,,the ,,thought ,,is ,,in ,,the ,,question ,,the ,,information ,,is ,,in ,,the ,,answer ,,hawkins ,,electrical ,,guide ,,number ,,two ,,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 ,,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 #b4 ,,the ,,armature #bba to #bbh ,definition--how continuous current is obtained--type of armature--comparison ring and drum armatures--why drum armature is the prevailing type--disc armatures--why disc armatures were abandoned4 ,,armature ,,windings #bbi to #bef ,preliminary considerations--winding diagrams and winding tables--lap and wave winding--angular pitch or spread of drum coils--parallel or lap winding--series or wave winding--double-windings--,siemens winding--objection to ,siemens winding--chord winding--multiplex windings--number of brushes required--number of armature circuits--equalizer rings--drum winding requirements4 ,,theory ,,of ,,the ,,armature #beg to #bhb ,current distribution in ring and drum armatures--connection of brushes--variation of voltage around the commutator--cross magnetization2 field distortion--remedies for field distortion--angle of lead--demagnetizing effect of armature reaction--effect of lead--eddy currents2 lamination--remedy for eddy currents--magnetic drag on the armature--smooth and slotted armatures--comparison of smooth and slotted armatures--magnetic hysteresis in armature cores--core loss or iron loss--dead turns--friction4 ,,commutation ,,and ,,the ,,commutator #bhc to #cjb ,period of commutation--commutating plane--normal neutral plane--neutral plane--plane of maximum induction--commutation--position of the brushes--sparking--effect of self-induction--construction of commutators--points relating to commutators--types of commutator4 ,,brushes ,,and ,,the ,,brush ,,gear #cjc to #cbj ,classification--gauze brushes--wire brushes--strip brushes--carbon brushes--adjustment--comparison of copper and carbon brushes--size of brushes--number--contact angle of brush---brush contact--drop in voltage at brushes--brush holders--brush rigging--multipolar brush gear4 ,,armature ,,construction #cba to to #cdh ,parts--shaft--core--slotted core--core laminations--core bolts--attachment to shaft--insulation of core discs--teeth--advantages and defects of slotted armatures--slotted cores2 built up construction--ventilation--insulation of core--armature windings--construction of inductors--objection to copper bars--various windings3 hand winding--evolute or butterfly winding--connectors--barrel winding--bastard winding--former winding--former coils--peculiarity of evolute coil--straight out coil--coil retaining devices--driving horns4 ,,motors #cdi to #chh ,definition--principles--propelling drag--essential requirements of construction--the reverse electromotive force--hydraulic analogy--action of current supplied to motor--armature reaction in motors--method of starting a motor---classes of motor2 series1 shunt1 and compound--power of a motor--brake horse power--mutual relations of motor torque and speed--speed regulation--series parallel controller--interpole motors4 ,,selection ,,and ,,installation #chi to #djf ,general conditions governing selection--construction--efficiency--ada ptation of series and shunt motors--location--foundations--erection of dynamos and motors--connecting up dynamos--marine generating set--belt clamp--belt lacing--belt speed--points relating to belts--gear drive--friction drive--electrical connections4 ,,auxiliary ,,apparatus #djg to #dcj ,switches--switch classification and construction--difficulty encountered in opening the circuit--various switches3 knife1 snap1 and quick break types--fuses--circuit breakers3 maximum1 minimum1 reverse current1 maximum and reverse current1 no voltage breaker--discriminating cut out--time limit attachments--rheostats--starting boxes--switchboards4 ,,chapter ,,xvii ,,the ,,armature ,the armature of a dynamo consists of coils of insulated wire wound around an iron core1 and so arranged that electric currents are induced in the wire when the armature is rotated in a magnetic field or the field magnets rotated and armature held stationary4 ,the commutator is in fact a part of the armature1 but is of sufficient importance to be considered in a separate chapter4 ,ques4 ,what are the practical objections to the elementary armature1 described in fig4 #afe8 ,ans4 ,it induces a very feeble current1 which is not of constant pressure1 but pulsating2 that is1 it consists of two pronounced impulses in each revolution as shown in fig4 #afh4 ,ques4 ,why does the elementary armature produce a pulsating current8 ,ans4 ,the pulsations are due to the coil moving alternately into1 and out of1 the positions of best and least action in the magnetic field4 ,ques4 ,how is a continuous current1 or one of uniform pressure obtained8 ,ans4 ,if an additional coil be added to the elementary armature1 at right angles to the existing coil1 and its ends suitably connected to a four part commutator1 as in fig4 #ahe1 so that one coil is in the position of best action1 while the other is in the position of least action1 the pulsations of the resulting current will be of less magnitude4 ,by increasing the coils and suitably altering the construction of the commutator to accommodate the ends of these coils1 the resultant current may be represented by practically a straight line1 indicating the so called continuous current1 instead of the wavy resultant curve ,no4 #f1 as illustrated in fig4 #ahg4 ,illustration3 ,,fig #bdg4--,ring armature of four pole dynamo3 diagram of winding and connections1 showing direction of the induced currents4 ,the currents in the windings under the upper ,n and ,s poles are opposed to each other and flow to the external circuit by the positive brush #a1 and back to this half of the armature by the negative brushes #c and #d4 ,at the same instant the opposed currents in the lower windings flow to the external circuit by positive brush #b and return to the armature through negative brushes #c and #d4 ,the armature is thus divided into four circuits and four brushes are required which must be placed between the poles so as to short circuit the coils as they pass through the neutral space4 ,in this form of winding there is no difference of potential between the brushes1 so that they are connected in parallel1 as are also the negative brushes1 and then to the external circuit4 ,in multipolar machines there are as many brushes as pole pieces4 ,since opposite commutator bars are of the same potential on this four pole dynamo they may be joined by a cross connecting wire and two brushes1 as #b and #d1 dispensed with4 ,this can only be done when there is an even number of coils4 ,the armature is said to be cross connected4 ,an armature for practical use has a large number of coils1 suitably arranged upon an iron core1 so that a large proportion of them are always actively cutting the lines of force1 or moving into the positions of best action in the magnetic field4 ,types of ,armature4--,although there are many forms of armature1 all may be divided into three classes1 according to the arrangement of the coils or winding on the core1 as3 #a4 ,ring armatures2 #b4 ,drum armatures2 #c4 ,disc armatures4 ,each of these forms of armature has its own special advantages for particular purposes1 the disc type being least in favor and not having had any extensive application in this country4 ,illustration3 ,,fig4 #bdh4--;,early form of ,gramme ring armature1 the core being shown cut through1 and some of the coils displaced to make it clearer4 ,the core1 ,f1 consists of a quantity of iron wire wound continuously to form a ring of the shape shown by the section4 ,over this is wound about thirty coils of insulated copper wire1 ,b ,c ,d1 etc41 the direction of the winding of each being the same1 and their adjacent ends connected together4 ,the commutator segments consist of a corresponding number of brass angle pieces1 m1 n1 which are fixed against the wooden boss1 o1 carried on the driving shaft4 ,the junction of every two adjacent coils is connected to one of the commutator segments1 as shown at n4 ,ques4 ,what is the comparison between ring and drum armatures8 ,ans4 ,the drum armature is electrically and mechanically the more efficient1 possessing1 as it does1 possibilities in the way of better mechanical construction of the core1 and in the arrangement and fixing of the inductors thereon not to be found in the ring form4 ,less wire and magnetizing current are required for the field magnets for a given output than with the ring armature4 ,drum winding is not so simple as ring winding1 and it is more difficult to ventilate a drum than a ring armature1 it being necessary to provide special ventilating ducts4 ,illustration3 ,,fig4 #bdi4--,modern form of ,gramme ring armature4 ,the core consists of a number of thin flat rings of well annealed charcoal iron1 the outer diameter of each ring or disc being #aa inches1 and its inner diameter #i inches4 ,sheets of thin paper insulate each disc from its neighbors to prevent the flow of eddy currents4 ,the armature is mounted on a steel shaft to which is keyed a four armed metal spider1 the extremities of whose arms fit into notches cut in the inner edges of the soft iron core rings1 so that a good mechanical connection is obtained between the core and the shaft4 ,the spider is made of a non-magnetic metal1 to reduce the tendency to leakage of lines of force across the interior of the armature4 ,the armature inductors consist of cotton covered copper wire of ,no4 #i standard wire gauge1 wound around the core in one layer1 and offering a resistance1 from brush to brush1 of #j4jdh ohm4 ,there are two convolutions in each section1 the adjacent ends of neighboring sections being soldered to radial lugs projecting from the commutator bars4 ,ques4 ,describe a ring armature4 ,ans4 ,it consists essentially of an iron ring1 around which is wound a number of coils4 ,these various coils are wound on separately1 the wire being carried over the outside of the ring1 then through the center opening and again around the outside1 this operation being repeated until the winding for that individual section is completed4 ,the adjacent coil is then wound in the same way1 the ends of each being brought out to the commutator side of the armature1 the arrangement of the coils on the ring and connections with the commutator being shown in fig4 #bdg1 examples of actual construction being shown in figs4 #bdh and #bdi4 ,ques4 ,for what conditions of operation is the ring armature specially adapted1 and why8 ,ans4 ,it is well suited to the generation of small currents at high voltage1 as for series arc lighting1 because the numerous coils can be very well insulated4 ,illustration3 ,,fig4 #bej4--;,distribution of magnetic lines of force through a ,gramme ring4 ,since the metal of the ring furnishes a path of least reluctance1 most of the magnetic lines will follow the metal of the ring and very few will penetrate into the aperture of the interior4 ,this condition causes a serious defect in the action of ring armatures rendering the winding around the interior useless for the production of electromotive force4 ,hence1 in ring armatures only about half of the winding is effective1 the rest or dead wire1 adding its resistance to the circuit1 thus decreasing the efficiency of the machine4 ,ques4 ,why does a ring armature require more copper in the winding than a drum armature8 ,ans4 ,for the reason that those inductors which lie on the inner side of the iron ring1 being screened from practically all the lines of force1 as shown in fig4 #bej1 do not generate any current4 ,numerous attempts have been made to utilize this part of the winding by making the pole pieces extend around the ring in such a manner that lines of force will pass to the inside of the ring1 also by arranging an additional pole piece on the inside of the armature1 but mechanical considerations have shown these methods to be impractical4 ,ques4 ,is any portion of the winding of a drum armature inactive8 ,ans4 ,yes2 the end connectors do not generate any current4 ,illustration3 ,,fig4 #bea4--;,illustrating the principle of ,siemens' drum winding4 ,in order to make the winding and connections clear1 one coil and the commutator is shown assembled1 although the latter is not put in place until after all the sections have been wound1 the ends of the wires being temporarily twisted together until all can be soldered to the risers4 ,the cores of these early machines were of wood overspun circumferentially with iron wire before receiving the longitudinal copper windings4 ,ques4 ,what is the chief advantage of the drum armature8 ,ans4 ,it reduces considerably the large amount of dead wire necessary with the ring type4 ,ques4 ,how is this accomplished8 ,ans4 ,by winding the wire entirely on the outer surface of a cylinder or drum1 as it is called1 as shown in fig4 #bea1 thus none of the wire is screened by the metal of the core4 ,illustration3 ,,fig4 #beb4--;,elementary four coil drum winding1 showing the connections with the commutator segments1 and directions of currents in the several coils4 ,the action of this type of armature is fully explained in the text4 ,fig4 #beb shows an elementary four coil drum armature4 ,starting from the point a and following the winding around without reference at first to the commutator1 it will be found that the rectangular turns of the wire form a closed circuit1 and are electrically in series with one another in the order of the numbers marked on them4 ,with respect to the connections to the four segments w1 x1 y1 z1 of the commutator it will be found that at two of these1 x and y1 the pressures in the windings are both directed from1 or both directed toward the junction with the connecting wire4 ,at the other two segments1 z and w1 one pressure is toward the junction and the other directed from it4 ,if1 therefore1 the brushes be placed on x and y they will supply current to an external circuit1 z and w1 for the moment being idle segments4 ,disc ,armatures4--,the inductors of a disc armature move in a plane1 perpendicular to the direction of the lines of force1 about an axis parallel to them as shown in fig4 #bec4 ,the main difficulty with this type has been in constructing it so that it will be strong and capable of resisting wear and tear4 ,it was introduced in an effort to avoid the losses due to eddy currents and hysteresis present in the other types of armature4 ,illustration3 ,,fig4 #bec4--;,disc armature of ,niaudet4 ,it is equivalent to a ring armature1 having the coils turned through an angle of #ij1 so that all the coils lie in a plane perpendicular to the axis of rotation4 ,the connections of the coils with each other and with the commutator remain the same1 the beginning and the end of adjacent coils leading to a common commutator bar as shown4 ,the magnetic field is arranged by the use of two magnets1 so arranged as to present the north pole of one to the south pole of the other1 and vice versa4 ,in the figure one of these magnets is considered as above the paper1 and the other below4 ,if this armature be rotated through the magnetic field as shown1 a reversal of current takes place in each coil1 when it is in such a position that one of its diameters coincides with the pole line1 ,,ns4 ,if the brushes be set so as to short circuit the coils that are in this position1 the armature will be divided into two branchings1 the current flowing in an opposite direction in each1 and a direct current will flow in the exterior circuit4 ,on account of the nature of the construction of a disc armature1 it is necessary that the coils subject to induction occupy as small a space as possible in the direction of their axes4 ,this requirement1 as well as the connection of the inductors with each other and with the commutator1 prevented the general adoption of this form of armature1 and subsequent experience failed to justify the existence of the type4 ,,chapter ,,xviii ,,armature ,,windings ,to connect up rightly the inductors on an armature so as to produce a desired result is a simple matter in the case of ring winding1 for bipolar or multipolar machines4 ,it is a less easy matter in the case of drum winding1 especially for multipolar machines4 ,often there are several different ways of arriving at the same result1 and the fact that methods which are electrically equivalent may be geometrically and mechanically different makes it desirable to have a systematic method of treating the subject4 ,the elementary arrangement of drum and disc armatures has already been considered1 which is sufficient explanation for small armature coils of only a few turns of wire1 but in the case of larger machines which require many coils1 further treatment of the subject is necessary4 ,for example1 in order to direct the winder how to make the connections for1 say a four pole machine having #ajj bars spaced around its armature1 some plain method of representing all the connections so that they may be easily understood is necessary4 ,from this the workman finds out whether he is to connect the front,a end of bar ,no4 #a across to #ej or across a quarter of the circumference to #bd1 or across three quarters of it to bar #ge4 ,again1 he ascertains to which bar he is to connect the back#a end of the bar1 and how the bars are to be connected to the commutator4 #a ,,note3--,the front end means the end at which the commutator is located4 ,armatures are most conveniently regarded from this end1 the opposite end being known as the back end4 ,winding ,diagrams and ,winding ,tables4--,in the construction of armatures1 instructions to winders are given in the form of diagrams and tables4 ,in the tables the letters ,f and ,b stand for front and back1 meaning toward the front end1 and from the front end respectively4 ,the letters ,u and ,d stand for up and down4 ,illustration3 ,,fig4 #bed4--;,end of ring winding for a four pole machine4 ,an end view is simply a view showing the arrangement of the armature inductors and connections looking from the front or commutator end4 ,a developed view of the above winding is shown in fig4 #beg4 ,there are three kinds of winding diagram3 #a4 ,end view diagram2 #b4 ,radial diagram2 #c4 ,developed diagram4 ,the end view is simply a view showing the arrangement of the armature inductors and connections looking from the front or commutator end1 such as shown in fig4 #bed4 ,in the radial diagram the inductors of the armature are represented by short radial lines1 while the end connectors are represented by curves or zigzags1 those at one end of the armature being drawn within1 those at the other end1 without the circumference of the armature4 ,with the radial diagram it is easier to follow the circuits and to distinguish the back and front pitch of the winding4 ,illustration3 ,,fig4 #bee4--,partial sketch of a four pole machine laid on its side4 ,if the observer imagine himself placed at the center1 and the panorama of the four poles to be then laid out flat1 the developed view thus obtained would appear as in fig4 #bef4 ,the developed diagram is a mode of representation1 originally suggested by ,fritsche of ,berlin1 in which the armature winding is considered as though the entire structure had been developed out of a flat surface4 ,this is best explained by aid of figs4 #bee and #bef4 ,if in fig4 #bee1 which represents an armature core and a four pole field1 wires a and c be placed parallel to the axis of the armature to represent two of the armature inductors1 and moved along the air gap space clockwise past the ,s poles1 they will cut magnetic lines inducing electromotive forces in the directions indicated4 ,to attempt to show a large number of inductors in a drawing of this kind would be unintelligible4 ,accordingly1 the observer is considered as being placed at the center of the armature1 and the panorama of the four poles surrounding him to be then laid out flat or developed as in fig4 #bef4 ,the faces of the ,n and ,s poles are shaded obliquely for distinction4 ,by choosing the proper directions for these oblique lines1 a piece of paper having a narrow slit to represent the wire may be laid over the drawing of the pole and when moved1 as indicated by the dotted arrows to the right1 the slit in passing over the oblique lines will cause an apparent motion in the direction in which the current in reality tends to flow4 ,it is easily remembered which way the oblique lines must slope1 for those on the ,n pole slope parallel to the oblique part of the letter ,n4 ,lap ,winding and ,wave ,winding4--,in winding armatures there are two distinct methods employed1 known respectively as lap and wave winding4 ,the distinction arises in the following manner3 ,since the inductors1 in passing a north pole generate electromotive forces in one direction1 and in passing a south pole generate electromotive forces in the opposite direction1 it is evident that an inductor in one of these groups ought to be connected to one in nearly a corresponding position in the other group1 so that the current may flow down one and up the other in agreement with the directions of the electromotive forces4 ,the order followed in making these connections gives rise to lap and wave windings4 ,illustration3 ,,fig4 #bef4--;,developed view of the four pole field shown in perspective in fig4 #bee4 ,ques4 ,what is lap winding8 ,ans4 ,one in which the ends of the coils come back to adjacent segments of the commutator2 the coils of such a winding lap over each other4 ,ques4 ,what is a wave winding8 ,ans4 ,one in which the coil ends diverge and go to segments widely separated1 the winding to a certain extent resembling a wave4 ,illustration3 ,,fig4 #beg4--;,development of ring winding of four pole machine shown in fig4 #bed4 ,the dead wire or inactive inductors on the inside of the ring are shown in dotted lines1 the full lines representing the active portion of the winding4 ,angular ,pitch or ,spread of ,drum ,coils4--,before taking up the winding as a whole1 the form of the individual coil should be considered4 ,fig4 #bfj shows an end view of one coil in position on a drum armature of a multipolar machine4 ,the two slots ,x and ,y contain the sides of the coil and the distance between them on the surface of the drum is called the angular pitch or spread of the coil4 ,theoretically this is equal to the pitch of the poles1 represented by the angle ,m1 which is the angle between the pole centres4 ,illustration3 ,,figs4 #beh and #bei4--,wooden armature core and winding table for practice in armature winding4 ,by using strings of different colors to represent the various coils1 the path of each coil is easily traced when the winding is completed1 as in fig4 #bfc4 ,for instance1 on a four pole machine the pitch would be #ij1 on a six pole machine1 #fj1 etc4 ,usually the angular pitch of the coil is made just a little less than the pole pitch of the machine1 in order to shorten the end connections of the coils from slot to slot4 ,however1 if the angular pitch be made too small trouble will be encountered in commutation4 ,in addition to the angular pitch there is the commutator pitch which relates to the distance around the commutator bridged by the ends of the coil4 ,thus1 if the commutator segments were numbered consecutively #a1 #b1 #c1 etc41 and the commutator pitch say is #aj1 it would signify that one end of the coil was connected to segment #a and the other end to segment #aa2 the ends of the next coil in order then would be connected to segments #b and #ab1 in each case there would be ten segments between the two segments connecting with the coil ends4 ,illustration3 ,,fig4 #bfj4--;,end view of drum armature of a multipolar machine showing one coil in position to illustrate the angular pitch or spread of drum coils4 ,parallel or ,lap ,drum ,winding4--,in order to avoid much of the difficulty usually experienced by students of drum winding1 the beginner should construct for himself a wooden armature core upon which he can wind strings of various colors1 or wires with distinctive insulation1 to represent the numerous coils that are used on real armatures4 ,a few windings attempted in this way will make clear many points that cannot be so easily grasped from a written description4 ,the type of drum core best adapted for this work is the slotted variety as shown in fig4 #beh1 as it will facilitate the winding4 ,the core as shown in the illustration has twelve slots and six commutator segments1 the number of each required for the example of lap winding indicated in the winding table fig4 #bei4 ,in making the wooden core1 the slots may be formed by nailing a series of thin strips around a cylindrical piece of wood1 thus avoiding the trouble of cutting grooves4 ,in the illustrations the commutator segments are shortened 7leaving no room for brushes7 in order to show the connections as clearly as possible4 ,illustration3 ,,fig4 #bfa4--;,developed view of a typical lap winding4 ,from the figure it is seen that at the back of the armature each inductor is united to one five places further on1 that is1 #a to #f1 #c to #h1 etc41 and at the front end of the winding1 after having made one element1 as for example d-#g-ab-;e1 then forms a second element e-#i-ad-;f which laps over the first1 and so on all around until the winding returns on itself4 ,ques4 ,describe the simple lap winding fig4 #bei4 ,ans4 ,as given in the table1 it consists of six loops of wire presenting twelve inductors on the cylindrical surface of the core or drum4 ,in the table1 six wires are shown1 having distinctive and varied insulation so as to readily distinguish the different coils4 ,opposite these are letters and figures designating the path and connections of each coil4 ,ques4 ,what is the path of the first coil8 ,ans4 ,according to the table it is3 ,a -- #a -- #f -- ,b that is1 one end of the wire is connected to commutator segment ,a 7fig4 #bfb7 and then wound to the back of the drum through slot #a1 across the back of the drum to slot #f1 returning through this slot1 and then connected with commutator segment ,b4 ,illustration3 ,,fig4 #bfb4--,skeleton view of wooden armature core showing in position the first two coils of the winding indicated in the table fig4 #bei4 ,ques4 ,describe the path of the second coil4 ,ans4 ,the second coil1 having the block insulation1 is wound according to the table1 in the order3 ,b--#c--h--;,c that is1 beginning at segment ,b1 thence to back of drum through slot #c1 across the back to slot #h1 returning through this slot and ending at segment ,c4 ,the completed winding of the first two coils are shown in fig4 #bfb1 the drum being shown in dotted lines so that all of each coil may be visible4 ,illustration3 ,,fig4 #bfc4--,view of completed winding as indicated in the table fig4 #bei4 ,thus the path of the first coil1 according to the table is ,a-#a-f-;,b which means that the coil begins at segment ,a of the commutator1 rises to slot #a1 and proceeds through the slot to the back of the drum2 thence across the back to slot #f1 through the slot and ending at segment ,b4 ,the other coils are wound in similar order as indicated in the table4 ,ques4 ,how are the remaining coils wound on the drum8 ,ans4 ,each of the succeeding coils are wound as indicated in the table1 the last connection being made to segment ,a1 the one from which the winding started4 ,ques4 ,what is the general form of the completed winding8 ,ans4 ,it may be considered simply as a wire wound spirally around the drum1 with loops brought down to the commutator segments1 and ending at the segment from which the start was made4 ,the completed winding as indicated by the table is shown in fig4 #bfc4 ,here the path of each coil is easily distinguished by means of the varied insulations although in part hidden by the drum4 ,fig4 #bfd shows a developed view of the winding4 ,illustration3 ,,fig4 #bfd4--;,developed view of the winding shown in perspective in fig4 #bfc4 ,ques4 ,what condition must obtain in winding an even number of coils8 ,ans4 ,the wire must not be wound around the drum to diametrically opposite positions1 as for instance #a to #g in fig4 #bfe4 ,ques4 ,why is this8 ,ans4 ,the reason will be clearly seen by attempting the winding on the wooden core4 ,a winding of this kind on the drum fig4 #beh1 would proceed as follows3 ,a--#a-- #g--;,b ,b--#c-- #i--;,c ,c--#e--aa--;,d ,in order now to continue winding in a regular way1 the wire from segment d should pass to the rear of the armature along space #g1 but this space is already occupied by the return of the first coil4 ,continuing the winding from this point1 it would be necessary to carry the wire from segment d to #f or #h1 resulting in an unbalanced winding4 ,illustration3 ,,fig4 #bfe4--,lap winding for bipolar machine1 with uneven number of coils2 in this case the rear connectors may be made directly across a diameter as shown4 ,ques4 ,how is a symmetrical winding obtained having an even number of coils8 ,ans4 ,the inductors1 in passing from the front to the rear of the armature1 fig4 #bfc1 must occupy positions #a1 #c1 #e1 #g1 #i1 #aa1 and the even numbered positions will then serve as the returns for these wires4 ,in the example here shown there are six coils1 comprising twelve inductors and six commutator segments2 it should be noted1 however1 that if there were an uneven number of coils1 the rear connections could be made directly across a diameter as shown in fig4 #bfe1 which would give a symmetrical winding4 ,with ten slots as shown in the figure1 the drum would be wound1 for a bipolar machine1 according to the following table3 ,a--#a-- #f--;,b ,b--#c-- #h--;,c ,c--#e--aj--;,d ,d--#g-- #b--;,e ,e--#i-- #d--;,f ,illustration3 ,,fig4 #bff4--;,developed view of a typical wave winding4 ,this winding1 instead of lapping back toward the commutator segment from whence it came1 as in lap winding1 turns the other way4 ,for instance1 d-#g-ab does not return directly to e1 but goes on to i1 whence another element i-#ag-d-;e continues in a sort of zigzag wave4 ,ques4 ,are coils such as shown in figs4 #bfc and #bfe used in practice8 ,ans4 ,no1 for practical use each coil would consist of several turns1 the diagram then merely indicates the end connections and slots for the several turns of each coil4 ,series or ,wave ,drum ,winding4--,in this mode of winding1 the inductors are arranged around the armature so that they do not turn back1 thus describing a zigzag or wave-like path2 that is1 the coil ends instead of connecting with adjacent segments of the commutator1 are attached to segments more or less remote4 ,ques4 ,describe the circuits of a simple or simplex wave winding4 ,ans4 ,only two sets of brushes are required for such a winding1 but as many brushes as there are poles can be used4 ,illustration3 ,,fig4 #bfg4--;,five coil wave winding for a four pole machine4 ,in this winding only two brushes are used1 there being only two paths through the armature4 ,ques4 ,for what service are wave windings adapted8 ,ans4 ,they are generally used on armatures designed to furnish a current of high voltage and low amperage4 ,an example of wave drum winding for a four pole machine is shown in fig4 #bfg4 ,for simplicity1 very few coils are taken1 there being only five as shown in the illustration4 ,to make the winding1 one strip should be removed from the wooden core and the others spaced equally around the cylindrical surface4 ,this will give ten slots1 the number required for the five coils4 ,the winding is indicated in the following table3 ,a--#a-- #d--;,c ,b--#c-- #f--;,d ,c--#e-- #h--;,e ,d--#g--aj--;,a ,e--#i-- #b--;,b ,accordingly the first coil starting at segment ,a1 is carried to the back of the drum through slot #a1 thence across the back and returning through slot #d1 ending at segment ,c the starting point of the second coil4 ,each coil is wound on in similar manner1 the last coil ending at segment ,a1 the starting point of the first coil4 ,a developed view of the winding is shown in fig4 #bfh4 ,double ,windings4--,in the various drum windings thus far considered1 each coil had its individual slots1 that is1 no two occupied the same two slots4 ,this arrangement gave twice the number of slots as commutator segments4 ,illustration3 ,,fig4 #bfh4--;,developed view of the five coil wave winding shown in fig4 #bfg4 ,in a double winding there are as many segments as slots1 each of the latter containing two inductors1 comprising part of two coils4 ,the ,siemens ,winding4--,in winding drum armatures for bipolar dynamos of two horse power or less1 and especially for very small machines as used in fan or sewing machine motors1 a form of winding1 known as the ,siemens winding1 which is shown in fig4 #bga1 is largely used4 ,it consists in dividing the surface of the armature core in one equal number of slots1 say #af1 and using a #af part commutator4 ,in the ,siemens winding1 the end of the wire used at the start is to be connected to the first commutator bar1 but must be fastened to the armature core out of the way so as not to interfere with the winding of the coils4 ,if eight turns of wire be required to fill a slot with one layer1 then the wire is carried from front to back and bent aside so as to clear the shaft2 after passing across the back or pulley end of the armature1 it is wound in the diametrically opposite section and brought to the front1 then across the commutator end and up close to the beginning of the coil4 ,illustration3 ,,fig4 #bfi4--,series connected wave wound ring armature for a four pole machine4 ,the coils are so connected that only two brushes are necessary4 ,since eight turns are to be used1 the process of