a' under charge will be left on open circuit4 ,the action of the device is briefly as follows3 ,owing to the direct current in the magnetizing coils ,c and ,c' one end of ,,sb will be permanently of north and the other of south polarity2 and since the polarities of the poles ,e and ,e' will alternate with the alternations of the transformer secondary current1 ,,sb will rock rapidly on its pivot1 and contact will be made by turns with ,,cs and ,,cs'4 ,the purpose of the condensers ,k and ,k' is to reduce the sparking at these points4 ,when contact is made at ,,cs1 the direct current terminals ,t and ,t' are connected to the ,s half of the secondary winding2 and when contact is made at ,,cs'1 they are connected to the ,s' half4 ,thus a rectified uni-directional current will flow from ,t and ,t'1 and it may be used to charge the battery ,a'1 work a small motor or for various other purposes requiring direct current4 ,when the rectifier is used for charging storage batteries1 the separate cell ,a may sometimes be dispensed with1 the winding ,c1,c' being connected to one of the cells under charge4 ,the rectifier is adjusted to suit the frequency of the supply circuit by altering the distance of the poles of ,e and ,e' from the ends of the polarized armature ,,sb2 and also by changing the tension of ,,sp1 ,,sp by means of the screw studs against which they bear4 ,,hawkins ,,practical ,,library ,,of ,,electricity ,,in ,,handy ,,pocket ,,form ,,price #a ,,each 99 99 99 99 99 ,they are not only the best1 but the cheapest work published on ,electricity4 ,each number being complete in itself4 ,separate numbers sent postpaid to any address on receipt of price4 ,they are guaranteed in every way or your money will be returned4 ,complete catalog of series will be mailed free on request4 ,,electrical ,,guide1 ,,no4 #a ,containing the principles of ,elementary ,electricity1 ,magnetism1 ,induction1 ,experiments1 ,dynamos1 ,electric ,machinery4 ,,electrical ,,guide1 ,,no4 #b ,the construction of ,dynamos1 ,motors1 ,armatures1 ,armature ,windings1 ,installing of ,dynamos4 ,,electrical ,,guide1 ,,no4 #c ,electrical ,instruments1 ,testing1 ,practical ,management of ,dynamos and ,motors4 ,,electrical ,,guide1 ,,no4 #d ,distribution ,systems1 ,wiring1 ,wiring ,diagrams1 ,sign ,flashers1 ,storage ,batteries4 ,,electrical ,,guide1 ,,no4 #e ,principles of ,alternating ,currents and ,alternators4 ,,electrical ,,guide1 ,,no4 #f ,alternating ,current ,motors1 ,transformers1 ,converters1 ,rectifiers4 ,,electrical ,,guide1 ,,no4 #g ,alternating ,current ,systems1 ,circuit ,breakers1 ,measuring ,instruments4 ,,electrical ,,guide1 ,,no4 #h ,alternating ,current ,switch ,boards1 ,wiring1 ,power ,stations1 ,installation and ,operation4 ,,electrical ,,guide1 ,,no4 #i ,telephone1 ,telegraph1 ,wireless1 ,bells1 ,lighting1 ,railways4 ,,electrical ,,guide1 ,,no4 #aj ,modern ,practical ,applications of ,electricity and ,ready ,reference ,index of the #aj ,numbers4 ,theo4 ,audel ,co41 ,publishers4 #gb ,,fifth ,,avenue1 ,,new ,,york ,,transcriber',s ,,notes ,silently corrected simple spelling1 grammar1 and typographical errors4 ,retained anachronistic and non-standard spellings as printed4 ,enclosed italics markup in underscores4 ,enclosed bold markup in tildes4