winding is continued until the section is full and the end of the coil will lie in a position ready to begin the next section4 ,sometimes the wire is cut at this part of the coil leaving #c or #d inches projecting for connecting to the commutator bar #b1 or next to the first bar where the winding was started4 ,the usual practice is1 however1 to make a loop of the wire of sufficient length to make the connection to the commutator and it has the advantage that since all of the coils on the armature are joined in series1 the ending of one coil is joined to the beginning of the next which avoids making mistakes in making the commutator connections4 ,if the ends be cut they should be marked beginning and end to avoid trouble1 because if they get mixed1 it will be necessary to test each coil with a battery and compass needle in order to determine the polarity produced and find which is the beginning of the coil and which the end4 ,with #cb ends of the wire projecting from the end of the armature1 it is confusing and mistakes are often made in the connections1 so that one or more coils may oppose each other which would reduce the voltage4 ,after the surface of the armature is covered with one layer it will be noticed that the number of leads from the coils to the commutator bars is only one-half the number of bars and that they lie on one-half of the armature4 ,in order to complete the winding the first layer should be insulated and the second layer wound on4 ,the beginning of the new coil will be directly over the first coil put on1 but the beginning of the new coil will be diametrically opposite the beginning of the first coil wound4 ,the winding is now continued section by section and as each coil is finished a loop or pair of leads is left to connect to each bar4 ,when the last coil is wound1 its end will be found lying next to the wire used in starting and should be joined to it and finally connected to bar number one where the start was made4 ,illustration3 ,,fig4 #bgj4--;,developed view of the series connected wave wound ring armature shown in fig4 #bfi4 ,with the winding and commutator connected1 all of the coils are in series and the beginning of the first coil joins the end of the last coil4 ,if a pair of brushes be now placed on the commutator at opposite points the current will flow into the bar and then divide between the two leads connected to it1 half of the current flowing around one side and the other half flowing around the other half of the armature or in other words1 the two halves of the armature are joined in parallel4 ,ques4 ,what is the objection to the ,siemens winding just described8 ,ans4 ,it produces an unsightly head where the wires pass around the shaft and requires considerable skill to make it appear workmanlike4 ,ques4 ,how may this be avoided8 ,ans4 ,by using the chord windings of ,froehlich or ,breguet1 which are improvements over the ,siemens in appearance and are more easily carried out4 ,illustration3 ,,fig4 #bga4--;,end view of an armature1 showing the distinction between ,siemens' winding and chord winding4 ,chord ,winding4--,in cases where the front and back pitches#b are so taken that the average pitch differs considerably from the value obtained by dividing the number of inductors by the number of poles1 the arrangement is called a chord winding4 #b ,,note--,the term back pitch means the number of spaces between the two inductors of a coil4 ,for instance1 in fig4 #bfg1 the pitch is #c2 that is1 there are three spaces between say inductors #a and #d which form part of the coil ,a--#a--d--;,c4 ,it is called the back pitch in distinction from the front or commutator pitch1 which in this instance is #b4 ,in this method each coil is laid on the drum so as to cover an arc of the armature surface nearly equal to the angular pitch of the poles2 it is sometimes called short pitch winding4 ,ques4 ,what is the difference between the ,siemens winding and the chord winding8 ,ans4 ,this is illustrated in fig4 #bga1 which shows one end of an armature4 ,in the ,siemens winding1 a wire starting1 say at ,a1 crosses the head and enters the slot marked ,b4 ,if it enters slot ,c it is a chord winding4 ,ques4 ,describe a chord winding4 ,ans4 ,the winding is started in the same manner as described in the ,siemens method1 only instead of crossing the head and returning in the section diametrically opposite1 the section ,a ,c1 fig4 #bga1 next to it is used for the return of the wire to the front end4 ,leads for connecting to the commutator are left at the beginning and end of each section as before stated and the only difference between the two methods will be noticed when the first layer is nearly complete in that two sections lying next to each other have no wire in them4 ,this will cause the winder to think he has made a mistake1 but by continuing the winding and filling in these blank spaces in regular order when the two layers are completed1 all the sections will be filled with an equal number of turns and there will be the required number of leads from the coils to connect up to the commutator bars4 ,ques4 ,how many paths in the chord winding just described8 ,ans4 ,two4 ,multiplex ,windings4--,an armature may be wound with two or more independent sets of coils4 ,instead of independent commutators for the several windings1 they are combined into one having two or more sets of segments interplaced around the circumference4 ,thus1 in the case of two windings1 the brush comes in contact alternately with segments of each set4 ,the brush then must be large enough to overlap at least two segments1 so as to collect current from both windings simultaneously4 ,both windings then are always in the circuit in parallel4 ,ques4 ,what is the effect of a multiplex winding8 ,ans4 ,it reduces the tendency to sparking1 because only half of the current is commutated at a time1 and also because adjacent commutator bars belong to different windings4 ,illustration3 ,,fig4 #bgb4--;,a progressive wave winding4 ,if the front and back pitches of a wave winding be such that in tracing the course of the winding through as many coils as there are pairs of poles1 a segment is reached in advance of the one from which the start was made1 the winding is said to be progressive4 ,the figure shows three coils of a winding having #ah inductors4 ,from the definition1 the number of coils to consider to determine if the winding be progressive is equal to the number of poles divided by #b1 which in this case is equal to #b4 ,these coils are shown in the figure as follows3 ,a--#a--d--;,f and ,f--#aa--ad--;,b4 ,the second coil ends at segment ,b which is in advance of segment ,a from which the winding began1 indicating that the winding is progressive4 ,fig4 #bgb is given simply to illustrate the definition of a progressive winding1 and not to represent a practical winding4 ,ques4 ,does an accident to one winding disable the machine8 ,ans4 ,no1 it simply reduces its current capacity4 ,ques4 ,can multiplex windings have more than two windings8 ,ans4 ,yes1 there may be three or four windings4 ,ques4 ,what is the objection to increasing the number of windings8 ,ans4 ,it involves an increased number of inductors and commutator segments1 which is undesirable in small machines1 but for large ones might be allowable4 ,illustration3 ,,fig4 #bgc4--;,a retrogressive wave winding4 ,if the pitches be such that in tracing the winding through as many coils as there are pairs of poles1 the first segment of the commutator is not encountered or passed over1 the winding is said to be retrogressive4 ,the number of coils to consider is two1 as follows3 ,a-#a-d-;,d and ,d-#g-aj-;,g4 ,the second coil ends at ,g1 hence1 since the segment ,a where the start was made has not been reached or passed over the winding is retrogressive4 ,fig4 #bgc is given simply to illustrate the definition of retrogressive winding1 and not to represent a practical winding4 ,when there are two independent windings the arrangement is called duplex1 with three windings1 triplex1 and with four1 quadruplex4 ,ques4 ,what loss is reduced with multiplex windings8 ,ans4 ,in these windings1 the division of what otherwise would be very stout inductors into several smaller ones1 has the effect of reducing eddy current loss4 ,ques4 ,for what service are machines with multiplex windings specially adapted8 ,ans4 ,multiplex windings are used in machines intended to supply large currents at low voltages1 such as is required in electrolytic work4 ,number of ,brushes ,required4--,the number of places on the commutator at which it is necessary or advisable to place a set of collecting brushes can be ascertained from the winding diagrams4 ,all that is necessary is to draw arrows marking the directions of the induced electromotive forces4 ,wherever two arrow heads meet at any segment of the commutator1 a positive brush is to be placed1 and at every point from which two arrows start in opposed directions along the winding1 a negative brush should be placed4 ,ques4 ,how many brushes are required for lap windings and ordinary parallel ring windings8 ,ans4 ,there will be as many brushes as poles1 and they will be situated symmetrically around the commutator in regular order and at angular distances apart equal to the pole pitch4 ,it should be noted that the number of brush sets does not necessarily show the number of circuits through the armature4 ,ques4 ,how many brushes are required for wave windings8 ,ans4 ,if arrows be drawn marking the direction of the induced electromotive forces to determine the number of brushes1 it will be found that only two brushes are required for any number of poles4 ,ques4 ,what is the angle between these two brushes8 ,ans4 ,it is the same as the angle between any north and south pole4 ,for instance1 in a ten pole machine with wave winding the pitch between the brushes may be any of the following angles3 #cfj #aj #cf #c #cf #ajh #e #cf #ahj ,illustration3 ,,figs4 #bgd and #bge4--,right and left hand windings4 ,these consist respectively of turns which pass around the core in a right or left handed fashion4 ,thus in fig4 #bgd1 in passing around the circle clockwise from a to b1 the path of the winding is a right handed spiral4 ,in fig4 #bge1 which shows one coil of a drum armature1 if a be taken as the starting point1 in going to b1 a must be connected by a spiral connector across the front end of the drum to one of the descending inductors such as ,m1 from which at the back end another connector must join it to one of the ascending inductors1 such as ,s1 where it is led to b1 thus making one right handed turn4 ,sometimes with lap winding it is desirable to reduce the number of brushes4 ,in fig4 #bgf1 is shown the distribution of currents in a four pole lap wound machine having four brushes and generating #abj amperes4 ,in each of the four circuits the flow is #cj amperes1 and the current delivered to each brush is #fj amperes4 ,if now two of the brushes be removed1 the current through each of the remaining two will be #abj amperes1 while internally there will be only two circuits as shown in fig4 #bgg4 ,it should be noted1 however1 that these two circuits do not take equal shares of the current since1 though the sum of the electromotive forces in each circuit is the same1 the resistance of one is three times that of the other1 giving #ij amperes in one and #cj amperes in the other1 as indicated in the figure4 ,if no spark difficulties occur in collecting all the current with only two brushes1 the arrangement will work satisfactorily1 but the heat losses will be greater than with four brushes4 ,illustration3 ,,fig4 #bgf4--;,distribution of armature currents in a four pole lap wound dynamo having four brushes and generating #abj amperes4 ,ques4 ,are more than two brushes ever used with wave winding8 ,ans4 ,it is sometimes advisable to use more than two brushes with wave windings1 especially when the current is very large4 ,for instance1 in the case of a singly re-entrant#c simplex wave winding for an eight pole machine1 whenever any brush bridges adjacent bars of the commutator1 it short circuits one round of the wave winding and this round is connected at three intermediate points to other bars of the commutator4 ,hence1 if the short circuiting brush be a positive brush1 no harm will be done by three other positive brushes touching at the other points4 ,if these other brushes be broad enough to bridge across two commutator bars1 they may effect commutation1 that is1 three rounds instead of one undergoing commutation together4 #c ,,note4--,a re-entrant winding is one in which both ends re-enter or lead back to the starting point2 a closed winding4 ,number of ,armature ,circuits4--,it is possible to have windings that give any desired even number of circuits in machines having any number of poles4 ,illustration3 ,,fig4 #bgg4--,showing effect of removing two of the brushes in fig4 #bge4 ,if no spark difficulties occur in collecting the current with only two brushes1 the arrangement will work satisfactorily1 but the heat losses will be greater than with four brushes4 ,ques4 ,how many paths are possible in parallel8 ,ans4 ,for a simplex spirally wound ring1 the number of paths in parallel is equal to the number of poles1 and for a simplex series wound ring1 there will be two paths4 ,in the case of multiplex windings the number of paths is equal to that of the simplex winding multiplied by the number of independent windings4 ,in large multipolar dynamos it is1 as a rule1 inadvisable to have more than #ajj or #aej amperes in any one circuit1 except in the case of special machines for electro-chemical work4 ,such considerations are factors which govern the choice of number of circuits4 ,equalizer ,rings4--,these are rings resembling a series of hoops provided in a parallel wound armature to eliminate the effects of unbalancing1 by which the current divides unequally among the several paths through the armature4 ,by means of leads1 equalizer rings connect points of equal potential in the winding and so preserve an equalization of current4 ,illustration3 ,,fig4 #bgh4--,rear view of armature of a large dynamo built by the ,general ,electric ,co41 showing equalizer rings4 ,ques4 ,in multipolar machines what points are connected by equalizer rings8 ,ans4 ,any two or more points in the winding1 that during the rotation1 are at nearly equal potentials4 ,if there were perfect symmetry in the field system1 no currents would flow along such connectors2 however1 owing to imperfect symmetry1 the induction in the various sections of the winding may be unequal and the currents not equally distributed4 ,drum ,winding ,requirements4--,there are several conditions that must be satisfied by a closed coil drum winding3 #a4 ,there cannot be an odd number of inductors2 ,an odd number of inductors would be equivalent to not having a whole number of coils4 ,the even numbered inductors may be regarded as the returns for the odd numbered inductors4 #b4 ,both the front and back pitches must be odd in simplex windings4 #c4 ,the average pitch should be approximately equal to the number of inductors divided by the number of poles4 ,this condition must obtain in order that the electric pressures induced in inductors moving simultaneously under poles of opposite sign1 will be added4 ,the smallest pitch meeting this condition would stretch completely across a pole face1 while the largest would stretch from the given pole tip to the next pole tip of like polarity4 ,the choice of front and back pitch for a given number of inductors should1 with lap and wave windings in general1 comply with the following conditions3 #a4 ,all the coils composing the winding must be similar1 both mechanically and electrically1 and must be arranged symmetrically upon the armature4 #b4 ,each inductor of a simplex winding must be encountered once only1 and the winding must be re-entrant4 #c4 ,each simplex winding composing a multiplex winding must fulfill the requirement for a simplex winding4 #d4 ,a singly re-entrant multiplex winding must as a whole satisfy the requirement for a simplex winding4 ,in addition to the above requirements for lap and wave windings in general1 lap windings must comply with the following conditions3 #a4 ,the front and back pitches must be opposite in sign2 #b4 ,the front and back pitches must be unequal2 ,if they be equal1 the coil would be short circuited upon itself4 #c4 ,the front and back pitches must differ by two2 #d4 ,in multiplex windings1 the front and back pitches must differ by two multiplied by the number of independent simplex windings composing the multiplex winding2 #e4 ,the number of slots on a slotted armature may be even or odd2 #f4 ,the number of inductors must be an even number2 it may be a multiple of the number of slots2 ,in the case of wave windings the several conditions to be fulfilled may be stated as follows3 #a4 ,the front and back pitches must be alike in sign2 #b4 ,the front and back pitches may be equal or they may differ by any multiple of two4 ,they are usually made nearly equal to the number of inductors divided by the number of poles4 ,,chapter ,,xix ,,theory ,,of ,,the ,,armature ,current ,distribution in ,ring and ,drum ,armatures4--,in studying the actions and reactions which take place in the armature1 the student should be able to determine the directions of the induced currents4 ,the basic principles of electromagnetic induction were given in chapter ,x1 from which1 for instance1 the distribution of current in the gramme ring armature1 shown in fig4 #bgi1 is easily determined by the application of ,fleming's rule4 ,tracing the current from the negative to the positive brush1 it will be seen that it divides1 half going through coils #a1 #b1 #c1 and half through coils ,i1 ,,ii1 ,,iii1 these two currents ascend to the top of the ring1 uniting at the positive brush4 ,ques4 ,in the ,gramme ring armature 7fig4 #bgi7 what is the distribution of armature currents8 ,ans4 ,there are two paths in parallel as indicated in fig4 #bgi4 ,ques4 ,how does the voltage vary in the coils8 ,ans4 ,it varies according to the position of the coils1 being least when vertical and greatest when horizontal in a two pole machine arranged as in fig4 #bgi4 ,the upper and lower coils in the right hand half of the ring armature1 fig4 #bgi1 will have about the same electromotive force induced in them1 say #b volts each1 while the two coils between them will have a higher electromotive force1 at the same instant1 say #d volts each1 since they occupy nearly the positions of the maximum rate of change of the magnetic lines threading through them4 ,these eight coils may be represented by two batteries connected in parallel1 each battery consisting of two #b volt cells and two #d volt cells as shown in fig4 #bhj4 ,the voltage of each battery then will be #b #d #d #b #ab volts ,illustration3 ,,fig4 #bgi4--;,current distribution in a gramme ring armature4 ,there are two paths for the current between the brushes1 half going up each side of the ring as indicated by the arrows1 thus giving two paths in parallel as indicated in fig4 #bha4 ,illustration3 ,,fig4 #bhj4--;,battery analogy illustrating current distribution in a ring armature4 ,the eight coils of the armature1 fig4 #bgi1 are represented by two batteries of four cells each4 ,the action of the two units thus connected is indicated by the arrows4 ,in the external circuit the voltage is equal to that of one battery and the current is equal to the sum of the currents in each battery4 ,the two batteries being connected in parallel1 the voltage at the terminals will be the same1 but the current will be the sum of the currents in each battery4 ,ques4 ,how may the number of paths in parallel be increased8 ,ans4 ,by increasing the number of poles4 ,for instance1 in a four pole machine1 as in fig4 #bhc1 there are four paths in parallel4 ,in this case the armature may be used to furnish two separate currents1 though this is not desirable4 ,illustration3 ,,fig4 #bha4--;,diagram showing distribution of current in the gramme ring armature of fig4 #bgi4 ,the current flows in two parallel paths as indicated4 ,illustration3 ,,fig4 #bhb4--;,diagram showing current distribution through armature of a four pole machine with like brushes connected4 ,there are four paths in parallel1 hence the induced voltage will equal that of one set of coils1 and the current will be four times that flowing in one set of coils4 ,ques4 ,how are the brushes connected8 ,ans4 ,usually all the positive brushes are connected together1 and all the negative brushes as in fig4 #bhc1 giving four paths in parallel through the armature as indicated in fig4 #bhb4 ,illustration3 ,,fig4 #bhc4--;,brush connections for four pole dynamo4 ,it is usual to connect all the positive brushes to one terminal and all the negative brushes to the other which gives four parallel paths as shown in the diagram1 fig4 #bhb4 ,in a four pole machine1 two separate currents can be obtained by omitting the parallel brush connections4 ,ques4 ,how does this method of brush connection affect the voltage8 ,ans4 ,the voltage at the terminals is equal to that of any of the sets of coils between one positive brush and the adjacent negative brush4 ,thus in the four pole machine1 fig4 #bhc1 the coils of the four quadrants are in four parallels1 which gives an internal resistance equal to one-sixteenth that of the total resistance of the entire ring4 ,when the coils are connected in two circuits or series parallel1 it requires only two brushes at two neutral points on the commutator1 for any number of poles2 this arrangement is shown in fig4 #bfi4 ,ques4 ,in general what may be said about the current paths through an armature8 ,ans4 ,the paths may be in parallel or series parallel according as the winding is of the lap or wave type4 ,illustration3 ,,fig4 #bhd4--,morday's method of measuring the variation of voltage around the commutator by use of a single exploring brush and volt meter4 ,it consists in connecting one terminal of the volt meter 7preferably an electrostatic one7 to one brush of the machine1 and the other terminal to the exploring brush1 which can be moved from point to point1 readings being taken at each point4 ,variation of ,voltage ,around the ,commutator4--,there are numerous ways of determining the value of the induced voltage in an armature at various points around the commutator4 ,in the method suggested by ,morday1 it can be measured by the use of a single exploring brush and a volt meter as shown in fig4 #bhd4 ,in this method1 one terminal of the volt meter is connected to one of the brushes of the dynamo1 and the other terminal is joined by a wire to a small pilot brush which can be pressed against the commutator at any desired part of its circumference4 ,with the machine running at its rated speed1 the exploring brush is placed in successive positions between the two brushes of the machine4 ,in each position a reading of the volt meter is taken and the angular position of the exploring brush noted4 ,illustration3 ,,fig4 #bhe4--;,cross magnetization4 ,this is defined as lines of magnetic force set up in the windings of a dynamo armature which oppose at right angles the lines of force created between the poles of the field magnet4 ,the figure shows this cross flux which is due to the armature current alone4 ,ques4 ,how does the voltage vary between successive pairs of commutator segments8 ,ans4 ,the variation is not constant4 ,cross ,magnetization2 ,field ,distortion4--,in the operation of a dynamo with load1 the induced current flowing in the armature winding1 converts the armature into an electromagnet setting up a field across or at right angles to the field of the machine4 ,this cross magnetization of the armature tends to distort the field produced by the field magnets1 the effect being known as armature reaction4 ,to understand the nature of this reaction it is best to first consider the effect of the field current and the armature current separately4 ,fig4 #bhe represents the magnetic flux through an armature at rest1 where the field magnets are separately excited4 ,if the armature be rotated clockwise1 induced currents will flow upward through the two halves of the winding between the brushes1 making the lower brush negative and the upper brush positive4 ,ques4 ,if1 in fig4 #bhe1 the current in the field magnet be shut off1 and a current be passed through the armature entering at the lower brush1 what is the effect8 ,ans4 ,the current will divide at the lower brush1 flowing up each side to the top brush4 ,these currents tend to produce north and south poles on each half of the core at the points where the current enters and leaves the armature4 ,hence1 there will be two north poles at the top of the ring and two south poles at the bottom4 ,ques4 ,what effect is produced by the like poles at the top and bottom of the ring8 ,ans4 ,the external effect will be the same as though there were a single north and south pole situated respectively at the top and bottom of the ring4 ,ques4 ,in the operation of a dynamo1 how do the poles induced in the armature affect the magnetic field of the machine8 ,ans4 ,they distort the lines of force into an oblique direction as shown exaggerated in the diagram fig4 #bhf4 ,illustration3 ,,fig4 #bhf4--;,distortion of magnetic field due to cross magnetization4 ,for clearness1 the effect is shown somewhat exaggerated4 ,a drag or resistance to the movement of the armature is caused by the attraction of the north and south poles on the armature and pole pieces respectively4 ,ques4 ,what effect has the presence of poles in the armature on the operation of the machine8 ,ans4 ,in fig4 #bhf1 the resultant north pole n1 n1 n1 where the lines emerge from the ring1 attracts the south pole1 s1 s1 s1 where the lines enter the field magnet1 hence a load is brought upon the engine1 which drives the dynamo1 in dragging the armature around against these attractions4 ,the stronger the current induced in the armature1 the greater will be the power necessary to turn it4 ,ques4 ,why does this reaction in the armature require more power to drive the machine8 ,ans4 ,the effect produced by the armature reaction is in accordance with ,lenz's law which states that3 ,in electromagnetic induction1 the direction of the induced current is such as to oppose the motion producing it4 ,illustration3 ,,fig4 #bhg4--;,actual distortion of field resulting from cross magnetization1 as shown by iron filings4 ,remedies for ,field ,distortion4--,since the distortion of the magnetic field of a dynamo causes unsatisfactory operation1 numerous attempts have been made to overcome this defect1 as for instance1 by3 #a4 ,experimenting with different forms of pole piece2 ,the reluctance of the pole piece should be increased in the region where the magnetic flux tends to become most dense4 ,the trailing horn of the pole piece may be made longer than the advancing horn and cut farther from the surface of the armature1 so as to equalize the distribution of the magnetic flux4 #b4 ,lengthening the air gap2 ,this increases the reluctance1 and also necessitates more ampere turns in the field winding4 ,the field distortion1 however1 will not be so great1 as it would be if the magnetic field of the machine were weaker4 #c4 ,slotting the pole pieces2 ,both longitudinal gaps and oblique slots have been tried4 ,the reduction of cross section of the pole piece causes it to become highly saturated and to offer large reluctance to the cross field4 #d4 ,the use of auxiliary poles4 ,these are small poles placed between the main poles and so wound and connected that their action opposes that of the cross field4 ,normal ,neutral ,plane4--,this may be defined as a plane passing through the axis of the armature perpendicular to the magnetic field of the machine when there is no flow of current in the armature1 as shown in fig4 #bhh4 ,it is the plane in which the brushes would be placed to prevent sparking when the machine is in operation were the field not distorted by armature reaction1 and there were no self-induction in the coils4 ,commutating ,plane2 ,lead of the ,brushes4--,it has been found that in order to reduce sparking to a minimum1 the brushes must be placed in certain positions found by trial and designated as being located in the neutral plane4 ,when the brushes are in the neutral plane1 they are in contact with commutator segments connecting with coils that are cutting the lines of force at the minimum rate4 ,ques4 ,define the term commutating plane4 ,ans4 ,this is a plane passing through the axis of the armature and through the center of contact of the brushes as shown in figs4 #bhi and #cjj4 ,ques4 ,what is the angle of lead8 ,ans4 ,the angle between the normal neutral plane and the commutating plane4 ,in the operation of a dynamo since the field1 on account of armature reaction1 is twisted around in the direction of rotation1 the proper position for the brushes is no longer in the normal neutral plane1 but lies obliquely across1 a few degrees in advance4 ,hence1 for sparkless commutation1 the commutating plane is a little in advance of the normal neutral plane1 the lead being measured by the angle between these planes1 as stated in the definition4 ,illustration3 ,,fig4 #bhh4--,normal neutral plane4 ,this is a reference plane from which the lead is measured4 ,as shown1 the normal neutral plane lies at right angles to the lines of force of an undistorted field4 ,ques4 ,what may be said with respect to the angle of lead8 ,ans4 ,for sparkless commutation1 the angle of lead varies with the load4 ,if the field be much altered at full load1 it is evident that at half or quarter load it will not be nearly so much twisted1 hence the necessity for mounting the brushes on some kind of rocking device which will allow them to be shifted in different positions for different loads4 ,a desirable point1 then1 in dynamo design is to make the angle of lead at full load so small that it will not be necessary to shift the brushes much for variation of load4 ,this can be accomplished by making the field magnet field considerably more powerful than the armature field4 ,illustration3 ,,fig4 #bhi4--;,diagram illustrating the demagnetizing effect of armature reaction4 ,this results from the forward lead given the brushes in order to secure sparkless commutation4 ,demagnetizing ,effect of ,armature ,reaction4--,in the operation of a dynamo1 as previously explained1 the position of the brushes for sparkless commutation must be varied with the load2 that is1 for light load they should occupy a position practically midway between the poles and for a heavy load they must be moved a few degrees in the direction of rotation4 ,in other words1 the commutating plane must be more or less in advance of the normal neutral plane as shown in fig4 #bhi4 ,ques4 ,what is the effect of lead8 ,ans4 ,it produces a demagnetizing effect which tends to weaken the field magnets4 ,ques4 ,describe this demagnetizing effect in detail4 ,ans4 ,tracing the armature currents1 in fig4 #bhi according to ,fleming's rule1 it will be seen that current in inductors #a to #ah flow from the observer indicated by crosses representing the tails of retreating arrows and in inductors #ai to #cf1 toward the observer from the back of armature1 indicated by dots representing the points of approaching arrows4 ,in determining these current directions the inductors to the right of the neutral line are considered as moving downward1 and those to the left as moving upward4 ,the current in inductors #a to #ae and #ai to #cc1 tends to cross magnetize the magnetic field of the machine1 but the current in inductors #cd to #cf and #af to #ah tends to produce north and south poles as indicated4 ,these poles are in opposition to the field poles and tend to demagnetize them4 ,hence1 the inductors lying outside the two upright lines are known as cross magnetizing turns1 and those lying inside1 as demagnetizing turns4 ,the breadth of the belt of demagnetizing turns included between the two upright lines is clearly proportional to the angle of lead2 therefore1 the demagnetizing effect increases with the lead4 ,eddy ,currents2 ,lamination4--,induced electric currents1 known as eddy currents1 occur when a solid metallic mass is rotated in a magnetic field4 ,they consume considerable energy and often occasion harmful rise in temperature4 ,armature cores1 pole pieces1 and field magnet cores are specially subject to these currents4 ,illustration3 ,,fig4 #bij4--;,arago's experiment illustrating eddy currents4 ,arago found that if a copper disc be rotated in its own plane underneath a compass needle1 the needle was dragged around as by some invisible friction4 ,the explanation of this phenomenon1 known as ,arago's rotations1 is due to ,faraday1 who discovered that it was caused by induction4 ,that is1 a magnet moved near a solid mass of metal1 induces in it currents1 which1 in flowing from one point to another1 have their energy converted into heat1 and which1 while they last1 produce 7in accordance with ,lenz's law7 electromotive forces tending to stop the motion4 ,thus1 in the figure1 there are a pair of eddies in the part passing between the poles1 and these currents oppose the motion of the disc4 ,foucault showed by experiment the heating effect of eddy currents1 but such currents were known years before ,foucault's experiments1 hence they are incorrectly called ,foucault currents4 ,ques4 ,describe the formation of eddy currents4 ,ans4 ,in fig4 #bia1 a bar inductor is seen just passing from under the tip of the pole piece ,n of the field magnet4 ,noting the distribution of the lines of force1 it will be seen that the edge c d is in a weaker field than the edge a b1 hence1 since the two edges move with the same velocity1 the electromotive force induced along c d will be less than that induced along a b4 ,this gives rise to whirls or current eddies in the copper bar as shown4 ,illustration3 ,,fig4 #bia4--;,formation of eddy currents in a solid bar inductor4 ,on account of its appreciable size1 the field is sometimes weaker at one point than another1 hence the unequal electromotive forces thus produced will induce eddy currents4 ,ques4 ,what should be noted in seeking a remedy for eddy currents8 ,ans4 ,it should be noted that eddy currents are due to very small differences of pressure and that the currents are large only because of the very low resistance of their circuits4 ,ques4 ,what is the best means of reducing eddy currents8 ,ans4 ,lamination4 ,ques4 ,explain this mode of construction with respect to the bar inductor fig4 #bia4 ,ans4 ,in the case of a large bar inductor such as shown in fig4 #bia1 it could be replaced by a number of small wires soldered together only at the ends4 ,the layer of dirt or oxide on the outside of the wires will furnish sufficient resistance to practically prevent the eddy currents passing from wire to wire4 ,illustration3 ,,fig4 #bib4--;,eddy currents induced in a solid armature core4 ,eddy currents always occur when a solid metallic mass is rotated in a magnetic field1 because the outer portion of the metal cuts more lines of force than the inner portion1 hence the induced electromotive force not being uniform1 tends to set up currents between the points of greatest and least potential4 ,eddy currents consume a considerable amount of energy and often occasion harmful rise in temperature4 ,ques4 ,how should an armature core be laminated to avoid eddy currents8 ,ans4 ,it should be laminated at right angles to its axis4 ,fig4 #bib shows the induced eddy currents in a solid armature core1 and fig4 #bic shows the manner in which the paths of these currents are interrupted and the losses due to their effect diminished by the use of laminated cores4 ,illustration3 ,,fig4 #bic4--;,armature core with a few laminations showing effect on eddy currents4 ,in practice the core is made up of a great number of thin sheet metal discs1 about #ah gauge1 which introduces so much resistance between the discs that the formation of eddy currents is almost entirely prevented4 ,in fig4 #bic1 only five laminations or plates are indicated1 so as to show the sub-division of the eddy currents1 but in practical armatures1 the number of laminations or punchings ranges from #dj to #ff to an inch1 and brings the eddy current loss down to about one per cent4 ,a greater increase in the number of laminations per inch is not economical1 however1 owing to the difficulties encountered in the punching and handling of extremely thin sheets of iron1 and the loss of space between the plates4 ,armature cores constructed of the number of plates stated1 and forced together by means of screws and heavy hydraulic pressure1 contain from #hj to #ij per cent4 of iron1 and have a magnetic flux carrying capacity only from #e to #ae per cent4 less than when they are made of an equal volume of solid iron4 ,magnetic ,drag on the ,armature4--,whenever a current is induced in an armature coil by moving it in the magnetic field so as to cut lines of force1 the direction of the induced current is such as to oppose the motion producing it4 ,hence1 in the operation of a dynamo1 considerable driving power is required to overcome this magnetic drag on the armature4 ,illustration3 ,,fig4 #bid4--;,circular concentric magnetic field surrounding a conductor carrying a current4 ,if this conductor be moved across a magnetic field1 as between the poles of a magnet1 the lines of force will be distorted as in fig4 #bie1 which will oppose the motion of the conductor4 ,a conductor carrying a current is surrounded by a circular concentric magnetic field4 ,if now such a conductor1 with current flowing toward the observer as in fig4 #bid1 be placed in a uniform magnetic field1 a distortion of the magnetic lines will occur as shown in fig4 #bie4 ,the resulting mechanical actions are easily determined by remembering that the magnetic lines act like elastic cords tending to shorten themselves4 ,there is in fact a tension along the magnetic lines and a pressure at right angles to both1 proportional at every point to the square of their density4 ,illustration3 ,,fig4 #bie4--;,illustrating drag on armature inductors4 ,in moving a wire carrying a current through a magnetic field1 the lines of force are distorted1 and the effect on the wire is the same as though the magnetic lines were elastic cords tending to shorten themselves4 ,they1 therefore1 oppose the motion of the wire2 hence1 in dynamo operation1 more or less power is absorbed in overcoming this drag on the numerous inductors4 ,in the figure the inductor is being moved upward against the drag due to the magnetic field4 ,it is evident by inspection of the lines in fig4 #bie1 that there is a drag upon the conductor in the direction shown by the arrow4 ,smooth and ,slotted ,armatures4--,the inductors of an armature may be placed on a smooth drum or in slots cut in the surface parallel to the axis4 ,in the first instance1 the magnetic drag comes on the inductors and in the case of slots1 upon the teeth4 ,the effect of embedding the armature inductors in slots is to distort the magnetic field as shown in fig4 #bif4 ,most of the lines of force pass through the teeth1 thus1 not only are the inductors better placed for driving purposes1 but1 being screened magnetically by the teeth1 the forces acting on them are reduced1 the greater part of the magnetic drag being taken up by the core4 ,it should be noted that1 although screened from the field1 the inductors in a slotted armature cut magnetic lines precisely as if they were not protected4 ,the effect is as though the magnetic lines flashed across the slots from tooth to tooth1 instead of passing across the intermediate slot at the ordinary angular velocity4 ,comparison of ,smooth and ,slotted ,armatures4--,the slotted armature has the following advantages over the smooth type3 #a4 ,reduced reluctance of the air gap2 #b4 ,better protection for the winding2 #c4 ,inductors held firmly in place preventing slippage2 #d4 ,no magnetic drag on inductors2 #e4 ,no eddy currents in inductors2 #f4 ,better ventilation2 #g4 ,opposition to armature reaction4 ,due to increased density of flux through the teeth4 ,the disadvantages of slotted armatures may be stated as follows3 #a4 ,tendency of the teeth to induce eddy currents in the pole pieces2 #b4 ,increased self-induction of the armature coils2 #c4 ,greater hysteresis loss on account of denser flux in the teeth2 #d4 ,leakage of lines of force through the core1 especially in the case of partially enclosed slots4 ,illustration3 ,,fig4 #bif4--;,effect of slotted armature4 ,the teeth1 as they sweep past the pole face1 cause oscillations of the magnetic flux in the iron near the surface because the lines in the pole piece ,,pp tend to crowd toward the nearest teeth1 and will be less dense opposite the slots4 ,this fluctuation of the magnetic lines produce eddy currents in the pole faces unless laminated4 ,the armature inductors1 being screened from the field1 are relieved of the drag which is taken by the teeth4 ,magnetic ,hysteresis in ,armature ,cores4--,when the direction or density of magnetic flux in a mass of iron is rapidly changed a considerable expenditure of energy is required which does not appear as useful work4 ,for instance1 when an armature rotates in a bipolar field1 the armature core is subjected to two opposite magnetic inductions in each revolution2 that is1 at any one instant a north pole is induced in the core opposite the south pole of the magnet and a south pole in the core opposite the north pole of the magnet as indicated in fig4 #big by n and s4 ,accordingly1 if the armature rotate at a speed of #a1jjj revolutions per minute1 the polarity of the armature will be changed #b1jjj times per minute1 and result in the generation of heat at the expense of a portion of the energy required to drive the armature4 ,this loss of energy is due to the work required to change the position of the molecules of the iron1 and takes place both in the process of magnetizing and demagnetizing2 the magnetism in each case lagging behind the force4 ,core ,loss or ,iron ,loss4--,these terms are often employed to designate the total internal loss of a dynamo due to the combined effect of eddy currents and hysteresis1 but as the losses due to the former are governed by laws totally different from those applicable to the latter1 special analysis is required to separate them4 ,the eddy current loss per pound of iron in the armature core diminishes with the thinness of the laminated sheets1 and may be made indefinitely small by the use of indefinitely thin iron plates1 were it not for certain mechanical and economical reasons4 ,the loss due to hysteresis per pound of iron in the core1 does not vary with the thinness of the core plates2 it can be reduced only by the use of a material having a low hysteretic coefficient4 ,dead ,turns4--,the voltage generated in a dynamo with a given degree of field excitation is not strictly proportional to the speed1 but somewhat below on account of the various reactions4 ,that is1 the machine acts as though some of its revolutions were not effective in inducting electromotive force4 ,the name dead turns is given to the number of revolutions by which the actual speed exceeds the theoretical speed for any output4 ,again1 this term is sometimes used to denote that portion of the wire on an armature which comes outside the magnetic field and is therefore rendered ineffective in inducing electromotive force4 ,the number of dead turns is about #bj of the total number of turns4 ,illustration3 ,,fig4 #big4--,magnetic hysteresis in armature core4 ,unlike poles are induced in the core opposite the poles of the field magnet4 ,since on account of the rotation of the core the induced poles are reversed a thousand or more times a minute1 considerable energy is required to change the positions of the molecules of the iron for each reversal1 resulting in the generation of heat at the expense of a portion of the energy required to drive the armature4 ,self-induction in the ,coils2 ,spurious ,resistance4--,self-induction opposes a rapid rise or fall of an electric current in just the same way that the inertia of matter prevents any instantaneous change in its motion4 ,this effect is produced by the action of the current upon itself during variations in its strength4 ,in the case of a simple straight wire1 the phenomenon is almost imperceptible1 but if the wire be in the form of a coil1 the adjacent turns act inductively upon each other upon the principle of the mutual induction arising between two separate adjacent circuits4 ,ques4 ,what effect has self-induction on the operation of a dynamo8 ,ans4 ,it prevents the instantaneous reversal of the current in the armature coils4 ,that is1 the current tends to go on and in fact does actually continue for a brief time after the brush has been reached4 ,illustration3 ,,fig4 #bih4--;,distribution of magnetic lines through a ring armature4 ,since the lines follow the metal of the ring instead of penetrating the interior1 no electromotive force is induced in that portion of the winding lying on the interior surface of the ring4 ,there is1 therefore1 a large amount of dead wire or wire that is ineffective in inducing electromotive force2 this is the chief objection to the ring type of armature4 ,ques4 ,what becomes of the energy of the current at reversal8 ,ans4 ,the energy of the current in the section of the winding undergoing commutation is wasted in heating the wire during the interval when it is short circuited1 and as it passes on1 energy must again be spent in starting a current in it in the reverse direction4 ,there is1 then1 a lagging of the current in the armature coils due to self-induction4 ,ques4 ,what is spurious resistance8 ,ans4 ,this is an apparent increase of resistance in the armature winding1 which is proportional to the speed of the armature1 and due to the lagging of the current4 ,illustration3 ,,fig4 #bii4--;,distribution of magnetic lines through solid drum armature of a four pole machine4 ,armature ,losses4--,the mechanical power delivered to the pulley of a dynamo is always in excess of its electrical output on account of numerous mechanical and electrical losses4 ,mechanical losses result from3 #a4 ,friction of bearings2 #b4 ,friction of commutator brushes2 #c4 ,air friction4 ,the electrical losses may be classified as those due to3 #a4 ,armature resistance2 #b4 ,hysteresis2 #c4 ,eddy currents4 ,ques4 ,how do the mechanical and electrical losses compare8 ,ans4 ,the mechanical losses are small in comparison with the electrical losses4 ,ques4 ,what may be said with respect to friction8 ,ans4 ,the bearing friction varies with the load4 ,in calculating this loss not only must the weight of the armature be considered but also the belt tension and magnetic attraction in order to get the resultant thrust on the bearing4 ,friction of the brushes is very small and may be neglected4 ,a small loss of power is caused by the friction of the air on the armature4 ,the latter1 since it revolves rapidly1 acts to some extent as a fan1 and in some machines this fan action is made use of for ventilation and cooling4 ,ques4 ,how are the other losses determined8 ,ans4 ,the loss of power due to armature resistance is easily found by ,ohm's law1 but the hysteresis and eddy current losses1 known collectively as iron losses1 are not so easily determined4 ,if the magnetization curve of the particular quality of iron used for armature plates be known1 the hysteresis loss may be calculated approximately4 ,eddy current losses are the most important1 especially in large machines4 ,as previously explained1 in all the moving metal masses unless laminated1 there will be eddy currents set up if they cut magnetic lines4 ,power may be lost from this cause even in the metal of the shaft if there be leakage of magnetic lines into it4 ,,chapter ,,xx ,,commutation ,,and ,,the ,,commutator ,the act of commutation needs special study4 ,if it be incorrectly performed1 the imperfection at once manifests itself by sparks which appear at the brushes4 ,in the study of this chapter on commutation it would be advisable for the student to first review the basic principles of commutation as given in chapter ,,xiv1 which contains a brief and simple explanation of how the alternating current in the armature is converted into direct current by the action of the commutator4 ,ques4 ,what is the period of commutation8 ,ans4 ,the time required for commutation1 or the angle through which the armature must turn to commute the current in one coil4 ,ques4 ,upon what does the period of commutation depend8 ,ans4 ,upon the width of the brushes as shown in fig4 #cjj4 ,this fixes the angle through which the armature must revolve to commute the current in one coil4 ,this angle is formed1 as shown in the figure1 by two intersecting planes1 ,m and ,s1 which pass through the axis of the armature and the two edges of the brush4 ,commutation then1 begins at ,m and ends at ,s4 ,ques4 ,what is the position of the commutating plane with respect to ,m and ,s1 in fig4 #cjj8 ,ans4 ,it bisects the angle formed by the planes ,m and ,s4 ,illustration3 ,,fig4 #cjj4--;,armature with one brush in position to illustrate the period of commutation and commutating plane4 ,the latter is called commutating line by some writers4 ,the period of commutation depends on the thickness of the brush end in contact with the commutator4 ,careful distinction should be made between commutating plane1 neutral plane1 and normal neutral plane as defined elsewhere4 ,ques4 ,what is the the commutating plane8 ,ans4 ,an imaginary plane passing through the axis of the armature and the center of contact of the brush4 ,ques4 ,what two planes are referred to in stating the position of the brushes8 ,ans4 ,the normal neutral plane and the commutating plane4 ,the angle intercepted by these two planes represents the lead1 thus in stating that the brushes have a lead of #f1 means that the angle intercepted by the normal neutral plane and the commutating plane is #f4 ,illustration3 ,,fig4 #cja4--,the proper position of the brushes1 if there were no field distortion and self-induction in the armature coils1 would be in the normal neutral plane4 ,in the actual dynamo these two disturbing effects are present which makes it necessary to advance the brushes as shown in figs4 #cjb and #cjc to secure sparkless commutation4 ,ques4 ,what is the difference between the normal neutral plane and the neutral plane8 ,ans4 ,this is illustrated in figs4 #cja and #cjb4 ,the normal neutral plane is the position of zero induction assuming no distortion of the field as in fig4 #cja4 ,the neutral plane is the position of zero induction with distorted field as in fig4 #cjb and as is found in the actual machine2 the distortion is exaggerated in the figure for clearness4 ,illustration3 ,,fig4 #cjb4--;,brush adjustment for field distortion4 ,the effect of the latter is to twist the lines of force around in the direction of rotation1 thus maximum induction takes place in an inclined plane4 ,the brushes then must be advanced to the neutral plane which is at right angles to the plane of maximum induction4 ,this gives the proper position of the brushes neglecting self-induction4 ,ques4 ,what is the normal plane of maximum induction8 ,ans4 ,a plane1 #ij in advance of the normal neutral plane1 being the position of maximum induction with no distortion of field1 as in fig4 #cja4 ,ques4 ,what is the plane of maximum induction8 ,ans4 ,a plane #ij in advance of the neutral plane1 being the position of maximum induction in a distorted field as in fig4 #cjb4 ,illustration3 ,,fig4 #cjc4--;,brush adjustment for self-induction4 ,for convenience an electric current is regarded as having weight and hence possessing the property of inertia4 ,the current then during commutation cannot be instantly brought to rest and started in the reverse direction but these changes must be brought about gradually by an opposing force4 ,hence by advancing the brushes beyond the neutral plane as illustrated1 commutation takes place with the short circuited coil cutting the lines of force so as to induce a current in the opposite direction2 this opposes the motion of the current in the short circuited coil1 brings it to rest and starts it in the opposite direction1 thus preventing sparks4 ,figs4 #cja to #cjc should be carefully compared and thoroughly understood4 ,ques4 ,what should be noted with respect to the different planes8 ,ans4 ,the commutating plane should be carefully distinguished from the normal neutral plane and from the neutral plane1 as shown in fig4 #cjc4 ,commutation4--,in order to understand just what happens during commutation1 a section of a ring armature may be used for illustration1 such as shown in fig4 #cjd4 ,here the coils ,a1 ,b1 ,c1 ,d1 ,e1 are connected to commutator segments #a1 #b1 #c1 #d1 and the positive brush is shown in contact with two segments #b and #c1 the brush being in the neutral position4 ,currents in the coils on each side of the neutral line flow to the brush through segments #b and #c2 the brush then is positive4 ,illustration3 ,,fig4 #cjd4--;,commutation4 ,this takes place during the brief interval in which any two segments of the commutator are bridged by the brush4 ,the coil connecting with the two segments under the brush is thus short circuited4 ,during commutation the current in the short circuited coil is brought to rest and started again in the reverse direction against the opposition offered by its so called inertia1 or effect produced by self-induction4 ,now1 as the armature turns1 the commutator segments come successively into contact with the brush4 ,in the figure1 segment #c is just leaving the brush and #b is beginning to pass under it1 hence1 for an instant the coil ,c is short circuited4 ,ques4 ,in fig4 #cjd1 what are the current conditions8 ,ans4 ,previous to contact with segment #b1 current flowed in coil ,c in the same direction as in coil ,b4 ,ques4 ,what occurs while the brush is in contact with segments #b and #c8 ,ans4 ,during this brief interval1 the current in ,c is stopped and started again in the opposite direction4 ,similarly each coil of the armature as it passes the brush will be short circuited and have its current reversed4 ,this is known as commutation4 ,ques4 ,what is the effect of field distortion with respect to commutation8 ,ans4 ,the neutral plane no longer coincides with the normal neutral plane but is advanced in the direction of rotation of the armature as shown in fig4 #cjb4 ,the reaction of the poles ,n' and ,s' of the armature field on the poles ,s and ,n of the main magnetic field tends to crowd the lines of force into the upper pole face of the south pole of the magnet1 and into the lower pole face of the north pole4 ,this effect is due to the strong magnetic attraction between the opposite poles ,s and ,n' and ,n and ,s'1 and the equally strong repulsion between like poles ,n and ,n' and ,s and ,s'4 ,hence1 the plane of maximum induction no longer coincides with the normal plane of maximum induction1 but is advanced in the direction of rotation1 depending upon the strength of the armature current1 being shifted forward for an increase of current1 and backward for a decrease of current4 ,this distortion of the field and the consequent shifting of the plane of maximum induction naturally results in the shifting of the neutral plane from the vertical position to the inclined position as shown4 ,position of the ,brushes2 ,sparking4--,in accordance with the laws of electromagnetic induction1 if the bipolar ring armature shown in fig4 #cja be rotated in the direction indicated by the arrow the armature current entering at the brush ,e will divide1 one part passing through the coils on the right half of the ring1 and the other part through the coils on the left half of the ring1 to the brush ,f1 from which the total current will pass out1 urged by the full value of the electromotive force induced in all the coils on both halves of the ring4 ,illustration3 ,,figs4 #cje to #cjh--;,improper brush adjustment resulting in excessive sparking4 ,when the brushes are not advanced far enough1 commutation takes place before the short circuited coil reaches the neutral plane1 hence1 its motion is not changed with respect to the magnetic field so as to induce a reverse current till after commutation4 ,there is then no opposing force1 during commutation1 to stop and reverse the current in the short circuited coil1 and when the brush breaks contact with segment #a1 as in fig4 #cjh1 the momentum of the current in coil ,f causes it to jump the air gap from segment #a to segment #b and the brush1 against the enormous resistance of the air1 thus producing a spark whose intensity depends on the momentum of the current in coil ,f4 ,sparking1 if allowed to continue1 will injure the brushes and commutator segments4 ,again1 if the brushes be placed at the points ,g and ,h1 each half of a current entering at ,g1 will pass through one-half of the coils on the left side and one-half of the coils on the right side of the ring1 so that each half of the current will be urged forward by an electromotive force equal to the electromotive force tending to force it back1 and therefore1 no current will pass in or out through the brushes4 ,from these considerations it is obvious that the proper position for the brushes would be in the normal neutral plane1 were it not for the disturbing effects of armature reaction and self induction of the current4 ,ques4 ,should the brushes of a dynamo be placed in the neutral plane8 ,ans4 ,no4 ,ques4 ,why not8 ,ans4 ,the brushes must be advanced beyond the neutral plane to prevent sparking4 ,ques4 ,what is the cause of sparking at the brushes8 ,ans4 ,it is due to self-induction in the coil undergoing commutation4 ,ques4 ,explain the effect of self-induction in detail4 ,ans4 ,when commutation takes place with the brushes in the neutral plane as in fig4 #cjd1 there will be no voltage induced in the short circuited coil ,c4 ,the current1 therefore1 which flowed in coil ,c before it was short circuited will cease1 and as segment #c breaks contact with the brush1 it will be thrown as a perfectly idle coil upon the right hand half of the ring in which a current is flowing toward the brush4 ,moreover1 the current which was flowing through ,d and #c directly to the brush1 must suddenly traverse the longer path through the idle coil ,c4 ,now1 on account of self-induction1 the current acts in precisely the same manner as though it had weight2 that is3 ,it cannot be instantly stopped or started4 ,illustration3 ,,figs4 #cji to #cac4--;,how sparkless commutation is obtained by advancing the brushes beyond the neutral plane2 commutation progressively shown4 ,illustration3 ,,fig4 #cji4--;,commutation begins2 current flows up both sides of the armature1 uniting at ,s and flowing to the brush through commutator segment #a as indicated by the arrow4 ,illustration3 ,,fig4 #caj4--,segment #b has come into contact with the brush and coil ,f1 in which commutation is taking place1 is now short circuited4 ,the current now divides at ,m1 part passing to the brush through segment #b1 and part through coil ,f and segment #a4 ,although coil ,f is short circuited and having passed the neutral plane1 is cutting the lines of force so as to induce a current in the opposite direction1 it still continues to flow with unchanged direction against these opposing conditions4 ,this is due to self-induction in the coil which resists any change just as the momentum of a heavy moving body1 such as a train of cars1 offers resistance to the action of the brakes in retarding and stopping its motion4 ,therefore1 when segment #c leaves the brush1 the current will not instantly change its path and flow through ,c1 but will be urged by its momentum1 and jump the air gap between the brush and segment #c1 thus producing a spark4 ,illustration3 ,,fig4 #caa4--,segment #b has moved further under the brush1 and the opposition offered to the forward flow of the current in the short circuited coil ,f by the reverse induction in the magnetic field to the right of the neutral plane has finally brought the current in ,f to rest4 ,the currents from each side of the armature now flow direct to the brush through their respective end segments #a and #b4 ,illustration3 ,,fig4 #cab4--,segment #a is now almost out of contact with the brush4 ,a current has now been started in the coil ,f in the reverse direction due to induction in the magnetic field to the right of the neutral plane2 it flows to the brush through segment #b4 ,the current has not yet reached its full strength in ,f1 accordingly1 part of the current coming up from the right divides at ,s and flows to the brush through segment #a4 ,illustration3 ,,fig4 #cac4--;,completion of commutation in segments #a and #b2 the brush is now in full contact with segment #b1 the current in coil ,f has now reached its full value1 hence the current flowing up from the right no longer divides at ,s but flows through ,f and segment #b to the brush4 ,if the current in ,f had not reached its full value1 at the instant segment #a left contact with the brush1 it could not immediately be made to flow at full speed any more than could a locomotive have its speed instantly changed4 ,this1 as previously explained1 is due to self-induction in the coil or the so called inertia of the current which opposes any sudden change in its rate of flow or direction4 ,accordingly that portion of the current which was flowing up from the right and passing off at ,s to the brush through segment #a as in fig4 #cab1 would1 when this path is suddenly cut off as in fig4 #cac1 encounter enormous opposition in coil ,f4 ,hence1 it would momentarily continue to flow through segment #a and jump the air gap between this segment and the brush1 resulting in a more or less intense spark depending on the current conditions in coil ,f4 ,ques4 ,how may this sparking be prevented8 ,ans4 ,if the brushes be given additional lead1 that is shifted further to the right to some position as ,n ,n1 fig4 #cjd1 coil ,c will not remain idle during the interval it is short circuited1 but will cut the magnetic lines in such a way as to induce a current in the reverse direction through it4 ,under these conditions1 when segment #c breaks contact with the brush1 the current flowing through ,d does not encounter an idle coil1 but one in which a current is flowing in the same direction1 hence1 the tendency to jump the air gap and produce a spark is reduced2 with proper adjustment of the brushes1 there will be no sparking4 ,ques4 ,what is the objection to very thin brushes8 ,ans4 ,time must be allowed for reversal of the current1 hence the brushes must not be so thin as merely to bridge the insulation between segments4 ,ques4 ,what is the effect of lead8 ,ans4 ,there is usually much sparking when the lead is too small2 a little sparking when too great1 and no sparking when just right4 ,if the lead be excessive1 there is a waste of energy due to the generation of a larger reverse current in the short circuited coil than is necessary4 ,fixed ,position of ,brushes4--,the condition for sparkless commutation is that the current in the short circuited coil be reduced to zero1 and increased in the opposite direction up to the same value as that in the next coil leading4 ,if the brushes are to remain in a fixed position1 this condition will only be realized at the particular load for which the brushes are set4 ,thus1 if the brushes be set for the average load1 the reversing field will not be correct for either a weaker or stronger load4 ,hence1 sparkless commutation with fixed brushes must be due to some other factor4 ,ques4 ,what may be said with respect to carbon brushes8 ,ans4 ,since carbon possesses a high resistance1 the drop will vary greatly with the contact area1 thus affecting a difference of potential in the two segments passing under the brush and it is largely to this that sparkless commutation is due4 ,ques4 ,what is the effect of resistance on commutation8 ,ans4 ,in fig4 #cjd during commutation1 that is1 while the brush contacts with any two segments1 as #b and #c1 the currents coming up through the winding on either side of the neutral plane are offered two paths to the brush3 #a1 direct to brush through the connecting segment1 or #b1 across the short circuited coil and adjacent segment4 ,thus1 on the right side3 to brush through segment #c1 or across coil ,c and adjacent segment #b4 ,the current will take the path of least resistance4 ,at the beginning of commutation1 almost the entire brush area being in contact with segment #c1 the contact resistance of this segment will be much less than for segment #b2 hence1 not only will the current at the right flow through #c1 but also the current at the left after first traversing the short circuited coil4 ,as commutation progresses1 the area of contact of #c decreases while that of #b increases1 and the respective resistances vary in inverse proportion4 ,likewise the tendency of the current in the left half of the winding to take the longer path through coil ,c and segment #c to the brush gradually decreases1 becoming zero when the two contact areas become equal4 ,during the second half of the period of commutation1 the contact area of segment #b becomes greater and of #c1 less2 thus the resistance of #b is lowered1 and that of #c increased4 ,accordingly1 all of the current at the left will flow through segment #b1 and the current at the right will flow through ,c and #b rather than through #c4 ,in this way the current is reversed in ,c1 and1 if the brush be broad enough to allow a sufficient time interval1 the current in ,c is built up to its full value before segment #c leaves the brush1 thus securing sparkless commutation4 ,this contact resistance factor in sparkless commutation is illustrated in figs4 #cad to #cah1 it being assumed that during commutation1 the brush contact resistance is inversely proportional to the area of contact1 and that the winding is free of resistance and inductance4 ,the current is taken as #dj amperes1 in which case #bj amperes will flow from each side of the winding to the brush4 ,in fig4 #cad the instant before commutation begins all the current will flow through segment ,a4 ,at the end of the first quarter of the period of commutation1 fig4 #cae1 #cj amperes will flow from the right to brush through ,a1 and from the left1 #aj amperes through the short circuited coil via ,a and #aj amperes through ,b4 ,at the end of the second quarter or half period1 fig1 #caf1 the current through each half of the winding will flow to the brush through these respective segments4 ,at the end of the third quarter1 fig4 #cag1 the current from the right will divide1 #aj amperes going through ,a1 and #cj amperes traversing the short circuited coil and out through ,b4 ,the entire current from the left will flow through segment ,b4 ,at the end of the fourth quarter1 fig4 #cah1 or completion of the period the current from each half of the winding will flow to the brush through ,b4 ,illustration3 ,,figs4 #cad to #cah4--;,brush contact resistance theory of commutation1 neglecting self-induction and resistance in the coils4 ,the total current is assumed to be #dj amperes made up of #bj amperes flowing toward the brush from the coils on the right and #bj amperes from the coils on the left4 ,during commutation1 that is1 the interval during which the brush contacts with any two adjacent segments of the commutator1 the current is assumed to vary directly as the contact area4 ,illustration3 ,,fig4 #cad4--;,beginning of commutation2 segment ,a is entirely under the brush1 and ,b is at the initial point of contact4 ,for this position the currents from both sides flow to the brush through segment ,a4 ,illustration3 ,,fig4 #cae4--,one-quarter period of commutation4 ,one quarter of the brush area is in contact with ,b and three quarters in contact with ,a2 hence1 #aj amperes will flow through ,b and #cj amperes through ,a4 ,illustration3 ,,fig4 #caf4--,second quarter of commutation period4 ,the brush now contacts equally with both segments1 hence #bj amperes will flow through each segment4 ,illustration3 ,,fig4 #cag4--,third quarter of commutation period4 ,three quarters of the brush area is in contact with segment ,b and one quarter with segment ,a2 accordingly1 #cj amperes will flow through ,b and #aj amperes through ,a4 ,illustration3 ,,fig4 #cah4--;,completion of commutation4 ,the brush is in full contact with segment ,b and at the point of breaking contact with ,a1 hence the entire current from both sides or #dj amperes will flow through ,b4 ,illustration3 ,,figs4 #cai to #cbc4--;,brush contact theory of commutation for case in which the brush covers two segments of the commutator4 ,fig4 #cai beginning of commutation2 fig4 #cbj one-quarter period2 fig4 #cba1 one-half period2 fig4 #cbb1 three-quarter period2 fig4 #cbc completion of commutation4 ,ques4 ,what is the effect of increasing the degree of contact of the brushes8 ,ans4 ,it lengthens the period of commutation1 and permits it to start in one coil before the preceding coil has entirely passed through this stage4 ,the effect of changing the degree of contact is shown in figs4 #cai to #cbc1 in which the width of the brush is made equal to that of two segments4 ,construction of ,commutators4--,the commutator for a closed coil armature consists of a number of segments or ,l-shaped bars ,c of drop forged hard drawn copper assembled around a tubular iron hub as shown in figs4 #cbd and #cbe4 ,the bars are held in position by the nuts ,e1 and washers ,f1 screwed on the ends of the tube ,d4 ,the bars are insulated from each other and from the washers by mica as shown by the heavy lines ,g1 and they are also insulated from the tube either by a tube of mica ,h1 or by a sufficient air space4 ,the ends of the sections of winding are connected to the vertical portions of the bars ,k1 by insertion in the slots ,l1 where they are securely held in place by means of the binding screws1 which for greater security are soldered together1 and may be released from the slots1 whenever necessary1 by the application of a hot soldering iron4 ,illustration3 ,,figs4 #cbd and #cbe4--,side and end sectional views of commutator showing construction4 ,the parts are3 ,c1 segments2 ,d1 tubular iron hub2 ,e1 end nuts2 ,f1 clamps2 ,g1 insulation2 ,l1 riser connection4 ,it is very important that all the parts of the commutator should be fitted together perfectly and screwed up tightly1 in order to prevent looseness4 ,commutator segments are often made with the washers ,e1 projecting beyond the ends1 but such construction reduces the effective length of the commutator1 therefore the under cut form of bar is preferable4 ,in the construction of commutators1 the conditions of operation require that there be3 #a4 ,adequate insulation2 ,it is necessary to have good insulation between each segment1 and a specially good insulation between the segments and the hub or sleeve on which they are mounted2 also between the segments and end clamps4 ,the insulating material must not absorb moisture1 hence asbestos1 plaster1 or vulcanized fibre are not used4 ,the end insulating rings are usually built up of mica and shellac1 moulded while hot under pressure to the correct shape4 ,illustration3 ,,fig4 #cbf4--;,front view of ,western ,electric ,commutator for bar wound armature4 ,this commutator is made of hard drawn copper and insulated throughout1 ventilating spaces being provided near the shaft4 #b4 ,rigidity against centrifugal force2 ,since the segments are subject to centrifugal force1 they must be securely clamped in place4 ,screws cannot be used1 for that would destroy the insulation4 ,they are therefore held in place by insulated clamps as shown in fig4 #cbd4 ,these clamps should be strong and capable of holding the segments firmly in position1 for if a segment should rise out of its place through centrifugal force1 it would disturb the action of the brush and cause sparking4 #c4 ,provision for wear4 ,the segments should be of considerable radial depth1 so that the commutator may be turned down from time to time to preserve its circular form4 ,illustration3 ,,fig4 #cbg4--,sectional view of a ,general ,electric commutator4 ,the segments are of rolled or forged copper and are separated by soft mica insulating sheets4 ,this mica must wear down evenly with the copper1 hence its consistency is important4 ,the segments are wedge shaped so that when drawn radially inward they support each other like the stones of an arch4 ,they are drawn together by hollow cone collars which bear upon lugs projecting from the ends of the segments4 ,these lugs are turned to form a smooth cone after the segments are assembled4 ,the collars are insulated with mica from the segments and they are held in place by nuts upon the commutator shell or by bolts passing from end to end under the segments4 ,the segments are also provided with lugs for connection to the windings4 ,points ,relating to ,commutators4--#a4 ,the number of commutator segments depends on the scheme of winding and on the number of sections in the armature winding4 #b4 ,increasing the number of bars diminishes the tendency to spark1 and lessens the fluctuations of the current4 ,there are two practical reasons for not using a very great number of segments3 it increases the cost1 and in small machines the segments would be so thin that a brush of the proper thickness to collect the current would lap over1 or bridge several segments4 ,types of ,commutator4--,commutators are made in various forms1 but they may be grouped into two general types3 #a4 ,commutators for closed coil armatures2 ,these consist of a large number of segments or bars1 insulated from each other and varying in number according to the scheme of armature winding1 and on the number of sections into which that winding is grouped4 ,illustration3 ,,fig4 #cbh4--;,a large current low voltage bipolar dynamo built for electrolytic work and here shown to illustrate the large size commutator and brushes necessary to collect the large current4 ,carbon brushes would not be suitable for this class of machine because even with copper brushes1 whose conductivity is much higher than carbon1 the commutator must be of considerable size to give the required brush contact area4 ,the contrast between the axial lengths of the armature and the commutator is very marked4 ,the rocker construction is of the ordinary type1 and heavy flexible cables conduct the current from the brush holders to the fixed terminals4 ,the machine here illustrated gives #caj amperes at #g volts when running at a speed of #adjj ,r4 ,p4 ,m41 corresponding to an output of #b4ag kilowatts4 #b4 ,commutators for open coil armatures2 ,this form of commutator is used on some machines designed especially for arc lighting1 such as the ,brush and ,thompson-,houston machines4 ,they consist of a comparatively small number of segments each of which covers a wide angle1 and are separated from each other by air gaps4 #c4 ,the segments should be of considerable depth to permit returning occasionally so that their circular form may be preserved2 #d4 ,the insulating material must be such that it will not absorb oil or moisture2 ,mica is best adapted for insulation1 but as there are a great many varieties1 differing greatly in hardness and other equalities1 it is important to select the kind that wears at the same rate as the segments4 ,if the mica be too hard1 the wearing of the segments will leave it projecting and prevent proper contact with the brushes2 again1 if the mica be too soft1 it will result in furrows or depressions between the segments into which copper dust will collect1 causing short circuits4 ,,chapter ,,xxi ,,brushes ,,and ,,the ,,brush ,,gear ,with respect to construction1 brushes may be broadly classified as3 #a1 those made of metal1 and2 #b1 those made of carbon4 ,there are several varieties of metal brush1 such as3 #a4 ,gauze brushes2 #b4 ,wire brushes2 #c4 ,laminated or strip brushes4 ,gauze ,brushes4--,these are very flexible and yielding1 their use being attended with little wear of the commutator4 ,ques4 ,what is the construction of a gauze brush8 ,ans4 ,a gauze brush is made up of a sheet of copper gauze1 folded several times1 with the wires running in an oblique direction1 so as to form a solid flat strip of from to inch in thickness1 increasing with the volume of the current to be collected4 ,ques4 ,what is the object of folding the gauze with the wires running in oblique directions8 ,ans4 ,it is to prevent the ends of the brush fraying or threading out1 which would be the case if the gauze were folded up in any other manner4 ,ques4 ,what are the features of gauze brushes8 ,ans4 ,they make good contact1 but are quite expensive4 ,they may be set either tangentially or radially1 the latter preferably1 since the point of contact remains the same as the brushes wear away4 ,illustration3 ,,figs4 #cbi to #ccb4--,various forms of brush4 ,fig4 #cbi gauze brush2 fig4 #ccj laminated or strip brush2 fig4 #cca strip and wire brush as used on the early ,edison machines2 fig4 #ccb carbon brush4 ,carbon is preferred to copper for brushes on account of the reduction of sparking secured by its use4 ,wire ,brushes4--,this class of brush1 which was extensively used before the invention of the gauze brush1 is made up of a bundle of brass or copper wires1 laid side by side and soldered together at one end4 ,since wire brushes are harder than the gauze brush1 they are more liable to cut or score the commutator1 and are also more troublesome to trim4 ,laminated or ,strip ,brushes4--,these probably represent the simplest form of brush1 but are not extensively used owing to the lack of flexibility4 ,they consist of a number of strips of copper or brass1 laid one upon the other and soldered at one end1 as in fig4 #ccj4 ,illustration3 ,,fig4 #ccc4--;,general ,electric brush holder4 ,the brush holder yoke consists of a cast iron ring of elliptical section1 supported from the bracket of the end shield in such a manner as to facilitate the shifting of the brushes4 ,it is provided with a suitable handle1 and may be fastened in any position by means of a thumb nut on the outside of the bracket4 ,it is so constructed that the tension on the individual brush can be adjusted without lifting the brush from the commutator and without the use of tools4 ,the brush can be removed while the machine is running1 without moving the holder on the stud and without disturbing any other brush4 ,removal of the brushes for inspection can be accomplished without permanent change in the adjustment of the tension of the brush holder spring4 ,the connection between the brush and stud is made through a flexible copper connection4 ,ques4 ,what name is generally given to strip brushes8 ,ans4 ,they are commonly and erroneously called tangential brushes1 but they are really beveled at the end and set inclined to the line of tangency so that the ends of all the sheets will make contact4 ,in the ,brush and ,thomson-,houston arc dynamos1 in which the current is limited to ten amperes1 the brushes consist of a simple strip of flexible sheet copper1 the ends of which are slit in a number of places so as to insure contact at several points4 ,illustration3 ,,fig4 #ccd4--;,crocker-,wheeler brush holder4 ,the carbon brush ,b is firmly clamped in the box ,c by two screws which bear on a sheet of brass to protect the carbon from being broken by the ends of the screws4 ,the box ,c is carried by four flexible springs ,s ,s1 one at each corner and formed of hard copper leaves4 ,these are fixed at one end to the box and at the other to the solid base which is in one piece with the spoke attached to the rocker ring4 ,an adjusting screw passes through appropriate lugs on the box ,c and loosely through the head ,a of a fixed arm a4 ,between the lower surface of a and the upper lug on the box ,c is placed the pressure spring4 ,carbon ,brushes4--,when metallic brushes are used upon the commutators of high tension machines1 they frequently give rise to excessive sparking and also heating of the armature1 the metallic dust given off appearing to lodge between the segments of the commutator1 thus partially short circuiting the armature4 ,to obviate this1 carbon brushes are extensively used in such dynamos1 this material being found very effectual in the prevention of sparking4 ,ques4 ,what is the usual form of carbon brushes8 ,ans4 ,they are usually in the form of oblong blocks4 ,illustration3 ,,fig4 #cce4--,perspective views of ,crocker-,wheeler brush holder4 ,this holder is of the parallel type in which the brushes may be adjusted without affecting the lead4 ,each brush is held rigidly in its box and there are no sliding contacts in the path of the current4 ,the holder is further described under fig4 #ccd4 ,ques4 ,how are they adjusted on the commutator8 ,ans4 ,they are set butt end on the commutator1 and fed forward as they wear away by means of a spring holder4 ,ques4 ,why are carbon brushes so extensively used8 ,ans4 ,because they are the only form of brush that will give good commutation with fixed lead4 ,ques4 ,what may be said of the different grades of carbon in use for brushes8 ,ans4 ,the very soft carbon leaves a layer of graphitic matter on the commutator1 and at high voltages1 this may cause sparking2 such grade of carbon should only be used on low voltage machines4 ,illustration3 ,,fig4 #ccf4--,western ,electric box type brush holder4 ,the box which holds the brush is broached to allow the brush to slide freely1 but not loosely1 to and from the commutator against which it is normally held by a lever acting directly upon the brush head4 ,this avoids the possibility of uneven bearing on the commutator1 as the brushes are allowed very slight lateral or angular motion4 ,the adjustment of a brush is also simplified after it has been removed and then replaced4 ,tension on the brush head is obtained by a special spring which maintains any given tension for which it may be set4 ,an auxiliary flat steel spring on the lower side of the lever acts as a shock absorber between the lever and the brush head1 absorbing all minor vibrations caused by a worn commutator4 ,side contact between brush and brush holder is not relied upon to carry the current1 flexible copper pigtails performing this function to the exclusion of sliding contacts or tension springs1 in order to reduce the brush loss4 ,it is not necessary to take the brush rigging apart or loosen cable connections when it is desired either to remove or reverse the brushes to change the direction of armature rotation4 ,ques4 ,how are the ends of carbon brushes treated and why8 ,ans4 ,they are usually covered at their upper part with a coating of electro-deposited copper to insure good contact with the holder4 ,comparison of ,copper and ,carbon ,brushes4--,copper brushes tend to tear and roughen the surface of the commutator1 while carbon brushes tend to keep the surface smooth4 ,copper causes more wear of the commutator than carbon4 ,with carbon brushes1 the armature may be run in either direction4 ,the resistance of carbon being greater than copper1 there is less short circuiting caused by carbon particles than by those of copper4 ,illustration3 ,,fig4 #ccg--,westinghouse brush holder4 ,it is made of brass1 cast in one piece1 and of standard sliding type with a shunt of braided copper wire directly connected to a clamp on each brush and to the solid portion of the holder1 where it is held by a screw4 ,this shunt relieves the spring of heavy currents4 ,the holder is so arranged as to be easily accessible for adjustment1 cleaning and renewal of carbons4 ,proper tension is provided by spiral strap springs so mounted as to eliminate friction and give uniform pressure over a wide working range4 ,the spring tension is readily adjusted by a simple ratchet arrangement4 ,ques4 ,what is the chief merit of carbon brushes8 ,ans4 ,they give less sparking than other types4 ,ques4 ,how has the construction of carbon brushes been varied8 ,ans4 ,since1 for minimum sparking1 it is only necessary that the brush have high resistance in the region near its edge1 attempts have been made to increase the conductivity of the other portions by combining with the carbon1 copper sheets or wires4 ,ques4 ,what are the objections to carbon brushes8 ,ans4 ,they are easily broken and not being flexible1 vibration1 or any roughness of the commutator will cause bad contact4 ,illustration3 ,,fig4 #cch4--;,holzer ,cabot multiple brush holder4 ,each brush is fastened securely to a machined surface by one or two machine screws1 making a positive contact4 ,several strips of flexible copper of ample section to carry the current are interposed between the part of the holder carrying the brush and the portion clamping the stud1 no sliding contact or spring being therefore required to carry any current4 ,the brushes are proportioned for a carrying capacity of not more than #be amperes per square inch of brush surface4 ,the brush can be adjusted to any degree of tension during the operation of the machine if necessary4 ,each holder is insulated in such a manner that no short circuit can occur if the holder be accidentally tipped backward while the operator is changing a brush or cleaning the commutator during a run4 ,ques4 ,for what class of machine are carbon brushes specially adapted8 ,ans4 ,for machines furnishing a small current at high pressure4 ,when carbon brushes are used1 it is desirable that the current be small1 because1 on account of the low conductivity of the carbon1 more contact area is necessary than with copper for equal current transmission4 ,for fixed lead and fluctuating currents1 carbon brushes should be used4 ,ques4 ,for what class of machine are copper brushes especially adapted8 ,ans4 ,for machines furnishing large current at low pressure1 as in fig4 #cbh4 ,size of ,brushes4--,the number of brush sets depends upon the number of poles of the machine1 but there may be several brushes in each set4 ,it is usual1 except in the smallest machines1 to place at least two brushes exactly similar side by side instead of one broad brush1 thus allowing one brush to be removed for trimming or renewal while the machine is running4 ,moreover1 better contact is secured by this sub-division1 because a slight elevation in the commutator surface at one point may slightly raise one brush of a set at each revolution without much harm1 while with one broad brush1 the entire brush would be lifted1 causing bad sparking4 ,ques4 ,what determines the number of brushes in each set8 ,ans4 ,it depends upon the current capacity1 size of machine1 and judgment of the designer4 ,ques4 ,what may be said with respect to the dimensions of the brushes8 ,ans4 ,no general rule can be given for breadth and thickness of brush4 ,the contact face must clearly be wider than the thickness of the insulation between commutator segments1 since the period of commutation must last an appreciable interval of time on account of self-induction4 ,ques4 ,what should be the minimum width of the brush contact face8 ,ans4 ,it may be taken as one and one-half times the thickness of the commutator segments4 ,ques4 ,how wide should a carbon brush contact be8 ,ans4 ,the brush should be thick enough to cover two and one-half commutator segments4 ,the thickness should in no case be excessive on account of the loss due to heating1 which results from the difference of potential at the forward and rear edge of the brush4 ,illustration3 ,,fig4 #cci4--;,contact angle for the different types of brush4 ,at ,a is shown a brush with tangential contact1 and at ,b1 a so called tangent brush2 the latter is properly called an inclined brush4 ,sheet copper brushes are set tangentially as at ,a1 and gauze brushes inclined as at ,b4 ,carbon brushes are placed radially as at ,c when mounted in box holders1 and inclined opposite to the direction of rotation when used with reaction holders4 ,contact ,angle of ,brush4--,this may be defined as the angle which the brushes make with the commutating plane as shown in fig4 #cci4 ,the several kinds of brush1 together with the varied conditions of operation require different contact angles ranging from zero to #ij4 ,thus in the figure1 a copper strip brush may lie at #ij or tangentially as at ,a4 ,wire or gauze brushes should make a more or less acute angle as at ,b1 in order to present the end and not the side of the brush to the commutator4 ,carbon brushes may be placed end on or radially as at ,c1 which is the position almost universally used in the case of traction or other reversing motors4 ,sometimes the carbon brush is inclined as at ,d1 in order that the revolving commutator may tend to push the brush against its supports and thus ensure better contact4 ,brush ,contact4--,the relation between contact pressure1 contact resistance1 and friction of brushes varies greatly for different kinds of brush4 ,copper brushes will carry from #aej to #bjj amperes per square inch of contact surface2 and carbon brushes from #dj to #gj amperes per square inch4 ,the usual contact pressure is #a4be to #a4e pounds per square inch for copper brushes1 and #a4e to #b pounds per square inch for carbon brushes4 ,the rim velocities of commutators vary from #a1ejj to #b1ejj feet per minute1 the velocity usually increasing with the size of the machine4 ,illustration3 ,,fig4 #cdj4--;,bissell double brush holder4 ,flexible cables carry the current between the brushes and holders4 ,this holder works equally well for forward or reverse rotation4 ,two or more holders are used on each stud except for the two smallest frames4 ,the construction permits of adjustment or renewal of brush while the machine is in operation4 ,sufficient contact area of brush is provided to permit running on one carbon at ordinary loads in case the other become worn or inoperative4 ,ques4 ,what is the drop in voltage at the brushes8 ,ans4 ,for carbon brushes it is about ,o4#h to #a4j volt at each contact1 or #a4f to #b4j volts for the two1 positive and negative1 contacts of a machine4 ,this value is not materially affected by placing a number of brushes in parallel or by using several sets1 as in the case of multipolar machines1 as such arrangement merely reduces the current density1 and since the contact resistance varies in the inverse ratio1 their product remains nearly constant4 ,ques4 ,what may be said of the friction of the brushes8 ,ans4 ,the coefficient of friction of brushes is about 4#b to4#be for copper and 4#c for carbon4 ,illustration3 ,,fig4 #cda--,western ,electric brush gear4 ,the brush holders carry carbon brushes and are so designed that the brushes may be firmly clamped in position and also be capable of independent adjustment4 ,any brush can be removed while the machine is in operation without disturbing the others and without moving the holder on the stud4 ,ques4 ,how many watts are lost at the brushes8 ,ans4 ,the watt loss is equal to #a4f to #b4 volts for carbon multiplied by the total current carried4 ,the watt lost on account of friction may be calculated by the formula3 774#c #gdf7_/ccjjj7 7,p ,s7 watts lost by carbon friction1 in which ,p is the total pressure in pounds on the commutator1 and ,s1 the rim velocity of the commutator in feet per minute4 ,the losses due to contact resistance and brush friction are very liable to be greatly increased above the values that may be obtained by the preceding methods1 if the commutator and brushes are dirty and rough1 or not in good condition4 ,illustration3 ,,figs4 #cdb to #cde4--,various types of brush holder4 ,fig4 #cdb1 arm or lever type2 fig4 #cdc1 spring arm type2 fig4 #cdd1 box type2 fig4 #cde1 reaction type4 ,brush ,holders4--,these are devices employed to hold the brushes against the commutator with the proper pressure4 ,they differ considerably in various types of machine1 hence1 no general rules can be given with respect to their construction or use1 but any brush holder must fulfill the following requirements3 #a4 ,it must hold the brush securely and at the same time feed it forward as it wears away so as to maintain a proper contact2 #b4 ,it must hold the brush at the proper contact angle2 #c4 ,it must be capable of being raised from the commutator1 and held out of contact by some form of catch2 #d4 ,it must be so constructed that the brush can be easily removed for cleaning or renewal2 #e4 ,the spring pressure must be adjustable2 #f4 ,the brush holders themselves must be carried on a rocker arm1 or rocker ring4 ,it is desirable that brush holders be capable of individual adjustment1 so that each may be set at its own point of minimum sparking4 ,a few forms of brush holder are illustrated in figs4 #cdb to #cde4 ,the various kinds of brush holder may be divided into four types3 #a4 ,arm or lever type2 #b4 ,spring arm type2 #c4 ,box type2 #d4 ,reaction type4 ,in the arm or lever type the brush is firmly attached to the extremity of a rigid arm capable of movement about the brush spindle1 except in so far as it is restrained by a spring as in fig4 #cdb4 ,fig4 #cdc shows a brush holder of the spring arm type4 ,the brush is firmly attached to the extremity of a spring arm1 the other end of which is secured to the brush spindle1 and when once adjusted is not capable of movement about the brush spindle4 ,in the box type of brush holder as illustrated in fig4 #cdd1 the brush is free to move up and down in the brush box1 so far as it is not restrained by a spring rigidly secured to the arm which carries the brush box at its extremity4 ,illustration3 ,,fig4 #cdf4--;,fort ,wayne type ,,mpl dynamo2 view showing details of armature1 commutator and brush rigging of large machine4 ,the laminations of the armature core are punched from thin sheet steel1 annealed and japanned4 ,spacing ribs are built into the core at proper intervals forming air passages for ventilation4 ,in addition1 there are recesses in the inside of the flanges which permit the passage of air from the interior around the ends of the core to the openings in the end flanges4 ,the armature coils are constructed of round or bar copper on standard forms4 ,the coils are laid in slots in the surface of the core4 ,the commutator is constructed of bars of hard drawn copper of uniform size and shape1 supported and clamped at either end between beveled rings and securely seated on the commutator drum4 ,the drum is connected by radial arms to the commutator sleeve which is mounted and keyed on the armature hub extension4 ,fig4 #cde shows the reaction type of brush holder1 in which the movement of the brush is constrained in one direction by the surface of a part rigidly secured to the brush spindle1 and is further constrained by a spring controlled arm1 the pressure of which is capable of ready adjustment4 ,among the special forms of brush holder may be mentioned #a4 ,scissor type of brush holder1 used for slip rings1 and consisting of two arms pivoted together like a pair of scissors4 ,the lower ends of the arms carry the brushes1 suitably mounted1 and the upper ends are drawn together by a spring1 which thus exerts pressure on the brushes4 #b4 ,clock spring type of brush holder in which the necessary contact pressure is applied to the brush by means of a clock spring1 which1 with the aid of a ratchet may be wound up and adjusted to any desired pressure4 ,illustration3 ,,fig4 #cdg4--,western ,electric brush holder4 ,this holder consists of a rugged iron casting1 elliptical in section1 and supported from the commutator end bearing bracket in such a manner as to provide for the shifting of the brushes4 ,a handle attached to the yoke aids in this shifting and a thumb nut on the outside holds the whole brush gear in the desired position4 ,the brush is fed through an accurately broached slot by a spring which maintains uniform pressure against the commutator throughout the wearing length of the brush4 ,the long lever arm of the spring is sufficiently flexible to take up any minor vibrations of the brush4 ,the tension of the brush may be adjusted without lifting it from the commutator or disturbing any of the other holders4 ,the brush1 may be removed for inspection by throwing the spring out of notch4 ,the brush is connected to the holder by flexible copper pig tails of ample current carrying capacity4 ,ques4 ,how are brush holders carried8 ,ans4 ,they are carried by a rocker arm for bipolar1 and by a rocker ring for multipolar machines1 which is mounted upon one of the main bearings1 or upon a support specially provided for it1 being pivoted to revolve from the same center as the shaft1 to permit shifting the brushes4 ,ques4 ,mention one trouble sometimes encountered with brush holders4 ,ans4 ,there is sometimes trouble resulting from the current passing through the spring which heats it and destroys its elasticity4 ,illustration3 ,,fig4 #cdh4--,western ,electric parallel spring brush holder as used on the larger machines4 ,ques4 ,how may this be avoided8 ,ans4 ,by insulating one end of the spring1 and carrying the entire current directly from the brush itself to the main conductors by a flexible copper strip or cable firmly connected to both4 ,ques4 ,what may be said with respect to brush construction on machines for electrolytic work8 ,ans4 ,the collection of large currents at low voltage1 generated by comparatively small machines1 requires careful design of brushes and brush holders4 ,the commutator is longer than the commutators on machines of equal capacity at higher voltages1 and as a rule the commutator segments are thicker and fewer in number4 ,each brush set is made up of numerous narrow brushes rather than two abnormally wide ones4 ,an example of brush and brush gear designed to meet such conditions is shown in fig4 #cbh4 ,in large machines for electrolytic work1 it is not unusual to find the current divided between two wide commutators1 one at each end of the armature1 thus giving a longer axial bearing surface for the brushes without inconveniently lengthening the pins upon which the separate brushes are threaded4 ,multipolar ,brush ,gear4--,the brush gear which includes the holders and carrier arm or ring1 becomes more complicated as the number of poles and magnitude of the current is increased4 ,in the early days of multipolar machines1 schemes of armature winding were devised such that all the necessary cross connections were made inside the machine1 and the number of brush holders reduced to two and placed at an angular distance apart depending upon the number of poles4 ,such windings1 though possible1 are not used much1 chiefly on account of their complexity1 which not only increases the danger of error in construction1 but also makes repairs costly4 ,in modern multipolar machines1 such complicated windings are avoided1 and the several sets of brushes are connected together in two groups1 positive and negative4 ,these connections are carefully designed as part of the brush gear4 ,ques4 ,how are the brushes held in large multipolar dynamos8 ,ans4 ,they are held at the proper points of commutation by arms offset from a cast iron rocker ring1 which is itself supported by brackets projecting from the magnet yoke as shown in fig4 #cdf4 ,ques4 ,what provision is made for shifting the ring to adjust the lead8 ,ans4 ,the ring is rotated by means of a worm gear and hand wheel4 ,,chapter ,,xxii ,,armature ,,construction ,the armature of a dynamo has been defined as3 a collection of coils of wire wound around an iron core1 and so arranged that electric currents are induced in the wire when the armature is rotated in a magnetic field4 ,from the mechanical point of view the armature may be said to be made up of the following parts3 #a4 ,shaft2 #b4 ,core2 #c4 ,spider 7in large machines72 #d4 ,winding2 #e4 ,commutator4 7broadly speaking74 ,of the two types of armature1 ring and drum1 the latter is almost universally used1 hence the examples of construction which follow will be confined chiefly to this type4 ,shaft4--,a typical armature shaft is shown in fig4 #cdi4 ,it is made of steel and1 except in the smaller machines1 is thicker in the middle than at the ends for stiffness to withstand the strong magnetic side pull on the core when the latter is slightly1 nearer one pole piece than the other4 ,ques4 ,what is the object of providing shoulders on the shaft as in fig4 #cdi8 ,ans4 ,they serve to keep the armature in the proper position with respect to the bearings4 ,ques4 ,how is the shaft proportioned8 ,ans4 ,if it be proportioned to secure the proper stiffness1 it will be found of ample size to resist the twisting strain4 ,illustration3 ,,fig4 #cdi4--,typical shaft for an armature4 ,the illustration shows the keyways for pulley armature and commutator4 ,in the smaller sizes1 there is usually a flange at ,a1 and threads at ,b and ,c for retaining nuts4 ,the shaft is subject also to bending by the weight of the armature1 by the magnetic drag on its core1 and in belt driven machines1 by the lateral drag of the pulley4 ,when running1 it is also subjected to bending stresses if the armature be not properly balanced4 ,if the bearings do not give1 it is evident that all such actions tend to bend the shaft at definite points4 ,core4--,in the small and medium size dynamos1 the core is attached direct to the shaft4 ,there are two kinds of core3 #a4 ,smooth2 #b4 ,slotted4 ,ques4 ,what may be said of the smooth type of core8 ,ans4 ,it has become obsolete1 except in special cases1 as for machines used for electrolytic work where a large current at low voltage is required4 ,ques4 ,what is necessary with a smooth core8 ,ans4 ,driving horns as later described4 ,illustration3 ,,fig4 #cej4--,laminated smooth core armature partly assembled4 ,it consists of numerous discs of thin sheet iron threaded on the shaft and pressed together by end plates4 ,the object of this construction is to prevent eddy currents4 ,ques4 ,what is a slotted core8 ,ans4 ,one having a series of parallel slots1 similar to the spaces between the teeth of a gear wheel1 and in which the inductors are laid4 ,ques4 ,what provision is made to avoid eddy current in cores8 ,ans4 ,they are laminated4 ,ques4 ,describe this method of construction4 ,ans4 ,the core is made of stampings of thin wrought iron or mild steel4 ,the numerous discs stamped from the sheet metal are threaded on the shaft as in fig4 #cej1 forming a practically solid metal mass4 ,illustration3 ,,fig4 #cea4--,sectional view of laminated smooth core armature showing end plates1 flange and retaining nut4 ,a key is provided to prevent rotation of the core with respect to the shaft4 ,ques4 ,how thick are the discs8 ,ans4 ,the thickness ranges from 4#jad inch to 4#jbe inch1 corresponding to #bg and #bb ,b and ,s gauge respectively1 #bg gauge being mostly used4 ,ques4 ,how are the discs held in place8 ,ans4 ,by two end plates pressed together either by large nuts screwed directly on the shaft as in fig4 #cea1 or by bolts passing through the core from end to end1 as in fig4 #ceb1 holes being punched in the discs for the purpose4 ,ques4 ,what precaution is taken with respect to the core bolts8 ,ans4 ,they are insulated from the core by tubes and washers of mica or other insulating material4 ,illustration3 ,,fig4 #ceb4--,laminated armature core with through retaining bolts4 ,in the larger sizes1 these bolts are used instead of a nut threaded on the shaft on account of the large size of the latter4 ,ques4 ,what is the construction of the core end plates1 and why8 ,ans4 ,the rims are beveled quite thin to avoid eddy currents4 ,ques4 ,how is the core connected to the shaft8 ,ans4 ,since the core has the full torque exerted upon it by the drag of the inductors1 it must be firmly connected to the shaft by means of a key1 as shown1 so that it may be positively driven4 ,core discs are stamped in one piece up to about #cj inches in diameter1 and for larger sizes they are built up from sections as later described4 ,figs4 #cec and #ced show two forms of disc stamped in one piece4 ,the first illustrates a solid disc1 and the second a ventilated disc in which more or less of the metal is cut away near the center1 thus providing passages for the circulation of air which carries away some of the heat generated in the armature4 ,illustration3 ,,figs4 #cec and #ced4--,solid and ventilated core discs4 ,in fig4 #cec1 the metal cut away near the center reduces the weight and provides passages for air circulation4 ,in some instances a forced circulation is secured by means of a fan attached to the armature1 as shown in fig4 #cff4 ,insulation of ,core ,discs4--,when the discs are stamped from very thin metal1 the mere existence of a film of oxide is sufficient insulation4 ,it is usual1 however1 to apply a quick drying varnish that will give a hard tough coat and not soften with heat or become brittle and crumble under vibration4 ,the varnish may be applied either by dipping or with a japanning machine2 it must be very thin1 and the solvent employed should be a very volatile spirit4 ,forms of ,armature ,teeth4--,the teeth stamped in the core discs are made in various shapes1 depending largely on the method of securing the inductors in the slots against electromagnetic drag and centrifugal force4 ,the teeth may be cut with their sides3 #a4 ,inclined2 #b4 ,projecting2 #c4 ,notched4 ,ques4 ,what may be said of teeth with inclined sides8 ,ans4 ,a tooth of this type is shown in fig4 #cef1 being slightly narrower at the root than at the top1 the resulting slot having parallel sides4 ,illustration3 ,,fig4 #cee4--,western ,electric slotted armature core4 ,the laminations are of sheet steel1 annealed and japanned4 ,they are mounted directly on the shaft1 7except in the large sizes7 and held in place by substantial end plates4 ,ques4 ,what are the features of the projecting type of tooth8 ,ans4 ,the projecting type is shown in figs4 #ceg and #ceh in which the tops project2 this gives a larger core area around the circumference of the armature which reduces the reluctance of the air gap1 and provides projecting surfaces for retaining the inductors in the slots by the insertion of wedges4 ,ques4 ,what is the object of cutting notches in teeth8 ,ans4 ,they are provided for the insertion of retaining wedges1 as in fig4 #cfa2 this results in less area at the top of the teeth4 ,ques4 ,how should teeth be proportioned to secure most efficient operation8 ,ans4 ,the width of the tooth should be about equal to the width of the slot minus twice the thickness of the slot insulation2 that is1 the cross sectional area of the teeth should be equal to that of the slots4 ,illustration3 ,,figs4 #cef to #cei4--,various forms of armature teeth2 fig4 #cef inclined type forming a slot with parallel sides2 figs4 #ceg and #ceh projecting type which provides a support for the retaining wedges2 fig4 #cei enclosed type which forms tunnels for the inductors4 ,advantages and ,defects of ,slotted ,armatures4--,the slotted armature1 sometimes called the ,pacinotti armature1 after its inventor1 has the following advantages over the smooth type3 #a4 ,the inductors are held more firmly in place to resist stresses due to electromagnetic drag and centrifugal force2 #b4 ,the inductors are protected by the teeth against mechanical injury2 #c4 ,less reluctance of the air gap2 #d4 ,the intermittent induction due to the presence of the teeth prevents the formation of eddy currents4 #e4 ,when the teeth are saturated they oppose the shifting of the lines due to armature reaction4 ,illustration3 ,,figs4 #cfj and #cfa4--,projecting and notched teeth2 cross sections showing inductors and retaining wedges in place4 ,the disadvantages of slotted armatures compared with the smooth type are3 #a4 ,greater hysteresis loss1 caused by denser flux in the teeth2 #b4 ,generation of eddy currents in the polar faces when the latter are not of laminated construction2 #c4 ,greater self-induction in the armature coils2 #d4 ,construction more expensive2 #e4 ,leakage of magnetic lines through core1 exterior to winding4 ,the generation of eddy currents in the polar faces may be overcome by making the air gap at least #ej per cent4 of the distance between the teeth1 so that the magnetic lines can spread from the corners of the teeth1 and become nearly uniformly distributed over the polar faces4 ,magnetic leakage through the core may be reduced by making the amount of metal above the inductors very small4 ,slotted ,cores2 ,built ,up ,construction4--,in the case of large dynamos1 the core discs are built up in order to reduce the cost of construction2 the following parts are used3 #a4 ,spider2 #b4 ,core rings split into sections4 ,illustration3 ,,figs4 #cfb and #cfc4--,side and end view of built up armature core4 ,the sheet metal ring sections containing the teeth are fastened into dovetail notches in the spider as shown4 ,the layers of ring sections are placed so as to break joints and are held by end clamps and through bolts ,b4 ,distance pieces are inserted at intervals to provide ventilating spaces ,d1 ,d1 ,d4 ,ques4 ,what is the approved method of core construction in large armatures8 ,ans4 ,the core should be of the built up construction to avoid waste of material in the stampings4 ,ques4 ,describe the construction of a built up core4 ,ans4 ,ring sections stamped1 from sheet metal are fastened to a central support or spider1 which consists of an iron hub with radiating spokes and a rim with provision for fastening the rings4 ,the rim of the spider is provided with dovetail notches into which fit similarly shaped internal projections on the core segments4 ,these features are shown in figs4 #cfb to #cfd4 ,each layer of core sections is placed on the spider so as to break joints and the core thus formed is firmly held in place by end clamps as shown4 ,the manner of fastening the rings to the spider is an important point1 for it must be done without reducing the effective cross section of the core in order not to choke the magnetic flux4 ,illustration3 ,,fig4 #cfd4--;,built up core with four spoke spider1 each spoke carrying two dovetail notches ,in this construction a little more air space is obtained for ventilation than where a separate spoke is provided for each notch4 ,in order to secure a better fit and reduce the machine work1 the spider hub in large machines is sometimes cored with enlarged section between the outer bearing surfaces1 and it is not unusual to find these surfaces turned to two different sizes as in fig4 #cfe1 to admit of easier erecting4 ,to avoid any trouble that may arise by unequal expansion1 the rim of the spider is not made continuous1 but in several sections as shown in fig4 #cfd4 ,the rim here consists of four sections each of which has two dovetail notches4 ,by thus dividing the rim into sections1 its weight is somewhat reduced and the ventilating spaces between the sections increased4 ,ventilation4--,in the operation of a dynamo more or less heat is generated1 depending on the load2 hence it is desirable that provision be made to carry off some of this heat to prevent excessive rise of temperature4 ,illustration3 ,,fig4 #cfe4--;,hub and shaft design on large machines to reduce the machine work and facilitate erecting4 ,ques4 ,why do armature cores heat8 ,ans4 ,they heat from these causes3 eddy currents1 hysteresis1 and heat generated in the inductors4 ,ques4 ,how is adequate ventilation secured8 ,ans4 ,the spider is constructed with as much open space as possible through which air currents may circulate4 ,the core is divided into several sections with intervening air spaces ,d as shown in fig4 #cfc1 the discs being kept apart at these points by distance pieces4 ,these openings between the discs are called ventilating ducts2 they are usually spaced from #b to #d inches apart4 ,illustration3 ,,fig4 #cff4--,western ,electric barrel wound armature1 having a fan attached at one end to induce a circulation of air for ventilation4 ,ques4 ,what other provision is sometimes made to secure ventilation8 ,ans4 ,in some machines a forced circulation of air is secured by means of a fan attached to one end of the armature as shown in fig4 #cff4 ,insulation of ,core4--,before the winding is assembled on the core1 the latter should be thoroughly insulated4 ,japan or enamel insulation is not sufficient because it is liable to have bubbles or minute holes in it1 or be pierced by particles of metal or by the rough edges of the core discs4 ,two or more layers of strong paper1 fibre1 canvas or mica1 should be applied to the core before placing the inductors in position4 ,the ends of the core should be insulated with thicker material1 since the strain upon it is greater1 especially at the edges4 ,illustration3 ,,fig4 #cfg4--;,holzer ,cabot partially wound barrel wound armature showing arrangement of coils4 ,the core is built up of thin discs of soft annealed steel1 which are slotted to allow the wire to sink below the surface1 this being sometimes called iron clad construction4 ,the discs are held by end plates1 clamped without through bolts4 ,the coils are machine formed of round ribbon or bar copper depending on the size and purpose of the machine1 being without joint except at the commutator4 ,they lie in insulated troughs1 the upper layers being insulated from the lower layers by fibre4 ,armature ,windings4--,the subject of windings has been fully treated from the theoretical point of view in chapter ,,xviii4 ,it remains then to explain the different methods employed in the shop and the mechanical devices used to construct the scheme of winding adopted4 ,ques4 ,what is the construction of the inductors8 ,ans4 ,they are made of copper2 the ordinary form consists of simple copper wire1 insulated with a double or triple covering of cotton1 and in some cases copper bars are used for large current machines4 ,ques4 ,what is the objection to copper bars8 ,ans4 ,they are liable to have eddy currents set up in them as illustrated in fig4 #bia4 ,illustration3 ,,fig4 #cfh4--;,holzer ,cabot iron clad band wound armature complete2 view showing openings for ventilation4 ,the advantage of the form of winding adopted1 is the ease with which a coil may be replaced in case of injury and the additional cooling surface4 ,the coils are held in place by maple wedges secured by binding wires which are soldered throughout their length4 ,ques4 ,what may be said with respect to the sizes of wire used for inductors8 ,ans4 ,wire larger than about number #h ,b and ,s gauge 74#abhe inch diameter7 is not easily handled1 hence for large inductors1 two or more wires may be wound together in parallel4 ,according to the mechanical features and manner of assembling on the core1 drum windings may be divided into several classes1 as follows3 #a4 ,hand winding2 #b4 ,evolute or butterfly winding2 #c4 ,barrel winding2 #d4 ,bastard winding2 #e4 ,former winding4 ,hand ,winding4--,the first windings were put on by hand and proved objectionable on account of the clumsy overlapping of the wires at the ends of the armature1 which stops ventilation and hinders repairs1 while the outer layers overlying those first wound1 bring into close proximity inductors of widely varying voltage4 ,the method is still used in special cases and for small machines4 ,such a winding has rarely1 if ever1 been made with one continuous wire4 ,illustration3 ,,figs4 #cfi and #cgj4--;,evolute and straight out connectors4 ,in small machines the connectors must be curved as in fig4 #cfi1 but in large machines1 especially where the teeth are wide1 they may be straight as in fig4 #cgj4 ,these connectors may take either of the following forms3 #a4 involute or evolute connectors--,an involute is the curve drawn by the extremity of a piece of string which is unwound from a cylinder2 #b4 spiral connectors--,these consist of double spirals1 the commutator being usually connected to the junction of the two spirals4 ,these connectors are also known as butterfly connectors4 ,evolute or ,butterfly ,winding4--,this mode of winding1 was introduced by ,siemens for electroplating dynamos to overcome the objections to hand winding4 ,it takes its name from the method of uniting the inductors by means of spiral end connectors as shown in fig4 #cgd1 also in figs4 #cfi and #cgj1 which show more modern forms4 ,illustration3 ,,fig4 #cga4--;,holzer ,cabot armature2 rear view showing back head and coil guard4 ,the construction of core and winding is described in fig4 #cfg4 ,the shaft is of crucible steel ground to gauge4 ,the commutator segments are of drop forged copper in the smaller and hard-drawn copper in the larger sizes4 ,the insulating material between the segments is mica4 ,on the larger sizes1 the commutator shell is fitted with a thread and mounted on a spider4 ,this construction provides openings between the commutator and shaft for ventilation4 ,ques4 ,what are evolute connectors8 ,ans4 ,the fork shaped strips used to connect bars at different positions on the armature1 as shown in fig4 #cfi4 ,in large machines1 especially where the teeth are wide1 these connections may be straight1 but in small cc machines they must be curved in the manner shown in the upper part of the figure1 as the room available may diminish by as much as half1 as the lowest point is reached1 and the room occupied by the strip is the width of a horizontal section at various points4 ,this width1 in the case of the straight connections1 is constant4 ,in place of the wooden block1 used in early machines1 for fastening the middle part of the connectors1 they may be anchored to an insulated clamping device built up like a commutator and for that reason called a false commutator4 ,illustration3 ,,figs4 #cgb and #cgc4--;,barrel and evolute windings2 end views showing placement of coils4 ,when all the coils are wound on the former1 the placing of them on the armature is a simple matter4 ,after insulating the slots1 the winder begins at any convenient slot1 and inserts the coils as shown4 ,before he can fill all the slots1 some of the first coils must be raised and the last ones inserted underneath4 ,there is not much difference between barrel and evolute winding and one style may be used at one end of the armature and the other at the opposite end4 ,ques4 ,how are the inductors arranged in evolute winding8 ,ans4 ,in fig4 #cgc1 it will be seen that the ends of the evolute connectors lie in two planes1 hence the inductors must project to different distances beyond the core4 ,accordingly1 one long and one short bar may be conveniently placed in each slot1 side by side4 ,in large machines1 especially where the teeth are wide1 the connectors may be straight as in fig1 #cgj4 ,evolute connectors may be used for either lap or wave windings4 ,illustration3 ,,fig4 #cgd4--,siemens' bar armature2 end view4 ,each inductor in the form of a bar is connected to the next by means of two evolute spiral copper strips1 one bending inwardly1 the other outwardly1 their junction being in some cases secured to a block of wood upon the shaft4 ,their outer ends are attached to the bars by rivets or silver solder4 ,barrel ,winding4--,this is a form of drum winding in which the inductors are arranged in two layers and carried out obliquely on an extension of the cylindrical surface of the drum to meet and connect with radial risers4 ,illustration3 ,,figs4 #cge and #cgf4--,single layer and double layer barrel winding4 ,barrel winding is a method of arranging the ends of armature coils as they pass from one pole to the next1 in which1 instead of using involute or butterfly connections1 ,v-shaped end connections are used which lie on a cylindrical surface1 which is a continuation of the armature surface4 ,the coil ends must of necessity be arranged in two layers1 but the method may be used for either one or two coils per slot1 the difference in arrangement for these is here illustrated4 ,barrel winding has been very widely adopted4 ,although it involves an increased length of armature1 this gives additional cooling surface and provides for good ventilation4 ,in barrel winding1 the coil ends must of necessity be arranged in two layers but the method may be used for either one or two coils per slot1 the difference in arrangement for these two cases being shown in figs4 #cge and #cgf4 ,in the single layer barrel winding1 fig4 #cge1 each slot is occupied by but one side of one coil4 ,in the double layer barrel winding1 fig4 #cgf1 the opposite sides of two separate coils occupy space in the same slot4 ,the coils1 on emerging from the slots bend in opposite directions1 and if one side of a coil occupy the bottom portion of a slot1 its other side usually occupies the top portion of a slot distant from the first slot by the polar pitch4 ,illustration3 ,,fig4 #cgg4--,westinghouse barrel wound armature4 ,the coils are former wound from copper strap and are interchangeable4 ,in the larger size machine they are of the single type4 ,the illustration shows plainly the characteristic feature of barrel winding1 namely the oblique end connectors carried out on the extended drum4 ,bastard ,winding4--,in this type of winding1 the end connectors project from the inductors in straight lines parallel to the shaft and then are bent inward4 ,it has the effect of being somewhat shorter than the barrel winding4 ,in order to secure better ventilation1 it is usual to combine a bastard winding at the rear end of the armature with a barrel winding at the commutator end4 ,this class of winding is used only with bar armatures4 ,illustration3 ,,fig4 #cgh4--,rear end of ,westinghouse wave-barrel wound armature2 view showing ventilation4 ,former ,winding4--,this relates to a method of winding coils1 and not to any particular type2 that is1 mechanical winding as distinguished from hand winding4 ,while hand winding is necessary for ring armatures1 a drum armature is wound better and more easily by the aid of machinery4 ,ques4 ,what is a former coil8 ,ans4 ,a former coil1 as its name suggests1 is one that is wound complete upon a former before being placed upon the armature4 ,ques4 ,what is the advantage of this method of winding coils8 ,ans4 ,by the use of formers much time is saved1 thus reducing the cost1 and also by their use all the coils are symmetrical which improves the appearance of the finished winding4 ,illustration3 ,,figs4 #cgi and #chj4--;,diagrams illustrating lap and wave barrel windings4 ,ques4 ,how is the required shape of the template or former for winding the coils determined8 ,ans4 ,by winding one coil on the armature in order to ascertain its dimensions and shape2 it is then removed from the armature and used as a pattern in constructing the former4 ,types of ,former ,coil4--,of the numerous shapes of former coil1 mention should be made of3 #a4 ,evolute coils2 #b4 ,straight out coil ,ques4 ,describe the evolute type of former coil4 ,ans4 ,the evolute coil is wound around eight pins inserted in a board as shown in fig4 #cha4 ,the required number of turns are taken around these pins and their ends ,g and ,h left projecting4 ,the coil thus formed is now covered with tape and after removal from the board1 is put into a clamp at ,c and ,f1 and opened up as shown in fig4 #chb1 which is the form required for insertion in the proper slots of the armature4 ,illustration3 ,fig4 #cha4--,method of winding evolute coils4 ,in preparing the former1 it is necessary to know the dimensions of the coil1 hence1 a pattern coil must first be made1 from which the spacing of the pins can be taken so that the completed coil will fit into the slots for which it is intended4 ,after the pins have been properly spaced on the board1 the wire is wound around them as indicated1 as many turns being taken as decided on for each coil4 ,when the coil is thus completely wound1 it is taken from the pins1 and the lower ends1 ,c and ,f1 placed in a suitable clamp4 ,the two halves of the coil are then spread apart1 the coil assuming the shape illustrated in fig4 #chb4 ,fig4 #chb4--;,appearance of an evolute former wound coil opened out4 ,the points ,a1 ,b1 ,c1 etc41 correspond to similar points in fig4 #cha4 ,ques4 ,what is the peculiarity of the evolute coil8 ,ans4 ,the two sides of the evolute coil have unequal dimensions4 ,the part marked ,,ab1 in fig4 #cha which is an upper layer inductor is longer than the part ,,de1 which constitutes a lower layer inductor4 ,the portions ,,dc and ,,ef act as parts of an inner layer of evolutes1 and the portions ,,af and ,,bc as parts of an outer layer of evolutes4 ,these features are shown in fig4 #chb4 ,ques4 ,how are evolute coils placed on the core8 ,ans4 ,they are placed in position as shown in figs4 #cgb and #cgc1 continuing around the core until all the slots are filled4 ,to complete the operation it is necessary to raise some of the first laid coils and insert the last ones below them4 ,the winding is thus completed and is symmetrical4 ,illustration3 ,,fig4 #chc4--,westinghouse combination bastard and barrel winding4 ,a bastard winding at the rear end is combined with a barrel winding at the commutator end1 as shown in the illustration1 to secure better ventilation4 ,ques4 ,describe the method of winding the straight out type of former coil4 ,ans4 ,the straight out coil may be wound on a former such as shown in fig4 #chd4 ,this consists of a board having four upright pins1 ,a1 ,b1 ,d1 ,e1 properly spaced and two horizontal pins ,c1 ,f1 attached to extensions at each end of the board4 ,a coil of the required number of turns is wound around these pins and then opened out as in fig4 #che4 ,after varnishing and baking it is ready to be placed on the armature4 ,ques4 ,for what class of winding are straight out former coils suitable8 ,ans4 ,for barrel winding4 ,illustration3 ,,fig4 #chd4--,method of winding straight out coils4 ,there are several ways of making these coils4 ,a former may be prepared1 as shown in the figure1 with a board having inserted four pins1 and having attached two blocks at the ends carrying horizontal pins as shown4 ,around the several pins1 the coil is wound to the required number of turns and taped4 ,this coil differs from the evolute coil in that the two halves are of equal size1 the parts which act respectively as upper and under inductor being of equal length4 ,the coil as shown is suitable for wave winding4 ,illustration3 ,,fig4 #che4--;,appearance of straight out coil after being opened out4 ,in opening out the coil1 the ends ,c and ,f are put into a clamp and twisted at right angles to the plane of the coil4 ,the letters correspond to the points indicated in fig4 #chd4 ,ques4 ,how are straight out coils placed on the core8 ,ans4 ,in the same manner as described for evolute coils2 when in position straight out coils appear as in fig4 #cgb4 ,ques4 ,what is the approved method of putting tape on a coil8 ,ans4 ,considerable time is saved by the use of a machine designed for the purpose1 such as shown in fig4 #chg4 ,illustration3 ,,fig4 #chf4 ,another and simpler method of winding a straight out coil4 ,a board with only two pins is employed as shown2 this plan1 however1 gives more trouble in the subsequent opening out of the coil4 ,the construction of these machines is such that a roll of tape placed on a split metal ring is revolved around the coil to be taped1 the coil being gradually moved until it is entirely covered4 ,coil ,retaining ,devices4--,in the operation of a dynamo there are two forces which tend to throw the inductors out of position3 #a4 ,armature drag2 #b4 ,centrifugal force4 ,both of these forces are present with smooth core armatures1 but only centrifugal force with slotted armatures4 ,the devices used to hold the inductors in position against these forces are3 #a4 ,driving horns2 #b4 ,binding ribbons2 #c4 ,retaining wedges4 ,ques4 ,what are driving horns8 ,ans4 ,they are simply pins or strips projecting from the surface of a smooth core as shown in fig4 #bea4 ,illustration3 ,,fig4 #chg4--;,armature coil taping machine4 ,numerous machines have been invented for taping armature coils4 ,they consist essentially of a device which revolves a roll of tape around the coil1 in such a direction that the tape is unwound from the roll and rewound on the coil4 ,the speed at which the coil is fed through the machine will determine the overlapping of the tape4 ,ques4 ,what other kinds of retainer are used on smooth core armatures8 ,ans4 ,they require several binding ribbons or brass bands placed around the winding to prevent the inductors being thrown off the core by centrifugal force4 ,ques4 ,with slotted armatures what provision must be made for retaining the inductors in position8 ,ans4 ,retaining wedges must be inserted into the notches or between the projecting tops of the teeth4 ,illustration3 ,,fig4 #chh4--;,front view of large armature for direct connected dynamo1 built by the ,general ,electric ,co4 ,ques4 ,how are the wedges made8 ,ans4 ,they are usually made of well baked hard wood1 such as hornbeam1 or hard white vulcanized fibre4 ,sometimes a springy strip of ,german silver is used4 ,,chapter ,,xxiii ,,motors ,an electric motor is just the reverse of a dynamo2 it is a machine for converting electrical energy into mechanical energy4 ,the electrical energy delivered by the dynamo must be obtained from a steam engine1 gas engine1 or other power2 the mechanical energy obtained from the motor comes from the energy of the current flowing through its armature4 ,ques4 ,what is the construction of a motor8 ,ans4 ,it is constructed in the same manner as a dynamo4 ,any machine that can be used as a dynamo will1 when supplied with electrical power1 run as a motor1 and conversely1 a motor when driven by mechanical power1 will supply electrical energy to the circuit connected to it4 ,dynamos and motors1 therefore1 are convertible machines1 and the differences that are found in practice are largely mechanical2 they arise chiefly from the conditions under which the motor must work4 ,hence1 the study of the motor begins with a knowledge of the dynamo1 and accordingly the student should understand thoroughly all the fundamental principles of the dynamo1 as already given1 before proceeding further with the study of the motor4 ,principles of the ,motor4--,all the early attempts to introduce motors failed1 chiefly because the law of the conservation of energy was not fully recognized4 ,this law states that energy can neither be created nor destroyed4 ,early experimenters discovered1 by placing a galvanometer in a circuit with a motor and battery1 that1 when the motor was running1 the battery was unable to force through the wires so strong a current as that which flowed when the motor was standing still4 ,moreover1 the faster the motor ran1 the weaker did the current become4 ,illustration3 ,,fig4 #chi4--;,conductor1 lying in a magnetic field and carrying no current2 the field is not distorted whether the conductor be at rest or in motion4 ,ques4 ,why does less current flow when the motor is running than when standing still8 ,ans4 ,because the motor1 on account of its rotation acts as a dynamo and thus tends to set up in the circuit a reverse electromotive force1 that is1 an electromotive force in opposite direction to the current which is driving the motor4 ,ques4 ,what is the real driving force which causes the armature of a motor to rotate8 ,ans4 ,the propelling drag1 that is1 the drag which the magnetic field exerts upon the armature wires through which the current is flowing1 or in the case of deeply toothed cores1 upon the protruding teeth4 ,the ,propelling ,drag4--,in fig4 #chi is shown the condition which prevails when a conductor carrying no current is placed in a uniform magnetic field4 ,the magnetic lines pass straight from one pole to the other4 ,the field is not distorted whether the conductor be at rest or in motion1 so long as there is no flow of current4 ,this represents the condition in the air gap of a motor or dynamo1 when no current is flowing in the armature4 ,ques4 ,what happens when a current flows in the conductor of fig4 #chi4 ,ans4 ,it sets up a magnetic field of its own as shown in fig4 #cij4 ,ques4 ,what is the effect of this magnetic field8 ,ans4 ,it distorts the original field 7fig4 #chi7 in which the conductor lies1 making the magnetic lines denser on one side and less dense on the other as in fig4 #cij4 ,ques4 ,what is the nature of these distorted magnetic lines8 ,ans4 ,they tend to shorten themselves to their original form of straight lines4 ,ques4 ,what effect has this on the conductor8 ,ans4 ,it produces a force on the conductor tending to push it in the direction indicated by the arrow1 fig4 #cij4 ,illustration3 ,,fig4 #cij4--;,conductor carrying a current in a magnetic field4 ,the current flowing in the conductor sets up a magnetic field which distorts the original field as shown1 making the magnetic lines denser on one side and less dense on the other4 ,this results in a force upon the wire1 which1 in the case of a dynamo 7fig4 #cia7 opposes its movement1 and which forms the propelling drag in the case of a motor 7fig4 #cib74 ,the distorted magnetic lines may be regarded as so many rubber bands tending to straighten themselves2 ,the result then is clearly to force the conductor in the direction indicated4 ,according to ,lenz' law1 the direction of the current in the armature of a dynamo is such as to oppose the motion producing it4 ,when the armature of a dynamo is rotated1 the bending of the lines of force of the main magnetic field due to armature reaction acts as a drag against the motion of the armature4 ,armature reaction increases with the increase of the armature current4 ,therefore1 the effect of the drag increases with the increase of load and requires an additional expenditure of power to drive the armature4 ,in a motor1 the direction of the actuating current is the reverse of that of the armature current of a dynamo1 consequently1 the armature reaction which constitutes a drag1 acting against rotation of the armature of a dynamo1 becomes a pull in the direction of rotation of the armature of a motor and constitutes its real turning effect or torque which is used at the pulley to do mechanical work4 ,the greater the load applied to the motor1 the greater will be the amount of current taken from the supply mains1 and consequently1 the greater the torque4 ,illustration3 ,,figs4 #cia and #cib4--;,action of the magnetic force in a dynamo and motor4 ,in the first instance1 according to ,lenz' law1 the direction of the current induced in the wire is such as to oppose the motion producing it4 ,in the operation of a motor1 the current supplied in flowing through the armature winding distorts the field and thus produces rotation4 ,in the figures1 the direction of the force is clearly indicated by remembering that the distorted lines of force act like rubber bands tending to straighten and shorten themselves4 ,ques4 ,what are the essential requirements of construction in a motor8 ,ans4 ,they are3 #a1 a magnetic field1 #b1 conductors placed perpendicular to the field1 and #c1 provision for motion1 of the conductors across the field in a direction perpendicular to both themselves and the field4 ,the ,reverse ,electromotive ,force4--,when an electric current flows through some portion of a circuit in which there is an electromotive force1 the current will there either receive or give up energy1 according to whether the electromotive force acts with or against the current4 ,illustration3 ,,fig4 #cic4--;,force exerted on a current carrying conductor placed across a magnetic field4 ,let ,n1 ,s1 be the pole pieces of an ordinary electromagnet1 having their faces flat and with only a narrow air gap between4 ,in this gap is stretched the vertical copper wire ,a ,b1 kept taut by a strong spring at ,a2 current can be passed into the wire from the leads ,c and ,d4 ,attached to the wire in the middle of the gap is a horizontal cord passing over a pulley ,p and kept taut by a weight ,w2 the pulley carries a pointer ,f which moves in front of a scale s s4 ,if the electromagnet be now excited and have the polarity indicated1 it will be found that on passing a strong current down the wire1 the index ,f moves toward the right1 showing a similar movement in the wire4 ,the index returns to zero when the current in the wire ceases1 and moves in the opposite direction if the current in the wire be reversed and sent up instead of down4 ,the experiment can be further varied by reversing the magnetizing current of the electromagnet4 ,this is illustrated in fig4 #cie1 which represents a circuit in which there is a dynamo and a motor4 ,each is rotating clockwise1 and accordingly1 each generates an electromotive force tending upward from the lower to the upper brush4 ,in both cases the upper brush is positive4 ,in the dynamo1 however1 where energy is being supplied to the circuit1 the electromotive force is in the same direction as the current1 and in the motor1 where work is being done1 the electromotive force is in the reverse direction to that of the dynamo4 ,illustration3 ,,fig4 #cid4--,showing relative directions of armature current and reverse electromotive force of a motor4 ,when a motor is in operation1 the wires around the periphery of its armature cut the magnetic lines of force produced by the field magnet exactly as in the case of the dynamo4 ,consequently1 an electromotive force is induced in each wire1 as in the dynamo armature4 ,this induced electromotive force is in opposition to the flow of current due to the electromotive force of the supply circuit1 and tends1 therefore1 to keep down the flow of current4 ,the figure shows a single loop of wire1 on the armature core connected directly to the source of electricity4 ,with current flowing in the loop in the direction indicated by the arrows marked c1 a magnetic field is set up in the direction indicated by the large arrow marked direction of armature flux4 ,with the field magnet energized so as to produce a field in the direction indicated by the large arrow ,f1 the reaction between the two fields will turn the armature core in the direction indicated by the arrow ,r4 ,as the core turns1 the upper wire of the loop will cut the flux under the south pole of the field magnet1 and the other side of the loop will cut the flux under the north pole4 ,the result will be the induction of a reverse electromotive force in the loop1 the direction being indicated by the small arrows marked e4 ,the actual flow of current in the armature is that due to the difference between the impressed and reverse voltage2 the latter is proportional to the speed of the armature1 the number of armature wires and the strength of the magnetic field in the air gaps between the armature and the pole faces4 ,the speed of a motor supplied with current at constant voltage varies directly with the reverse electromotive force1 also with other conditions fixed1 the stronger the field1 the slower the speed4 ,weakening the field will increase the speed up to the point where the increase in reverse electromotive force due to the increased speed cuts down the armature current below the value necessary to give the requisite pull at the armature periphery4 ,when this point is reached1 any weakening of the field will reduce the speed of the armature4 ,the pull or torque of a motor armature is directly proportional to the strength of the magnetic field1 and to the strength of the armature current1 the number of armature inductors being fixed4 ,in a field of constant strength1 therefore1 the pull of the armature depends on the amount of current passing through the winding4 ,the torque must be just sufficient to overcome the load2 if in excess1 the speed will increase until the increase of the reverse electromotive force reduces the current and the increase of speed increases the load to the point of equilibrium between load and torque4 ,if the torque be insufficient for the load1 the speed will diminish until equilibrium is established1 assuming the motor is running on constant voltage circuit4 ,ques4 ,describe similar conditions which prevail in the operation of a dynamo4 ,ans4 ,when no current is being generated by the dynamo1 little power is required to drive it1 but when the external circuit is closed and current is forced through it against more or less resistance1 work is being done1 hence more power is required4 ,in other words1 there is an opposition to the mechanical force applied at the pulley which is proportional to the electric power delivered by the dynamo4 ,an opposing reaction or reverse force then is set up in a dynamo when it does work4 ,illustration3 ,,fig4 #cie4--;,circuit with generator and motor4 ,whenever current flows through some portion of a circuit in which there is an electromotive force1 the current will there either receive or give up energy according to whether the electromotive force acts with the current or against it4 ,in the figure1 the generator and motor are rotating clockwise1 and hence each generates an electromotive force tending upwards from the lower brush to the higher4 ,in each case the upper brush is the positive one4 ,in the dynamo1 where energy is being supplied to the circuit1 the electromotive force is in the same direction as the current1 while in the motor where work is being done and energy is leaving the circuit1 the electromotive force is in a direction which opposes the current4 ,ques4 ,in the operation of a motor what is the nature of the reverse electromotive force8 ,ans4 ,it is proportional to the velocity of rotation1 the strength of the magnets1 and to the number and arrangement of the wires on the armature1 that is1 the reverse voltage depends on the rate at which the lines of force are cut4 ,illustration3 ,,figs4 #cif and #cig4--,water and electric circuits4 ,diagrams showing comparison between water motor and electric motor4 ,in the diagrams3 ,the pump corresponds to the dynamo4 ,the high level pipe positive conductor4 ,the low level pipe negative conductor4 ,the valve switch4 ,the water motor electric motor4 ,the water pressure 7called head7 electric pressure 7called voltage74 ,the flow in gallons per minute amperes4 ,the size of pipe size of conductor4 ,the foot pounds watts4 ,the greater the difference between the height of the two pipes the higher the pressure1 and the greater the difference between the pressures of the two conductors the higher the voltage4 ,the larger the diameter of the pipes the less resistance is offered to the flow of water1 and the larger the diameter of the conductors the less resistance is offered to the flow of electricity4 ,the more water required by the water wheel1 the more power is required to drive the pump4 ,the more electricity required by the motor the more power is required to drive the generator4 ,illustration3 ,,fig4 #cih4--;,fairbanks-,morse standard ,,tr type motor4 ,this type is built in the smaller sizes and the design is such that the motor can be installed upon the floor1 wall or ceiling1 the bearing yokes being attached to the frame by four equally spaced bolts so that they can be turned to provide for proper operation of the oiling devices in either position4 ,a substantial base is provided with a thrust screw for adjusting the belt tension4 ,this base has clamping bolts which permit adjusting the position of the motor while suspended4 ,there is a cast ring type frame having steel side pieces which press firmly together1 the steel laminations making up the pole pieces4 ,the field coils armature1 and armature coils are illustrated in detail in figs4 #cii to #dja4 ,the commutator bars are of drawn copper1 insulated with mica4 ,the lugs which extend outward from the bars to receive the lead wires from the armature windings are formed in one piece with the bars1 and are of the full width of the bars with the insulator extending outward between them1 so that when assembled a solid flange is formed to receive the armature connections4 ,self-oiling bearings are provided and the location of the bearing sleeves in the housing is adjustable so that the armature may be centered in the magnetic field4 ,the brush rigging is carried on a skeleton rocker supported in a groove1 turned in the edge of the frame4 ,the brush holders are of the box type with independently adjustable tension spring for each4 ,standard shunt windings are for #aae1 #bcj and #eej volts4 ,the compound wound motors operate at approximately the same full load speeds as the shunt wound1 but the no load speeds will be about #bj per cent4 higher than the full load speeds4 ,they have1 however1 the ability to exert a more powerful starting effort than shunt motors without drawing such a heavy current from the line1 and are1 therefore1 especially adapted for driving apparatus that has to be frequently started and stopped under load and where close speed regulation is not required4 ,ques4 ,describe an experiment which shows the existence of a reverse electromotive force in a motor4 ,ans4 ,the apparatus required consists of a small motor1 battery1 and ammeter4 ,they should be connected in one circuit and the deflection of the ammeter observed when the armature is held stationary1 and when it rotates with various loads4 ,in an experiment of this kind made on a motor with separately excited magnets1 the following figures were obtained3 ,revolutions per minute #j #ej #ajj #afj #ahj #aie ,amperes #bj #af4b #ab4b #g4h #f4a #e4a ,apparently1 if the motor had been helped on to run at #bfa revolutions per minute1 the current would have been reduced to zero4 ,in the last result obtained1 the current of #e4a amperes was absorbed in driving the armature against its own friction at the speed of #aie revolutions per minute4 ,illustration3 ,,fig4 #cii4--;,fairbanks-,morse field coil and pole piece4 ,the field coils are wound upon iron forms1 each layer treated with insulating compound4 ,afterward they are removed from the forms and baked hard and dry and finally wrapped with insulating materials2 all but the three smaller sizes are wrapped with a protecting cord4 ,the series and shunt coils of the compound winding here shown are wound separately1 the smaller one being the series coil and the larger the shunt coil4 ,ques4 ,explain the action of the current supplied to a motor for its operation4 ,ans4 ,the motor current passing through the field magnets polarizes them and establishes a magnetic field1 and entering the armature1 polarizes its core in such a way that the positive pole of the core is away from the negative pole of the magnetic field1 and the negative pole is away from the positive pole of the magnetic field4 ,the magnetic repulsions and attractions thus created cause the armature to rotate in a position of magnetic equilibrium or so as to bring its positive and negative poles opposite the negative and positive poles respectively of the magnetic field4 ,it is evident that unless suitable means were provided to reverse the polarity of the armature core at the instant it reached the position of the magnetic equilibrium1 the armature would not rotate any further4 ,the construction is such that the polarity of the armature core1 or the direction of the current in the armature coils is reversed at the proper instant automatically by the commutator1 thus giving continuous rotation4 ,illustration3 ,,fig4 #djj4--;,fairbanks-,morse armature for #g ,h4,p41 #acjj ,r4,p4,m41 ,,tr type motor4 ,the armature core is built up of thin sheet steel laminations with notches in the circumference1 which1 when the discs are placed together1 form grooves or slots to receive the armature coils4 ,the armature cores for the larger machines are mounted on a cast iron spider1 which also carries the commutator1 making the two parts entirely self-contained1 and with this construction1 it is possible to remove the armature shaft1 without disturbing the core1 commutator or windings4 ,cores of all sizes are provided with ventilating spaces1 running from the surface to the central opening of the core1 so that air is drawn through the core and blown out over the windings by the revolution of the armature4 ,direction of ,rotation of ,motors4--,in the case of either a motor1 or a dynamo used as a motor1 the direction in which the armature will rotate is easily found by the left hand rule1 as illustrated in fig4 #daa1 when the polarity of the field magnets and the direction of currents through the armature are known4 ,ques4 ,how may the rotation of a motor be reversed8 ,ans4 ,by reversing either the current through the fields1 or the current through the armature4 ,ques4 ,what will happen if both currents be reversed8 ,ans4 ,the motor will run in the same direction as before4 ,illustration3 ,,fig4 #dja4--;,fairbanks-,morse wire wound armature coils4 ,these coils are form wound and are thoroughly insulated and baked before assembling in the slots4 ,material of great mechanical strength as well as high insulating value is used1 and the coils are subjected to dippings in insulating compound and to bakings1 thus driving out all moisture and making a coil which is practically waterproof and which will withstand rough handling4 ,these coils1 when completed1 are placed in the slots1 where they are retained by bands on the three smaller sizes and by hardwood wedges on the larger sizes4 ,ques4 ,what is the effect of supplying current to a series dynamo8 ,ans4 ,it will run in a direction opposite to its motion as a dynamo4 ,ques4 ,what is the result of reversing the direction of current at the terminals of a series motor8 ,ans4 ,it will not change its direction of rotation1 since the current still flows through the armature in the same direction as through the field4 ,illustration3 ,,figs4 #djb to #daj4--;,diagrams showing relative direction of rotation of motors and dynamos4 ,from figs4 #cia and #cib1 it is seen that the direction of the current in a motor armature must be such as will increase1 by the flux it produces1 the intensity at the leading polar edge and decrease the intensity at the trailing polar edge4 ,in a dynamo1 the armature has to be moved by mechanical force1 against a magnetic force1 hence the leading polar edge is weakened1 while the trailing edge is strengthened4 ,the magnetomotive force in a motor armature is1 therefore1 opposed to the direction of that in a generator armature1 when the direction of rotation and the direction of the field magnetomotive force are the same4 ,upon this depends all the relations existing between the direction of rotation of a machine when acting as a motor or as a dynamo4 ,ques4 ,what is the behavior of a shunt dynamo when used as a motor8 ,ans4 ,its direction of rotation remains unchanged4 ,ques4 ,why is this8 ,ans4 ,because if the connections be such that the current supplied will flow through the armature in the same direction as when the machine is used as a dynamo1 the current through the field will be reversed1 since the field windings are in parallel with the brushes4 ,illustration3 ,,fig4 #daa4--,the left hand rule for direction of motion in motors4 ,place the left hand1 as shown1 so that the thumb points in the direction of the current1 the #crd1 #dth and #eth fingers in the direction of the lines of force1 then will the #bnd or forefinger1 at right angles to the others1 point in the direction in which the conductor is urged4 ,armature ,reaction in ,motors4--,in the operation of a motor the reaction between the armature and field magnets distorts the field in a similar manner as in the operation of a dynamo4 ,a current supplied from an outside source magnetizes the armature of a motor and transforms it into an electromagnet1 whose poles would lie nearly at right angles to the line joining the pole pieces1 were it not for the fact that negative lead must be given to the brushes4 ,illustration3 ,,fig4 #dab4--,principle of the electric motor as illustrated by experiment showing effect of a magnetic field on a wire carrying an electric current4 ,let a vertical wire ab be rigidly attached to a horizontal wire gh1 and let the latter be supported by a ring or other metallic support as shown1 so that ab is free to oscillate about gh as an axis4 ,let the lower end of ab dip into a trough of mercury4 ,when a magnet is held in the position shown and a current from a cell is sent through the wire as indicated1 the wire will move in the direction shown by the arrow f1 that is1 at right angles to the direction of the lines of magnetic force4 ,let the direction of the current in the wire be reversed1 then the direction of the force acting on the wire will be found to be reversed also4 ,the conclusion is that a wire carrying a current in a magnetic field tends to move in a direction at right angles both to the direction of the field and to the direction of the current4 ,the relation between the direction of the magnetic lines1 the direction of the current1 and the direction of the force1 is often remembered by means of the following rule1 known as the motor rule1 and which differs from the dynamo rule only in that it is applied to the fingers of the left hand instead of to those of the right4 ,let the forefinger of the left hand point in the direction of the magnetic lines of force and the middle finger in the direction of the current sent through the wire1 then will the thumb1 at right angles to the other two fingers1 point in the direction in which the wire is urged4 ,negative lead is the amount of backward advance of the brushes against the direction of the rotation of the armature1 measured in degrees from the neutral plane4 ,if the brushes be given positive lead1 that is1 placed in advance of the neutral plane in the direction of rotation1 the cross magnetizing force is converted into one that tends to increase that of the field magnet1 while if they be given negative lead1 it tends to demagnetize the field magnet4 ,since with positive lead the armature polarity strengthens that of the field magnet1 it is possible1 disregarding sparking1 to operate a motor without any other means being taken to magnetize the field magnets1 because the armature will induce a pole in the field magnet and then attract itself towards this induced pole4 ,ques4 ,what effect has the cross magnetizing force on the field8 ,ans4 ,it tends to shift the field around in a direction opposite to that of the rotation4 ,illustration3 ,,fig4 #dac4--;,current commutation in a motor4 ,considering the coil ,w which is ascending1 current is flowing through it from the top brush1 while it is itself the seat of an electromotive force that tends to stop or reverse its current4 ,the condition for sparkless commutation requires that during the interval the coil is short circuited by the brush1 the coil should be passing through a field that is not only sufficiently strong but one that tends to reverse the direction of its current4 ,the coil is already in such a field1 hence1 commutation must take place before it passes put of this field4 ,to accomplish this the brushes must be shifted backward1 that is1 given negative lead1 to overcome sparking4 ,in other words1 the commutating plane must be shifted back of the neutral plane in a motor instead of being placed in advance as in a dynamo4 ,ques4 ,what are the conditions of minimum sparking8 ,ans4 ,the same conditions must obtain as in a dynamo1 that is1 the current in the coil undergoing commutation must be brought to rest and started again in the opposite direction4 ,this involves that while the coil is short circuited by the brush1 it should be passing through a field that tends to reverse the direction of the current4 ,since the coil is already in such a field1 the act of commutation must take place before it passes out of this field4 ,accordingly1 a negative lead must be given the brushes4 ,illustration3 ,,fig4 #dad4--,railway motor4 ,this type of motor1 since it must operate under cars1 has taken on the peculiar form under which it is most familiar4 ,as illustrated1 the case is of such shape that compactness and water proofing are secured1 and the means of attachment to the car axle and support from the axle and truck frame are provided4 ,method of ,starting a ,motor4--,although motors and dynamos are practically similar in general construction and either one of them will act as the other when suitably traversed by an electric current1 there are certain differences between the connections and accessories of a machine operated as generator and one employed as a motor4 ,for instance1 when a machine is operated as a dynamo1 it is first driven up to speed until it has excited itself to the right pressure1 and then it is connected to the circuit2 but when a machine is used as a motor it will not start until it has been connected to the circuit1 and this must not be done until the proper precautions have been taken to ensure that the current1 which will pass through it when so connected1 will not be excessive and thereby result in serious injury to the motor4 ,for this reason a rheostat or variable resistance1 commonly called a starting box is usually inserted in the armature circuit of a motor to prevent an undue rush of current before the motor attains its speed1 and subsequently the speed is regulated by the cutting in or out of the circuit of certain extra resistances which constitute the controller used on a series motor requiring variable torque at variable speed1 as in the case of elevator or electric traction service4 ,illustration3 ,,fig4 #dae4--,view of railway motor1 open4 ,the frame is of cast steel for lightness1 and which serves as magnetic circuit and protecting case4 ,it is circular or octagonal in form except in very large motors4 ,four short magnets project from the case4 ,the armature is large in order to secure the required torque4 ,it is always series wound1 requiring two brushes4 ,the brush holders are mounted upon a frame of insulating material which is attached to the upper half of the case4 ,the brushes are adjustable radially1 but usually it is not necessary to provide for shifting as they remain in the neutral plane4 ,in motors which receive so little attention as these1 special attention must be given to the design of devices for keeping oil and grease out of the case4 ,these would injure the insulation of the coils and produce sparking at the commutator4 ,oil rings are1 therefore1 placed on the shaft1 and these discharge into chambers connected to the oil wells or allow the oil to overflow on the track4 ,the bearings are made self-oiling or self-greasing by means of rings or wicks and will run for weeks without attention4 ,classes of ,motor4--,motors are classified in the same manner as dynamos4 ,the fields may be either bipolar or multi-polar1 and with respect to the type of armature winding employed1 motors are classed as3 #a4 ,series wound2 #b4 ,shunt wound2 #c4 ,compound wound4 ,illustration3 ,,fig4 #daf4--,series motor connections4 ,a series motor on a constant voltage circuit does not have a constant field strength1 and does not run at uniform speed4 ,if the load be taken off it will run at excessive speed4 ,to start the motor1 the circuit is completed through a variable resistance or rheostat by moving the switch ,s so that the resistances ,r1 ,r#a1 ,r#b1 ,r#c1 are gradually cut out of the circuit4 ,to stop1 the switch ,s is moved back to its off position4 ,series ,motors4--,a series motor is one in which the field magnet coils1 consisting of a few turns of thick wire1 are connected in series with the armature so that the whole current supplied to the motor passes through the field coils as well as the armature4 ,fig4 #daf is a diagram of a series motor showing the connections and rheostat4 ,ques4 ,what are the characteristics of a series motor8 ,ans4 ,the field strength increases with the current1 since the latter flows through the magnet coils4 ,if the motor be run on a constant voltage circuit1 with light load1 it will run at a very high speed2 again1 if the motor be loaded heavily1 the speed will be much less than before4 ,illustration3 ,,fig4 #dag4--;,general ,electric type ,,cl-,b motor for slow and moderate speeds4 ,it is of multipolar construction1 having six pole pieces4 ,the advantages of slow speed machinery are generally understood1 and in motors the additional outlay to secure slow speeds is warranted1 inasmuch as it results in diminished wear and friction losses in gearing1 belting1 bearings1 and commutators1 and decreased brush renewals4 ,the comparatively slow speeds of these motors are of importance in that they permit belting or gearing the motors directly to ordinary slow speed line shafting without employing intermediate counter shafting4 ,when motors are geared to heavy duty machines1 it is considered better practice to supply an outboard bearing to take up the additional strain that would otherwise be put on the gearing and bearing4 ,ques4 ,for what kinds of service are series motors unsuited8 ,ans4 ,series motors should not be employed where the load may be entirely removed because they would attain a dangerous speed4 ,they should not be used for driving by means of belts1 because a sudden release of the load due to a mishap to the belt would cause the motor to run away4 ,very small series motors may be used with belts since their comparatively large frictional resistance represents an appreciable load1 restraining the motor from reaching a dangerous speed4 ,ques4 ,for what service are series motors adapted8 ,ans4 ,for gear drive4 ,in the case of a sudden release of the load the gears provide some load on account of the frictional resistance of the gear teeth4 ,illustration3 ,,fig4 #dah4--,shunt motor connections4 ,a shunt motor runs at constant speed on a constant voltage circuit4 ,in connecting the motor in circuit1 the field coils must be placed in circuit first1 so that there is a certain amount of field strength to produce rotation of the armature and thus prevent excessive current through the armature4 ,if the field magnets were not put in the circuit first1 the armature1 at rest on receiving current1 would probably burn out1 because it is of low resistance1 and would take practically all the current supplied1 especially since no reverse voltage is generated in the armature at rest4 ,the method of starting is shown in the illustration4 ,to start1 the switch is closed1 and the rheostat lever pushed over so as to make contact with ,a and ,b1 thus first exciting the magnets4 ,on further movement of the lever1 the rheostat resistances ,r1 ,r#a1 ,r#b1 ,r#c1 etc41 are gradually cut out as the speed increases1 until finally all the resistance coils are cut out4 ,to stop1 the lever is brought back to its original position4 ,ques4 ,what advantage is obtained with series motors with respect to the connections8 ,ans4 ,a single wire only proceeds from the rheostat to the motor1 so that1 with the return wire1 only two wires are required4 ,ques4 ,for what service are series motors specially adapted8 ,ans4 ,series motors are used principally for electric railways1 trolleys1 and electric vehicles1 and similar purposes where an attendant is always at hand to regulate or control the speed4 ,they are also used on series arc light circuits in which the current is of constant strength4 ,very small motors are generally provided with series windings4 ,illustration3 ,,fig4 #dai4--,speed regulation of a shunt motor4 ,the speed of a motor depends on the voltage of the current supplied and the field strength4 ,the motor tends to rotate so fast as to produce a reverse voltage nearly equal to that supplied to the brushes2 hence1 the speed varies with the voltage supplied4 ,by decreasing this voltage then1 the speed is decreased4 ,accordingly1 the speed may be reduced by inserting1 by means of a rheostat1 a resistance in series with the motor4 ,by inserting this resistance in the field circuit1 the voltage at the terminals of the motor is lowered1 thus giving the condition necessary to reduce the speed4 ,the arrangement for speed regulation shown in the figure includes a starting regulator and a shunt regulator4 ,shunt ,motors4--,a shunt motor may be defined as one in which the field coils are wound with many turns of comparatively fine wire1 connected in parallel with the brushes4 ,the current then is offered two paths3 one through the armature1 and one through the field coils4 ,illustration3 ,,figs4 #dbj to #dbb4--,reversing the direction of rotation of a series motor4 ,fig4 #dbj shows the connections for counter clockwise rotation4 ,the motor may be reversed3 #a1 by allowing the current to flow in its original direction 7from ,d to ,c7 in the field magnet coils1 and altering the direction of the armature current by changing the two connections on the brushes ,a and ,b1 thus connecting ,c to ,a and ,b to the return wire as in fig4 #dba1 or #b1 by leaving the direction of the current in the armature in its original direction1 and reversing that of the field current1 as in fig4 #dbb4 ,if the wires leading to the rheostat and motor directly1 were reversed there would be no reversal of the motor1 because by so doing1 both the armature and field magnet currents would be reversed4 ,illustration3 ,,figs4 #dbc to #dbe4--,reversing the direction of rotation of a shunt motor4 ,fig4 #dbc shows the connections for counter clockwise rotation4 ,the motor may be reversed3 #a1 by allowing the current to flow in its original direction through the field magnet coils 7from ,d to ,c71 and reversing its direction through the armature 7from ,a to ,b7 as in fig4 #dbd1 or #b1 by allowing the armature current to flow in its original direction 7from ,b to ,a7 and reversing the current through the field coils 7from ,c to ,d7 as in fig4 #dbe4 ,ques4 ,what may be said with respect to the speed of a shunt motor8 ,ans4 ,it is practically constant with varying loads4 ,the variation of speed ranges from #a_/aj to #e per cent41 except in the case of small motors1 in which the variation may be much greater4 ,ques4 ,how should a shunt motor be started8 ,ans4 ,to properly start the machine1 the field coils must be fully excited4 ,it is1 therefore1 necessary to switch the magnet coils immediately on to the voltage of supply1 while a variable resistance must be provided for the armature circuit4 ,to get both connections at the same time1 rheostats for shunt motors are arranged as shown in fig4 #dah4 ,influence of ,brush ,position on ,speed4--,in the case of a shunt motor supplied with current at constant pressure1 the speed is a minimum when the brushes are in the neutral plane1 and the effect of giving the brushes either positive or negative lead is to increase the speed1 especially with little or no load4 ,ques4 ,why does the speed increase8 ,ans4 ,when the brushes are shifted from the neutral plane1 the reverse voltage between the brushes is decreased1 speed remaining unchanged4 ,accordingly1 the pressure in the supply mains forces an increased current through the armature thus producing an increased armature pull which causes the speed to increase until the reverse voltage reaches a value sufficiently large to reduce the current to the value required to supply the necessary driving torque4 ,compound ,motors4--,this type of motor has to a certain extent1 the merits of the series motor without its disadvantages1 and is adapted to a variety of service4 ,if the current flow in the same direction through both of the field windings1 then the effect of the series coil strengthens that of the shunt coil2 this strengthening is greater1 the larger the armature current4 ,illustration3 ,,fig4 #dbf4--;,compound motor connections for starting from a distant point4 ,a compound winding may be used on motors for many different purposes4 ,if the current flow in the same direction through both windings1 then the effect of the series coil strengthens that of the shunt coil4 ,this strengthening increases with the load4 ,thus the motor gets1 at increasing load1 a stronger magnetic field1 and will therefore1 if the voltage remain constant run slower than before4 ,accordingly1 for a given current1 the starting power will be greater than that of a shunt motor4 ,with a decreasing load the motor will run faster4 ,the compound motor has1 to a certain extent1 the merits of the series motor without its disadvantages4 ,by means of compound motors the starting at a distance with only two mains may be effected1 just as in the case of the series motor4 ,the connections are shown in the diagram4 ,if the motor be regarded as being without the shunt coil1 then it is connected up exactly as the series motor in fig4 #daf4 ,the current coming from the starter enters the series coil at ,f1 flows through the series coil and leaves it at ,e1 flowing from there to the armature brush ,b1 through the armature to brush ,a1 and from there through the second main back to the generator4 ,the shunt winding is connected directly with the armature brushes ,a and ,b1 and gets at starting1 therefore1 only a very small voltage1 hence its field is nearly ineffective4 ,but on account of the series winding1 the motor starts as a series motor4 ,obviously such a motor will not develop a very large starting power like a real series motor1 for1 on account of the large space occupied by the shunt coils1 there is less space available for the series coils than with a series motor4 ,a compound motor may1 however1 even with this arrangement1 be easily started1 provided the load on starting be not too heavy4 ,when once running the armature will produce a reverse voltage and the shunt coil will be supplied with nearly the full terminal voltage4 ,ques4 ,mention some characteristics of the compound motor4 ,ans4 ,since it is a combination of the shunt and series types1 it partakes of the properties of both4 ,the series winding gives it strong torque at starting 7though not as strong as in the series motor71 while the presence of the shunt winding prevents excessive speed4 ,the speed is practically constant under all loads within the capacity of the machine4 ,ques4 ,describe the connections for starting a compound motor at a distance4 ,ans4 ,control at a distance can be effected with only two wires1 just as in the case of a series motor4 ,in the diagram fig4 #dbf1 the current coming from the rheostat enters the series coil at ,f1 and leaves it at ,e1 thence it flows to the armature brush ,b1 through armature to brush ,a1 and from here back to the dynamo4 ,the shunt winding1 which is connected across the brushes1 gets a very small voltage at starting and is accordingly very ineffective4 ,the motor then starts as a series motor4 ,the starting effect is smaller than in a series motor because of the fewer turns in the series winding1 most of the available space being occupied by the shunt coils4 ,power of a ,motor4--,the word power is defined as the rate at which work is done1 and is expressed as the quotient of the work divided by the time in which it is done1 thus3 work power ------ time ,the difference between power and work should be clearly understood4 ,work is the overcoming of resistance through a certain distance4 ,it is measured by the product of the resistance into the space through which it is overcome1 thus3 work distance space ,for instance1 in lifting a body from the earth against the attraction of gravity1 the resistance is the weight of the body1 and the space1 the height to which the body is raised1 the product of the two being the work done4 ,the unit of work is the foot pound1 which is the amount of work done in overcoming a pressure or weight equal to one pound through one foot of space4 ,the unit of power is the horse power which is equal to #cc1jjj foot pounds of work per minute1 that is3 foot pounds per minute horse power ------------------------ #cc1jjj ,the unit of power was established by ,james ,watt as the power of a strong ,london draught horse to do work during a short interval1 and used by him to measure the power of his steam engines4 ,in order to measure the mechanical power of a motor1 it is necessary to first determine the following three factors upon which the power developed depends3 #a4 ,pull of the armature1 in pounds2 #b4 ,distance in feet at which the pull acts from the center of the shaft2 #c4 ,revolutions per minute4 ,,example4--,if the armature pull of a motor having a two foot pulley1 be such that a weight of #ejj lbs4 attached to the rim1 is just balanced1 and the speed be #a1jjj revolutions per minute1 what is the horse power8 ,here1 the distance that the pull acts from the center of the shaft is one foot1 hence for each revolution the resistance of #ejj pounds is overcome through a distance equal to the circumference of the pulley or diameter #c4adaf #b #f4bhcb feet4 ,illustration3 ,,fig4 #dbg4--;,general ,electric type ,,cq ,motor4 ,these motors range in capacity from #a_/f to #bj horse power4 ,the small sizes are bipolar1 and the larger sizes have four poles4 ,for installations where the motor is exposed to dust1 mechanical injury or moisture1 it may be partially or entirely enclosed by means of hand hole covers4 ,the standard voltages are #aae1 #bcj and #eej4 ,the work done in one minute is expressed by the following equation3 work weight circumference revolutions per in of pulley per foot pounds minute lbs4 in feet minute #ejj #f4bhcb #a1jjj #c1ada1fjj4 ,hence1 the power developed is #c1ada1fjj #cc1jjj #ie4b horse power4 ,ques4 ,what is brake horse power8 ,ans4 ,the net horse power developed by a machine at its shaft or pulley2 so called because a form of brake is applied to the pulley to determine the power4 ,ques4 ,describe the apparatus used in making a brake test4 ,illustration3 ,fig4 #dbh4--,prony brake for determining brake horse power4 ,it consists of a friction band ring which may be placed around a pulley or fly wheel1 and attached to a lever bearing upon the platform of a weighing scale in such a manner that the friction between the surfaces in contact will tend to rotate the arm in the direction in which the shaft revolves4 ,this thrust is resisted and measured in pounds by the scale4 ,in setting up the brake the distance between the center of the shaft and point of contact 7knife edge7 with the scales must be accurately measured1 the knife edge being placed at the same elevation as the center of the shaft4 ,an internal channel permits the circulation of water around the interior of the rim as shown1 to prevent overheating4 ,ans4 ,tests of this kind are usually made with a ,prony brake as shown in fig4 #dbh4 ,it consists of a band of rope or strip iron--the latter is the arrangement shown--to which are fastened a number of wooden blocks1 several carrying shoulders to prevent the contrivance from slipping off the wheel rim4 ,the brake band is drawn tight1 as shown1 so that the blocks press against the surface all around4 ,the brake thus formed is restrained from revolving with the pulley by two arms attached near the top and bottom centers of the wheels1 and joined at the opposite ends to form a lever which bears upon an ordinary platform scale1 a suitable leg or block being arranged to keep its end level with the center of the shaft4 ,by this arrangement the amount of friction between the brake band and the revolving wheel is weighed upon the scales4 ,since the brake fits tightly enough to be carried around by the wheel1 but for the arms bearing upon the scale1 the amount of frictional power exerted by the wheel in turning free within the blocks may be transmitted and measured1 just as would be the case were a machinery load attached1 instead of a friction brake4 ,ques4 ,why must the point of contact of the brake with the scales be level with the center of the shaft8 ,ans4 ,in order to determine the force acting at right angles to the line joining the point of contact and center of the shaft4 ,ques4 ,what is the distance between the center of the shaft and point of contact with the scales called8 ,ans4 ,the lever arm4 ,ques4 ,what three quantities must be determined in a test in order to calculate the brake horse power8 ,ans4 ,the lever arm1 the force exerted on the scales1 and the revolutions per minute4 ,ques4 ,how is brake horse power calculated8 ,ans4 ,from the following formula3 ,b4 ,h4 ,p4 #b ,l ,n ,w ----------- #cc1jjj in which ,b4 ,h4 ,p4 brake horse power2 ,l lever arm1 in feet2 ,n number of revolutions per minute2 ,w force in pounds at end of lever arm as measured by scales4 ,,example--,in making a brake test on a motor1 the lever arm of the brake is #c ft41 and the reading of the scales is #cj lbs4 ,when the motor is running #a1jjj revolutions per minute1 what is the brake horse power8 ,substituting the given values in the formula1 #b #c #a1jjj #cj ,b4 ,h4 ,p4 --------------------- #ag4a #cc1jjj ,now1 if the voltmeter and ammeter readings be #bbj and #fe respectively1 what is the efficiency of the motor at this load8 ,the amount of power absorbed by the motor1 or in other words1 the input is #bbj #fe ,e4 ,h4 ,p4 -------- #ai4af #gdf and since the output is #ag4a horse power1 output brake horse power #ag4a efficiency ------ ---------------------- ----- #hi4 input electrical horse power #ai4af ,speed of a ,motor4--,the normal speed at which any motor will run is such that the sum of the reverse electromotive force and the drop in the armature will be exactly equal to the electromotive force applied at the brushes4 ,the drop in the armature is the difference between the applied voltage and the reverse voltage4 ,mutual ,relations of ,motor ,torque and ,speed4--,the character of the work to be done not only determines the condition of the motor torque and speed required1 but also the suitability of a particular type of motor for a given service4 ,there are three general classes of work performed by motors1 and these require the following conditions of torque and speed3 #a4 ,constant torque at variable speed2 ,suitable for driving cranes1 hoists1 and elevators1 etc41 where the load is constant and has to be moved at varying rates of speed4 ,illustration3 ,,fig4 #dbi4--,two path method of speed regulation of series motor4 ,a rheostat is connected in shunt to the field coils as shown4 ,the current passing from a to b divides between the magnet coils and the rheostat coils2 the higher the resistance of the rheostat the less current passes through it1 and the more through the magnet coils1 hence the stronger the field magnet4 #b4 ,variable torque at constant speed2 ,suitable for driving line shafting in machine shops1 which must run at constant speed regardless of variations of torque due to variations in the number of machines in operation at a time1 or the character of work being performed4 #c4 ,variable torque at variable speed4 ,suitable for electric railway work4 ,for example3 when a car is started1 the torque is at its maximum value and the speed zero1 but as the car gains headway1 the torque decreases and the speed increases4 ,speed ,regulation of ,motors4--,the speed of motors connected to constant voltage circuits is usually regulated by the two following methods3 ,illustration3 ,,fig4 #dcj4--,variable field method of speed regulation of series motor4 ,the field winding is divided into a number of sections with leads connecting with switch contact points as illustrated4 ,the speed then is regulated by cutting in or out of the circuit sections of the field winding thus varying the strength of the field4 #a4 ,by inserting resistances in the armature circuit of a shunt wound motor2 #b4 ,by varying the strength of the field of a series motor4 ,the first method is sufficiently explained under fig4 #dah and the second method is illustrated in fig4 #dcj4 ,the controller switch ,s is so arranged that a greater or lesser number of field coils can be inserted in the field circuit4 ,when the switch arm is on point #a1 the motor current will flow through all the field windings1 and the strength of the field will be at its maximum4 ,when the switch arm is moved so as to successively occupy positions #b1 #c1 and #d1 thus cutting out of circuit a greater and greater number of field coils the strength of the field will be gradually decreased until practically all of the motor current is led or wired through the armature4 ,under these conditions1 when the field of a motor is at its maximum strength1 the motor torque will be at a maximum for any given strength of current1 and the reverse electromotive force will also be at a maximum for any given speed1 therefore1 when the field strength is increased the speed will decrease and vice versa4 ,ques4 ,what results are obtained by this method of regulation8 ,ans4 ,the speed of a series motor may be nearly doubled1 that is1 if the lowest permissible speed of the motor be #bej revolutions per minute it can be readily increased to #ejj revolutions per minute by changing the field coil connections from series to parallel4 ,it is on this account1 as much as on their powerful starting torque1 that series motors have been until recently almost exclusively employed for electric traction purposes4 ,series ,parallel ,controller4--,when two motors are used in electric railway work1 their armatures are connected in series with each other and an extra resistance which prevents the passage of an excessive current through the armature before the motor starts4 ,as the speed of the car increases1 the extra resistance is gradually cut out of circuit and the field winding connections changed from series to parallel by means of a series parallel controller1 which finally connects each motor directly across the supply mains1 or between the trolley line and the track or ground return4 ,efficiency of a ,motor4--,the commercial efficiency of a motor is the ratio of the output to the input4 ,as a rule1 the power developed by a motor increases as the reverse voltage generated by it decreases1 until this voltage equals one half of the voltage applied at the brushes4 ,after this point is reached1 the power developed by the motor decreases with the decrease of the reverse voltage4 ,therefore1 a motor performs the largest amount of work when its reverse voltage is equal to one half the impressed voltage4 ,illustration3 ,,fig4 #dca4--;,double throw1 double pole switch for reversing direction of rotation of a motor4 ,the direction of rotation can be reversed by changing the direction of current in either the armature or the field coils4 ,it is preferable1 however1 to reverse the direction of rotation by changing the direction of current through the armature4 ,the switch is wired as shown1 means of reversal being provided by running the wires as indicated by the dotted lines4 ,the efficiency of a motor as just stated is the ratio of the output to the input2 this is equivalent to saying that the efficiency of a motor is equal to the brake horse power divided by the electrical horse power4 ,the electrical horse power is easily obtained by multiplying the readings taken from volt meter and ammeter1 which gives the watts1 and dividing the product by #gdf1 the number of watts per horse power4 ,that is3 volts amperes watts ,electrical horse power --------------- ----- #gdf #gdf ,illustration3 ,,fig4 #dcb4--,wiring diagram1 showing electrical connections between the armature1 field1 and interpoles of an interpole motor4 ,as the name implies1 an interpole motor has in addition to the main poles1 a series of interpoles which are placed between the main poles1 and whose function is to assist in the reversal of the current under the brushes4 ,they provide a separate commutating field of a correct value at all loads and speeds1 and their windings are for this purpose connected in series with the armature4 ,the proper functioning of the interpoles is independent of the direction of rotation of the armature1 also of the load carried over the whole speed range4 ,in an ordinary motor without interpoles1 commutation is assisted by a magnetic fringe emanating from the main poles1 but as the value of this fringe is altered by the load of the motor and by rheostatic field weakening1 if higher speeds be desired from such a machine1 commutation becomes imperfect and sparking results1 making a readjustment of the brushes necessary4 ,interpole ,motors4--,an interpole motor has in addition to the main poles1 a series of interpoles1 placed between the main poles4 ,the object of these poles is to provide an auxiliary flux or commutating field at the point where the armature coils are short circuited by the brush4 ,illustration3 ,,figs4 #dcc to #dcg4--,parts of the type ,s interpole motor built by ,electro-,dynamic ,co4 ,they are as follows3 #a4 yoke--commutator view2 #b4 interpole coil2 #c4 top ,r4,h4 main coil2 #d4 bottom ,r4,h4 main coil2 #e4 main pole2 #f4 interpole2 #g4 armature shaft1 ,r4,o4 bearing2 #h4 commutator2 #i4 armature wedge2 #aj4 armature coil2 #aa4 brush ring2 #ab4 brush carrier insulation2 #ac4 brush carrier2 #ad4 brush guard2 #ae4 carbon brush2 #af4 brush holder2 #ag4 cross connecting cable2 #ah4 oil ring2 #ai4 commutator end bearing bushing2 #bj4 pulley end bearing bushing4 ,ques4 ,what is the object of the commutating field produced by the interpoles8 ,ans4 ,its object is to assist commutation1 that is1 to help reverse the current in each coil while short circuited by the brush1 and thus reduce sparking4 ,illustration3 ,,fig4 #dch4--;,interpole motor as built by the ,electro ,dynamic ,co4 ,this type of motor is devised to prevent sparking at all loads by the use of interpole magnets1 that is1 small magnets placed between the field magnets4 ,the interpoles set up a field in a direction to stop and reverse the current in the armature coils while they are short circuited by the brushes4 ,ques4 ,what is the nature of the commutating field8 ,ans4 ,the excitation of the interpoles being produced by series turns1 the field will vary with the load1 and will1 if once adjusted to give good commutation at any one load1 keep the same proportion for any other load1 provided the iron parts of the circuit be not too highly saturated4 ,ques4 ,state briefly how sparking is reduced or prevented by the action of the interpoles4 ,ans4 ,sparking is due to self induction in the coil undergoing commutation1 which impedes the proper reversal of the current4 ,the action of the interpoles corrects this in that they set up a field in a direction that causes a reversal of the current in the coil while it is short circuited4 ,thus1 the coil at the instant it leaves the brush1 is not an idle coil1 but has a current flowing in it in the right direction to prevent sparking4 ,ques4 ,mention some of the claims made for interpole motors4 ,ans4 ,constant or adjustable speed1 and momentary overloads without sparking2 constant brush position2 operation at adjustable speeds on standard supply circuits of #aaj1 #bbj1 and #ejj volts2 constant speed with variable load2 reversal without changing the position of the brushes4 ,,chapter ,,xxiv ,,selection ,,and ,,installation ,,of ,,dynamos ,,and ,,motors ,general ,conditions ,governing ,selection4--,in any particular case1 the voltage1 current capacity1 and type of dynamo selected will depend upon the system of transmission or distribution to which it is to be connected1 and the character of the work which it is required to perform4 ,the suitability of the different types of dynamo for various kinds of work has already been considered to some extent1 but there are certain general conditions which are applicable to almost all cases1 such as3 #a4 ,construction2 #b4 ,operation2 #c4 ,cost2 #d4 ,number and size of units4 ,construction4--,this should be as simple as possible and of the most solid character4 ,all parts should be interchangeable1 and have a good finish4 ,all machines should be provided with eye bolts or other means by which they can be lifted or moved1 as a whole or in parts1 easily and without injury4 ,these features are so carefully attended to and guaranteed by the manufacturers as to leave little choice in this direction4 ,operation4--,the considerations relating to the operation of a machine involve an examination of the details of its construction1 in order to determine the amount of attention it will require1 the character of its regulating device1 its capacity1 form1 and weight4 ,ques4 ,what may be said regarding capacity8 ,ans4 ,dynamos and motors should not be overloaded1 because the efficiency is greater when the working load does not exceed the rated capacity of the machine4 ,form4--,as a rule1 there is not much choice in the matter of form between standard machines1 as they are uniformly symmetrical1 well proportioned and compact4 ,it is a mistake1 however1 to select a light machine for stationary use1 as the weight of a machine increases its strength1 stability and durability4 ,cost4--,in some cases1 the matter of first cost is important and deserves careful consideration4 ,it should be remembered1 however1 that high grade electric machinery cannot be built out of low grade materials and with poor workmen2 therefore1 when necessity compels the selection of a cheap machine1 it should not be expected that its service will be as satisfactory as that of a first class machine4 ,number and ,size of ,units4--,the best number and size of units for an electrical plant is usually governed by the requirements of the driving engines4 ,as a rule1 dynamos and motors are not much less efficient at quarter load than at full load1 and the smaller dynamos are fully equal to the larger machines in this respect1 therefore1 a generating plant can be subdivided1 and if so desired1 without any detrimental results except those to a multiplicity of units4 ,ques4 ,what is the important consideration with respect to efficiency8 ,ans4 ,efficiency at maximum load is not so important as efficiency at average load4 ,for instance1 in the diagram1 fig4 #dci1 the rated efficiency of one dynamo as shown by the curve ,a1 is #ie per cent41 and that of another1 as shown by curve ,b1 is #ia per cent41 but it will be observed that the average efficiency of ,b is much higher1 being #ge per cent4 at quarter-load1 #hi per cent4 at half-load1 and #ia per cent4 at three-quarter load1 to #ee1 #gg and #hi per cent4 of ,a1 at the corresponding loads4 ,in this case1 ,a is higher than ,b only at full load1 and as full load is a limit which should not be reached except in special cases1 and then only for short intervals of time1 the service rendered by ,b would be much more satisfactory in the long run4 ,in order to avoid the difficulties possible under these conditions1 a guarantee to carry #be per cent4 overload for two hours without injury should be required1 and either this or the rated load be taken1 as the full load1 so as to give a factor of safety of #be per cent4 ,illustration3 ,,fig #dci4--;,efficiency curves for #ajj ,k4 ,w4 dynamos4 ,the efficiency of a dynamo at maximum load is not so important as at average load4 ,for instance1 if in the figure the curve ,o ,b ,c represent the efficiency of a #ajj ,k4 ,w4 dynamo and ,o ,a ,d1 that of another machine1 it would be in accordance with common practice to compare them at rated load1 at which the efficiency of the first is only #ia1 while the other is #ic4 ,the first machine1 however1 is far better than the second1 since its average efficiency is much higher1 being nearly #ia between half load and #be overload4 ,it should be noted that full load is a limit which should be but occasionally reached1 and then only for short periods of time4 ,ques4 ,upon what does the choice of field winding of a dynamo depend8 ,ans4 ,the different classes of field winding have already been discussed1 but in general the conditions governing selection are as follows3 ,the series dynamo is used where a constant current at variable voltage is desired1 as in series arc lamp circuits4 ,a shunt dynamo is used on constant voltage circuits1 where the distance from the machine to the load is not great1 that is1 where there is small line loss4 ,with a compound dynamo there is compensation for line loss1 that is1 it can be constructed so that the voltage at its terminals1 or at the load can be maintained constant or allowed to increase or decrease with a change in load4 ,it can thus operate lamps at constant voltage though they be located at some distance1 or the voltage at the end of the line can be made to increase with an increase of load1 as is frequently the case in railway work4 ,illustration3 ,,fig4 #ddj4--;,holzer-,cabot performance curves of standard #bj ,h4 ,p4 motor1 showing efficiency1 speed regulation1 and amperes input4 ,ques4 ,for what conditions of service are series motors adapted8 ,ans4 ,they are used on constant current circuits1 and also on constant voltage circuits as in railway work and similar purposes where an attendant is always at hand to regulate the speed4 ,ques4 ,name some advantages and disadvantages of series motors4 ,ans4 ,they are easily started even under heavy loads1 the winding is cheaper than the other types and the speed is nearer constant than shunt motors when operated on constant current circuits4 ,when used on constant pressure circuits1 such as is employed for incandescent lighting1 the speed will depend on the load4 ,ques4 ,what kind of circuit is suitable for shunt motors8 ,ans4 ,they are used on constant voltage circuits4 ,ques4 ,what are the advantages of shunt motors8 ,ans4 ,the speed remains nearly constant for variable load4 ,ques4 ,state the disadvantages4 ,ans4 ,they start less easily under a heavy load than do series motors1 and the speed cannot be varied through any wide range without considerable loss4 ,the shunt motor requires more attention than the series type and is more liable to be burnt out4 ,location4--,the place chosen for the dynamo or motor should be dry1 free from dust1 and preferably where a cool current of air can be had4 ,it should allow sufficient room for a belt of proper length when a belt drive is used4 ,foundations4--,it is most important to secure a good foundation for every dynamo1 and great care should be taken to have them entirely separate from those of the walls of the building in which the machine is installed1 and if the dynamo be directly driven1 but not on the same bed plate as the engine1 a foundation large enough for both together should be laid down4 ,stone or concrete may be used1 or brick built with cement1 having a large thick stone bedded at the top4 ,for small machines the holding down bolts may be set with lead or sulphur in holes in the stone top1 but for large machines the bolts should be long enough to pass down to the bottom1 where they should be anchored with iron plates4 ,setting up of ,dynamos and ,motors4--,in unpacking the machines care should be taken to avoid injury to any part1 and in putting the parts together1 each part should be carefully cleaned1 and all the parts put together in exactly the right way4 ,the shafts1 bearings1 magnetic joints1 and electrical connection should receive especial attention and be thoroughly cleaned of every particle of dirt1 grit1 dust1 metal clippings1 etc4 ,ques4 ,who should preferably assemble the machines8 ,ans4 ,whenever possible1 they should be assembled by someone thoroughly familiar with the construction2 but if the services of such a person cannot be had1 no one should attempt to put a machine together unless he has a drawing or photograph of the same for a general guide4 ,ques4 ,what precaution should be taken with the armature8 ,ans4 ,it should be handled carefully to avoid any injury to the wires of the winding and their insulation4 ,if it become necessary to lay the armature on the ground it should be laid on clean paper or cloth1 but it is better to support it by the shaft on two wooden horses or other supports1 and thus avoid any strain on the armature body or commutator4 ,illustration3 ,,fig4 #dda4--;,foundation4 ,it may be made either of concrete1 stone or brick4 ,the machinery is held firmly in place on the foundation by anchor bolts built into it2 the proper position for the bolts are determined by a wooden template suspended above the foundation as shown4 ,the bolts are surrounded by iron pipe that fixes them vertically but permits a little side play to allow for any slight errors in locating the centers on the template4 ,connecting ,up ,dynamos4--,the manner in which the connections of the field magnet coils1 brushes1 and terminals1 are connected to one another depends entirely upon the type of machine4 ,the field magnet shunt coils of shunt and compound wound dynamos1 are invariably arranged in series with one another1 and then connected as a shunt to the brushes or terminals of the machine4 ,the series coils of series and compound wound machines are arranged either in series or in parallel with one another1 according to conditions of operation1 and then connected in series to the armature and external circuit4 ,coupling ,up ,field ,magnet ,coils4--,in coupling up the coils of either salient or consequent pole field magnets1 assume each of the pole pieces to have a certain polarity 7in bipolar dynamos two poles only1 a north and south pole respectively1 are required2 in multipolar dynamos the poles must be arranged in alternate order around the armature1 the number of ,n and ,s poles being equal71 then apply ,flemming's rule as given under fig4 #acb1 to each of the coils1 and ascertain the direction in which the magnetizing current must flow in each in order to produce the assumed polarity in each of the pole pieces4 ,having marked these directions on the coils1 they can be coupled up in either series or parallel connection according to requirements1 so that the current flows in the proper direction in each4 ,illustration3 ,,fig4 #ddb4--;,comparison of space occupied by direct and belt connected dynamos4 ,in office buildings space is of value and the room required by belt connected dynamos can always be put to profitable use4 ,for this reason the direct connected unit has become generally adopted in the best type of office buildings4 ,in large factories the direct connected unit is generally adopted also to save space4 ,where these conditions do not obtain1 belted type of dynamo can be used to advantage as a given output can be obtained with a smaller size machine than where it is direct connected to the engine4 ,this is due to the limited rotative speeds at which engines can be run4 ,the illustration shows the relative space required by the two types4 ,the ,drive4--,various means are employed to connect the engine or other prime mover with the dynamo1 or the motor with the machinery to driver4 ,among these may be mentioned the following3 #a4 ,direct drive2 #b4 ,belt drive2 #c4 ,rope drive2 #d4 ,gear drive2 #e4 ,friction drive4 ,illustration3 ,,fig4 #ddc4--;,general ,electric type ,m ,p1 marine generating set with tandem compound engine4 ,the requirements of such units are compactness1 light weight1 simplicity1 freedom from vibration and noise at high speed1 perfect regulation and durability4 ,by adopting a short stroke for the engines and a special armature winding for the dynamos1 the height and length of the sets have been reduced4 ,the bed is carried out to the full width of the dynamo frame1 making an ample base surface for foundation without increasing the floor space required4 ,while the construction gives a massive appearance1 the bed has been cored out and the various parts so designed that the complete sets have an approximate capacity of #c watts per pound4 ,all of the moving parts are enclosed by the engine column1 excluding dust and reducing wear and attention to a minimum4 ,the bearing are oiled automatically under pressure4 ,these sets are made in sizes from #be ,k4 ,w4 to #ge ,k4 ,w41 the cylinder dimensions for the smallest size being #f and #aj by #e1 and for the largest size #aj and #ah by #h4 ,single cylinder sets are made in sizes ranging from #b ,k4 ,w4 to #ej ,k4 ,w41 the cylinder dimensions ranging from #c #c to #ab #aa4 ,see fig4 #gcj4 ,ques4 ,what is a direct drive8 ,ans4 ,one in which the driving member is connected direct to the driven member1 without any interposed gearing4 ,fig4 #ddc shows a direct connected unit1 which is an example of direct drive4 ,ques4 ,what may be said with respect to direct drive8 ,ans4 ,it is the simplest method and the space required is less than with belt drive4 ,with direct drive the engine and dynamo must run at the same speed2 this is a disadvantage because the desirable speeds of the two machines may not agree4 ,since the usual engine speeds are slower than dynamo speeds1 direct drive involves the use of a larger dynamo for a given output than would be necessary with belt connection1 and involves a corresponding increase in cost and greater friction loss due to the rotation of larger and heavier parts4 ,illustration3 ,,fig4 #ddd4--;,belt clamp for stretching belt and holding the ends while making joint4 ,it consists of a stretching frame1 the two ends of which are coupled by screwed bars2 used for pulling the ends of a belt together with the proper tension1 when lacing or joining the ends4 ,ques4 ,mention some of the features of belt drive4 ,ans4 ,greater flexibility in the original design of a plant is possible and new arrangements of old apparatus can be made at any time4 ,it gives conveniently any desired speed ratio and permits the use of high speed dynamos and motors4 ,ques4 ,state some of the disadvantages of belt drive4 ,ans4 ,considerable space is required and the action is not positive4 ,belts exert a side pull on the bearings which results in wear1 also loss of power by friction4 ,illustration3 ,,figs4 #dde and #ddf4--,two methods of lacing a belt4 ,in fig4 #dde two rows of oval holes should be made with a punch1 as indicated4 ,the nearest hole should be inch from the side1 and the first row #g_/h inch from the end1 and the second row #a inches from the end of the belt4 ,in large belts these distances should be a little greater4 ,a regular belt lacing 7a strong1 pliable strip of leather7 should be used1 beginning at hole ,no4 #a1 and passing consecutively through all the holes as numbered4 ,in fig4 #ddf the holes are all made in a row4 ,this method has the advantage of making the lacers lie parallel with the motion on the pulley side4 ,the lacing is doubled to find its middle1 and the two ends are passed through the two holes marked #a and #a;a precisely as in lacing a shoe4 ,the two ends are then passed successively through the two series of holes in the order in which they are numbered1 #b1 #c1 #d1 etc41 and #b;a1 #c;a1 #d;a1 etc41 finishing at #ac and #ac;a1 which are additional holes for fastening the ends of the lacer4 ,ques4 ,give a rule for determining the proper size of belt4 ,ans4 ,a single belt travelling #a1jjj feet per minute will transmit one horse power per inch of width2 a double belt will transmit twice this amount4 ,,example4--,what size of double belt is required to transmit #ej horse power at #d1jjj ft4 speed1 and what diameter pulley must be used for #ied revolutions per minute at #d1jjj ft4 speed of belt8 ,illustration3 ,,fig4 #ddg4--,wrong way to run a belt4 ,the pull should not come on the top side1 because1 with slack at bottom there is a tendency to slip4 ,the horse power transmitted per inch is #d1jjj ----- #b #h #a1jjj accordingly1 the width of belt required to transmit #ej horse power is #ej #h #f4be1 say #f4 ,for #d1jjj ft4 per minute belt speed1 the distance in inches travelled by the belt per revolution of the pulley4 #d1jjj #ab ---------- #ej4ca inches #ied ,this is equal to the circumference of the pulley1 and the corresponding diameter is #ej4ca ---------- #af4a1 say #af inches4 ,ques4 ,what is the proper speed for a belt8 ,ans4 ,from #c1jjj to #e1jjj feet per minute1 depending on conditions4 ,points ,relating to ,belts4 #a4 ,the amount of power that a belt of given size can transmit is not a very definite quantity4 ,the rule just given is conservative and will give an amply large belt for ordinary conditions4 #b4 ,a belt should make a straight run through the air and over the pulleys without wabbling2 it should maintain an even and perfect contact with that part of the pulley with which it comes in contact4 ,in order to do this it should be kept soft1 pliable1 and have no abrasions or rough places4 ,illustration3 ,,fig4 #ddh4--,right way to run a belt4 ,the pull should come on the lower side bringing the slack on top4 #c4 ,when belt fasteners give way there is too much strain upon belt4 ,the greatest amount of slack in a belt is found where it leaves the driving pulley1 hence the tightener should be near the driving pulley1 as it takes up the slack1 prevents vibration and diminishes strain on belts and bearings4 ,more than #ajj degrees of heat is injurious to belts4 #d4 ,double belts should always run with the splices1 and not against them4 ,quarter turn belts should be made of two ply leather1 so as to diminish the side strain4 #e4 ,friction is greatest when the pulleys are covered with leather4 ,friction depends upon pressure1 but adhesion depends upon surface contact2 the more a belt adheres to pulley surface without straining1 through too much tightening1 the better the driving power4 ,slipping occurs on wet days because the leather absorbs dampness4 #f4 ,a leather covered pulley will produce more resistance than polished or rough iron ones4 ,a good belt dressing makes a smooth1 resisting surface1 and as it contains no oils which create a slippery surface to belts1 it increases belt adhesion4 ,the friction of leather upon leather is five times greater than leather upon iron4 #g4 ,moisture and water distend the fibres1 change the properties of the tanner's grease and softening compounds4 ,repeated saturation and drying will soon destroy leather4 ,leather well filled with tanner's grease or animal oil1 if allowed to hang in a warm room for several months without handling1 will dry out1 become harsh1 and will readily crack4 #h4 ,a running belt is stretched and relaxed at different times and unless there be perfect elasticity in all its parts there will not be uniform distension4 #i4 ,there should be #be per cent4 margin allowed for adhesion before a belt begins to slip4 ,illustration3 ,,figs4 #ddi to #dea4--,method of aligning engine and dynamo4 ,in fig4 #ddi1 a line is stretched from ,a to ,e and the dynamo shifted until the line contacts with points ,a1 ,d1 ,i1 and ,e4 ,in a small dynamo1 the pulley may be loosened and set back on the shaft as in fig4 #dej1 while lining up the faces1 and then moved back to its original position as in fig4 #dea4 ,when the pulley is not easily shifted the distances at ,a and ,d 7fig4 #ddi7 may be measured4 ,rules for ,calculating ,speed and ,sizes of ,pulley4--,when two pulleys are working together connected by a belt1 the one which communicates the motion is called the driver and the other which receives it1 the driven pulley4 ,to ,find the ,size of the ,driving ,pulley3 ,multiply the diameter of the driven pulley by its required number of revolutions1 and divide the product by the revolutions of the driver4 ,the quotient will be the diameter of the driver4 ,to ,find the ,number of ,revolutions of the ,driven ,pulley3 ,multiply the diameter of the driver by its number of revolutions1 and divide by diameter of driven4 ,the quotient will be the number of revolutions of the driven4 ,to ,find the ,diameter of the ,driven that shall ,make a ,given ,number of ,revolutions1 the ,diameter and ,revolutions of the ,driver ,being ,given3 ,multiply the diameter of the driver by its number of revolutions1 and divide the product by the number of revolutions of the driven pulley4 ,the quotient will be the diameter of the driven pulley4 ,rope ,drive4--,in this method of power transmission1 rope is run in ,v-shaped grooves in the rims of the pulleys2 this form of drive1 in some cases1 is more desirable than others4 ,illustration3 ,,fig4 #deb4--;,general ,electric ,c ,q back geared motor driving ,hamilton sensitive drill4 ,when slowly moving machines are to be driven1 or where1 for any reason1 very moderate belt speeds are required1 the back geared motor is desirable4 ,two ratios of gear reduction have been adopted as standard2 they are3--#d to #a and #h to #a4 ,ques4 ,what are some of the advantages of rope drive8 ,ans4 ,more power can be transmitted with a given diameter and width of pulley1 on account of the increased grip in the grooves4 ,rope drive can be employed for long or short distances by reason of its lightness and the action of the grooves4 ,gear ,drive4--,this method is used where a positive drive is desired1 as for elevator or railway motors4 ,it admits of any degree of speed reduction without attending difficulties as would be encountered with belt drive4 ,thus1 with the worm type of gear as used on elevator motors a great reduction in velocity can be made without incurring the expense of countershaft as with a belt4 ,illustration3 ,,fig4 #dec4--,watson vertical motor designed to operate a vertical shaft1 either through belt connection1 or by direct drive4 ,hess-,bright ball bearings are used1 taking the downward thrust due to the weight of the armature4 ,for mounting on the floor or ceiling1 a tripod base 7as shown7 is furnished1 the standard sliding base being used on a side wall4 ,the armature shaft may be extended for pulley or coupling either above or below the motor4 ,friction ,drive4--,this is a very simple mode of transmitting power and has the advantages of simplicity and compactness4 ,in operation1 the driving wheel is pressed against the wheel to be driven1 transmitting motion to the latter by the frictional grip4 ,the drive is thrown out of gear by slightly moving the machine on its sliding base4 ,in construction1 the friction may be increased by making one wheel of the pair of wood1 compressed paper1 or leather4 ,electrical ,connections4--,circuits for dynamos and motors should be carefully planned so as to secure the simplest arrangement1 and to avoid unnecessary expense and delay1 the wiring should be installed in accordance with the requirements of the ,national ,electrical ,code4 ,illustration3 ,,fig4 #ded4--,sling for handling armatures4 ,in raising an armature it should be supported by the shaft to avoid any strain on the armature body or commutator4 ,ques4 ,what may be said with respect to exposed and concealed wiring8 ,ans4 ,exposed wiring is cheap and accessible2 a short circuit or ground is easily located and repaired4 ,concealed wiring1 especially when placed under the floor1 has the advantage of being out of the way1 and thus protected from injury4 ,ques4 ,in wiring a dynamo what are the considerations with respect to size of wire8 ,ans4 ,all conductors1 including those connecting the machine with the switchboard1 as well as the bus bars on the latter1 should be of ample size to be free from overheating and excessive loss of voltage4 ,the drop between the generator and switchboard should not exceed one-half per cent4 at full load1 because it interferes with proper regulation and adds to the less easily avoided drop on the distribution system4 ,,chapter ,,xxv ,,auxiliary ,,apparatus ,there are numerous devices that must be used in connection with dynamos and motors for proper control and safe operation4 ,among these may be mentioned3 #a4 ,switches2 #b4 ,fuses2 #c4 ,circuit breakers2 #d4 ,rheostats2 #e4 ,switchboards4 ,switches4--,a switch is a device by means of which an electric circuit may be opened or closed4 ,there are numerous types of switch2 they may be either single or multi-pole1 single or double throw and either of the snap or knife form4 ,ques4 ,what is the difference between a single and double pole switch8 ,ans4 ,a single pole switch controls only one of the wires of the circuit1 while a double pole switch controls both4 ,ques4 ,what is the difference between a single break and a double break switch8 ,ans4 ,the distinction is that the one breaks the circuit at one point only1 while the other breaks it at two points4 ,ques4 ,what is the advantage of a double break8 ,ans4 ,if the circuit be opened at two points in series at the same instant1 the electromotive force is divided between the two breaks and the length to which the current will maintain an arc at either break is reduced to one-half2 thus there is less chance of burning the metal of the switch4 ,another reason for providing two breaks is to avoid using the blade pivot as a conductor1 the contact at this point being too poor for good conductivity4 ,illustration3 ,,figs4 #dee to #deg4--;,adam's single throw knife switches without fuse connections4 ,fig4 #dee1 single pole switch2 fig4 #def1 double-pole switch2 fig4 #deg1 three-pole switch4 ,illustration3 ,,figs4 #deh to #dfj4--;,adam's single throw knife switches with fuse connections at the handle end4 ,fig4 #deh1 single pole switch2 fig4 #dei1 double pole switch2 fig4 #dfj1 three pole switch4 ,ques4 ,when should a knife switch be used8 ,ans4 ,when the capacity of the circuit in which it is to be placed exceeds #aj amperes4 ,ques4 ,describe a knife switch4 ,ans4 ,fig4 #dfa illustrates a knife switch of the double pole1 single throw type4 ,it consists of the following parts3 base1 hinges1 blades1 contact jaws1 insulating cross bar1 and handle1 as shown4 ,ques4 ,how should knife switches be installed8 ,ans4 ,they should be placed so that gravity tends to open them4 ,otherwise if the hinges become loose1 the weight of the blades and handle would tend to close the switch1 thus closing the circuit and possibly resulting in considerable damage4 ,illustration3 ,,fig4 #dfa4--;,a single throw1 two pole knife switch4 ,as usually constructed it is made of hard drawn copper with cast terminal lugs and fibre cross bar4 ,ques4 ,how should switches be proportioned8 ,ans4 ,the minimum area of the contact surfaces should not be less than 4#ja square inch per ampere1 and in those used on arc lighting or other high voltage circuits where the current is usually small1 the area of the contact surfaces are usually from 4#jb to 4#je inch per ampere4 ,since dirt or oxidation would prevent good contact under a simple pressure between the contact surfaces1 the mechanism of a switch provides a sliding contact4 ,in the general design of switches1 all parts which carry current are given a cross sectional area of at least one square inch per #a1jjj amperes if they be made of copper1 and about three times as much if made of brass1 as the conductivity of the latter is only one-third that of the former4 ,furthermore1 the current should never be permitted to pass through springs1 as the heat generated will destroy their elasticity4 ,illustration3 ,,fig4 #dfb4--,triple pole1 double break double throw knife switch for very heavy current4 ,the blades are made up of numerous strips to give adequate contact area4 ,a double throw switch is used when it is desirable to open one circuit and immediately close another1 or to transfer one or more connections from one circuit to another in the least practical interval of time1 also1 when one connection is to be broken and another closed and it is undesirable to allow both to be closed at the same time4 ,ques4 ,what difficulty is experienced in opening a circuit in which a heavy current is flowing8 ,ans4 ,it is impossible to instantly stop the current by opening the switch1 consequently the current continues to flow and momentarily jumps the air gap1 resulting in a more or less intense arc which tends to burn the metal of the switch4 ,ques4 ,how is this remedied to some extent8 ,ans4 ,the contact pieces are so shaped that they open along their whole length at the same time1 so as to prevent the concentration of the arc at the last point of contact4 ,this feature is clearly shown in fig4 #dfa4 ,illustration3 ,,fig4 #dfc4--;,a quick break knife switch of the single throw1 single break1 one pole1 type4 ,the contact blade is held between the jaws by their clamping friction until the handle compresses the spring sufficiently to force the blade out4 ,as soon as it breaks contact with the jaws1 the spring expands and drives the blade away from the jaws with greater rapidity than could be done by hand4 ,the object of this action is to break the arc as quickly as possible to prevent burning the metal of the switch4 ,illustration3 ,,figs4 #dfd and #dfe4--,snap switch2 views showing switch with cover on1 and exposed to show mechanism4 ,the switch is provided with indicating dial which registers on and off positions4 ,,fig4 #dff4--;,gas ,engine snap switch4 ,the first snap makes connection so that igniter is run from storage battery2 second snap connections are changed so that igniter is supplied from dynamo2 third snap makes connections so that dynamo supplies igniter and charges storage battery2 fourth snap1 all off4 ,ques4 ,for what service are snap switches suitable8 ,ans4 ,they are used on circuits containing lamps in comparatively small groups1 and other light duty service4 ,ques4 ,what is a quick break switch8 ,ans4 ,a form of switch in which the contact pieces are snapped apart by the action of the springs1 as shown in fig4 #dfc1 so as to make the duration of the arc as short as possible4 ,the current allowed in each branch circuit of an electric lighting system is limited by the insurance rules to a maximum of #ffj watts equivalent to #ab lamps of #af c4p4 each at #aaj volts4 ,they are also employed to control lamps in groups in theatres and other places where many lamps are turned on or off at about the same time4 ,illustration3 ,,fig4 #dfg4--,spool of fuse wire4 ,the wire is usually made of an alloy of tin and lead1 such as half and half solder4 ,bismuth is frequently added to the alloy to lower the melting point ,for half and half solder the melting point is #cgj ,fahr4 ,the quickness with which a fuse will melt after the current has reached the limit depends upon the specific heat and latent heat of the metal4 ,the current required to blow a fuse increases somewhat with the age of the fuse owing to oxidation and molecular changes4 ,fuses are sometimes rated according to the number of amperes to be taken normally by the circuit they are to protect4 ,thus1 a #aj ampere fuse is supposed to protect a circuit whose regular current should not exceed #aj amperes1 and to blow if the current rise to say #ab amperes4 ,the ,underwriters' rule requires that the rating be about #hj of the maximum current it can carry indefinitely1 thus allowing about #be overload before the fuse melts4 ,the fusing current varies considerably according to circumstances4 ,the temperature of the surrounding air or other substances affects the melting current greatly1 because the rate at which heat from the fuse will be transferred to the surroundings depends upon the difference of temperance between them and the fuse4 ,hence a fuse in a warm place will be melted by a smaller current than a similar fuse in a cold place4 ,for a similar reason1 a fuse in an enclosed place where there is little chance for the heat to be dissipated1 will melt with a smaller current than the same in an open place4 ,if the current increase gradually to that which would ordinarily melt the fuse1 the high temperature makes the fuse wire oxidize rapidly2 this sometimes makes a sort of tube of oxide which will not break even after the fuse wire inside has melted1 and so the fuse carries more than its rated current4 ,open fuses are so unreliable that circuit breakers are preferable for large currents2 when fuses are used1 the enclosed type as shown in figs4 #dfh to #dgj1 is usually the more desirable4 ,fuses4--,all circuits subject to abnormal increase of current which might overheat the system1 should be protected by fuses which will melt and thus open the circuit4 ,a fuse is simply a strip of fusible metal1 often consisting of lead with a small percentage of tin1 connected in series in the circuit4 ,experiments have shown that for large fuses1 a multiple fuse is more sensitive than a single one4 ,a one hundred ampere fuse may be made by taking four wires of twenty-five amperes capacity4 ,a fuse block may be overloaded1 not because the metal of the terminals is not of sufficient cross section to carry the current1 but because of insufficient area of1 contact1 or loose contact of fuse and wires2 the overload thus caused results in heating and frequently melts the fuse4 ,illustration3 ,,figs4 #dfh to #dgj4--;,d ,w1 enclosed cartridge fuses4 ,fig4 #dfh1 type for #c to #fj amperes2 fig4 #dfi1 type for #fa to #ajj amperes2 fig4 #dgj1 type for #aja to #a1jjj amperes4 ,ques4 ,where should fuses be placed8 ,ans4 ,they should be inserted wherever the size of wire changes or wherever there is a branch of smaller size wire connected1 unless the next fuse on the main or larger wire is small enough to protect the branch or small wire4 ,illustration3 ,,figs4 #dga to #dgh4--;,interior construction of ,d4 ,w4 fuses4 ,in the manufacture of these fuses1 four types of fuse link are used according to capacity of fuse1 and classified as3 #a1 air drum link2 #b1 flat link2 #c1 multiple link2 #d1 cylinder link4 ,in the air drum link1 figs4 #dga and #dgb1 a capsule provides an air space about the center of the link1 the rate of heat conduction through the confined air being very slow1 the temperature of that portion of the link rises rapidly with increasing current1 rendering the blowing point practically constant2 fig4 #dgc shows a section through the complete fuse4 ,in the flat link1 fig4 #dgd1 the section is reduced in the center1 cutting down as far as possible the volume of metal to be fused4 ,figs4 #dge to #dgh show various form of multiple link construction4 ,by sub-dividing the metal1 increased radiating surface is obtained which permits a reduction in the volume of fusible metal necessary1 and the metal vapor formed when the fuse blows on heavy overload is more readily dissipated4 ,figs4 #dgg and #dgh show two forms of the cylinder link1 the plain cylinder fig4 #dgg1 being used for low voltage and large current1 and fig4 #dgh1 for certain high tension service4 ,the corrugated cylinder presents more surface to the fuse filling than the plain type and secures a maximum radiating surface with resulting minimum volume of metal for a given current4 ,ques4 ,how should fuses be mounted8 ,ans4 ,they should be placed on a base of slate1 porcelain1 marble1 or other incombustible material4 ,ques4 ,what is the objection to copper fuses8 ,ans4 ,they heat perceptibly soon after their rated capacity is passed4 ,the melting temperature is higher than lead alloy4 ,ques4 ,upon what consideration does the choice between switches and circuit breakers depend8 ,ans4 ,simple knife switches are suitable for use when the circuit is not liable to be opened while carrying large current4 ,a circuit breaker1 operated automatically or by hand should be used for interrupting heavy currents4 ,illustration3 ,,figs4 #dgi and #dhj4--;,d ,w fuse indicator4 ,the operation is illustrated in the figures which show appearance of the label before the blowing of the fuse1 fig4 #dgi1 and the same fuse blown1 as indicated by the appearance of the black spot within the circle fig4 #dhj4 ,circuit ,breakers4--,a circuit breaker is a switch which is opened automatically when the current or the pressure exceeds or falls below a certain limit1 or which can be tripped by hand4 ,ques4 ,what is the construction of a circuit breaker8 ,ans4 ,it is composed of a switch and a solenoid in the main circuit4 ,when the current1 flowing through the circuit1 exceeds a certain value1 the core of the solenoid is drawn in and trips a trigger which allows the switch to fly open under the action of a spring4 ,illustration3 ,,figs4 #dha to #dhf4--,various open fuses4 ,fig4 #dha1 fuse for main and branch blocks2 fig4 #dhb1 standard railway fuse2 fig4 #dhc1 ,edison main style2 fig4 #dhd1 ,w4,u4 pattern2 fig4 #dhe1 ,bell telephone style2 fig4 #dhf1 sneak current fuse4 ,when an open fuse blows as a result of overloading1 the rupture is accompanied by a flash1 and by spattering of the fused material4 ,with large currents this phenomenon is a source of danger1 and the use of enclosed fuses is accordingly recommended whenever the rating of the fuse exceeds #be amperes4 ,various types of enclosed fuse are shown in figs4 #dfh to #dgj4 ,there are numerous kinds of circuit breaker to meet the varied conditions of service of which may be mentioned the following3 #a4 ,maximum circuit breaker2 #b4 ,minimum circuit breaker2 #c4 ,reverse current circuit breaker2 #d4 ,maximum and reverse circuit breaker2 #e4 ,no voltage circuit breaker4 ,illustration3 ,,figs4 #dhg and #dhh4--,reverse current circuit breaker2 fig4 #dhh1 view looking at end of coils of cut out1 showing direction of current4 ,a to bus bar2 ,b1 resistance lamp2 ,c1 brush of cut out2 ,d1 shunt coil2 ,e1 series coil2 ,f1 core that trips cut out2 ,g1 to - bus bar2 ,h1 to pole of dynamo4 ,of these the maximum1 reverse1 and maximum and reverse types are the more important4 ,a maximum circuit breaker is equivalent to a fuse1 but has the advantage that it can be at once reset1 whereas a fuse must be replaced4 ,a reverse breaker is used in connection with dynamos in parallel1 to automatically cut out a machine if it take more than say1 #aj per cent4 motor current4 ,maximum and reverse circuit breakers are frequently used on dynamo panels4 ,illustration3 ,,figs4 #dhi and #dij4--;,front and top views of ,i-,t-,e automatic overload circuit breaker4 ,in fig4 #dhi the current in the circuit enters at ,a1 passes through the solenoid coil ,b 7which in its iron jacket becomes a powerful magnet71 through the copper terminal ,c1 to the contact blades ,d1 across the bridge at ,e to the contact blades ,f1 and out into the line at ,g4 ,the path of the current as indicated above is more clearly indicated in the top view fig4 #dij4 ,when the current in the solenoid coil produces sufficient magnetism to overcome the weight of the plunger1 the latter is drawn up with constantly increasing velocity until it strikes a restraining latch or trigger which forces the arm out of the switch1 thus automatically opening the circuit4 ,the device is so constructed that in opening the circuit the arc is broken on the carbon contacts instead of the copper contacts4 ,ques4 ,describe a reverse current circuit breaker or discriminating cut out4 ,ans4 ,this type of circuit breaker is arranged to open a circuit in the event of current flowing in the circuit in a direction reverse to the normal4 ,this is sometimes effected by winding the electromagnet of the circuit breaker with two coils1 one connected as a shunt across the main circuit and the other in series with the main circuit1 the two coils being so arranged that when the main current flows in the normal direction their effects assist one another1 whereas1 when the main current reverses1 the effects of the coils are neutralized and the breaker opens4 ,illustration3 ,,fig4 #dia4--,roller-,smith ,s4,e4 plain overload circuit breaker4 ,in operation1 current entering through the lower studs flows through the laminated strap windings ,c1 from this into the arm ,d1 through the contact plate ,e1 into the stationary brush ,f1 and finally out through the upper stud ,q4 ,in its passage through the laminated windings ,c1 the square core ,a is of course magnetized to a degree dependent on the current strength4 ,when this magnetization reaches a pre-determined value1 the attraction exerted on the ends ,k of the pivoted armature causes the same to rise with great and increasing velocity1 finally bringing the finger ,d which forms part of the armature into violent contact with the face ,r of the corresponding projection on the housing which carries the handle and the roller ,h4 ,this heavy blow causes ,h1 in its rotation about the shaft ,j1 to go over the center and consequently allows the strong outward pressure of the brush ,f and the resilient coil ,c to throw the arm outward with a high velocity and so break the circuit1 first between the brush fingers and the contact plate and finally between the carbons ,s and ,f1 the one of which is rigidly secured to the arm and the other of which is resiliently mounted on its supporting spring4 ,to reset the breaker1 the handle1 which the act of opening has raised1 is pulled down1 thus bringing roller ,h into engagement with roller ,g once more and in that way forcing the arm back into its initial position4 ,illustration3 ,,fig4 #dib4--,roller ,smith ,s4,e4 combination overload and underload circuit breaker4 ,attached to the supporting frame ,b is the extension ,z1 which like ,b1 is of non-magnetic material and carries a rectangular magnetic core around which there are wrapped laminated copper conductors4 ,hinged at ,u is a heavy cup-shaped mass of magnetic material1 and hinged at ,v is a flat lever ,x which bears against the extension ,y secured to the housing which carries the operating handle4 ,the circuit through the breaker conveys the current around the windings of this underload coil carried by frame ,z and passes from it to the regular overload winding ,c from which it pursues the same course and exercises the same function as in a plain overload breaker4 ,the core of ,z being thus magnetized1 the cup-shaped member ,w is held in firm contact therewith and the lever ,x hangs free4 ,should1 however1 the current fall below the minimum value1 ,w is no longer sustained by the magnetic attraction but drops away1 swinging on its hinge ,u until the projection on the heel thereof strikes the lever ,x1 which blow is transmitted through ,y to the handle and thus trips the breaker4 ,when closing to reset the breaker1 the handle is manipulated just as in the case of a plain overload breaker1 that is1 it is pulled down1 thus not only closing and locking the breaker as before but through the pressure exerted by ,y on ,x and by ,x on ,w1 putting the latter into contact with its rectangular core to which it will adhere if the necessary current be present4 ,ques4 ,state some disadvantages of a discriminating cut out4 ,ans4 ,if one current reverse very rapidly1 and soon reach a large value in the opposite direction1 it is possible the cut out may not open at the desired instant1 and thereafter the effect of the heavy reverse current will be so great that the breaker will be held in more and more strongly2 a second disadvantage is that should the supply fail1 the breaker will open in any case1 and have to be reset before the supply can be resumed1 though in certain cases1 as1 for instance where there is a motor load1 this feature is an advantage and not a disadvantage1 since the breaker acts as a no-voltage cut out as well as a reverse current cut out4 ,reverse breakers1 however1 can be made positive in their action2 that is1 they can be so arranged that a reverse current exerts a positive pull on the tripping gear1 so that the greater the reverse current1 the greater the tripping effect4 ,ques4 ,what are time limit attachments8 ,ans4 ,devices which are fitted to circuit breakers and which act as dampers and prevent the too sudden operation of the breakers on what may be only a temporary overload or reverse current4 ,by having different time limits on feeder and dynamo breakers it can be ensured that the former operate before the latter1 and suitably in other cases where it is desired that one breaker shall operate before another4 ,ques4 ,describe a time limit attachment4 ,ans4 ,there are numerous types4 ,it may consist of a clockwork device1 a weight acting on a small drum or pulley1 a modified dash pot arrangement1 or a device operating by the expansion of a conductor due to the heat generated by a current passing through it4 ,ques4 ,how should a time limit device be arranged8 ,ans4 ,it should be so arranged that the heavier the overload the quicker the device acts1 until with a short circuit the device is almost instantaneous in its action4 ,illustration3 ,,fig4 #dic4--;,diagram showing connections of a rheostat4 ,the various resistance coils are connected to brass buttons or contacts4 ,the rheostat is connected in series in the circuit that it is to control4 ,in operation when the lever is on contact #a1 the current is opposed by all the resistance of the rheostat so that the flow is very small4 ,as the lever is moved over contacts #a1 #b1 #c1 etc41 the coils are successively cut out1 thus diminishing the resistance1 and when contact #aa is reached all the resistance is short circuited allowing the full current to flow4 ,in some types of rheostat the wire is wound around an iron frame-work which has been previously dipped into a fireproof insulating enamel4 ,the advantage of this construction is that the heat from the wire is dissipated much more rapidly1 so that a much smaller wire can be used to carry a given current4 ,the size of such an enameled rheostat required for absorbing a given amount of energy is much smaller than one made of coils of wire stretched between an iron supporting framework4 ,rheostats4--,these devices consist of conductors inserted into a circuit for the purpose of diminishing1 either constantly or in a variable degree1 the amount of current flowing1 or to develop heat by the passage of a current through them4 ,rheostats designed to be used in starting electric motors are frequently called starting boxes4 ,ques4 ,describe the construction of a rheostat4 ,ans4 ,in fig4 #dic1 resistance coils1 ,a1 ,b1 ,c1 etc41 are mounted in a frame or box1 and are connected at intervals to the contacts #a1 #b1 #c1 etc4 ,the rheostat arm or lever ,l is pivoted at ,s1 and when moved over the contacts1 inserts more or less of the resistance in the circuit thus regulating the flow of the current4 ,one terminal ,m of the rheostat is connected to the first contact and the other terminal ,o1 to the lever at ,s4 ,illustration3 ,,fig4 #did4--,starter with no voltage release for a series motor4 ,a helical spring coiled around the lever pivot ,p1 and acting on the lever ,a1 tends to keep it in the off position against the stop ,s4 ,this lever carries a soft iron armature ,i1 which is held by the poles of the electromagnet ,e1 when1 in starting the motor1 the arm has been gradually forced over as far as it will go4 ,should anything happen to interrupt the current while the motor ,m is running1 ,e will lose its magnetism and ,a will be released1 and will fly over to the off position4 ,e is usually shunted by a small resistance ,r1 so that only a portion of the main current flows through it4 ,this device constitutes the no voltage release1 and ensures that all the resistance is in circuit every time the motor is started4 ,ques4 ,how is a starting box connected to a motor8 ,ans4 ,in series4 ,ques4 ,why should a starting box be used with a motor8 ,ans4 ,if the line voltage should be applied directly to the terminals of the armature when not running1 an excessive flow of current will result1 on account of the low resistance4 ,accordingly1 to prevent injury to the winding1 a variable resistance or starting box is inserted between one supply terminal and the armature so that the pressure may be applied gradually while the motor is coming up to speed4 ,illustration3 ,,fig4 #die4--,starter with no voltage release for a shunt motor4 ,the terminals of the motor are at ,m1 ,m'1 m1 and those of the starter at ,s1 ,s'1 s4 ,the lever ,,sa is shown in the on position4 ,the current enters the motor at the terminal ,m1 and there divides1 part going through the field coil ,f1 and the main current through the motor armature ,a4 ,the armature current enters the starter at the terminal ,s'1 and traversing the lever ,,sa1 leaves by the terminal ,s4 ,the field current enters the starter at the terminal s1 traverses the coil of the magnet ,e 7which holds up the armature a linked to the lever7 and thence completes its journey through the whole of the resistance ,r1 and through the lever ,,sa1 to the terminal ,s4 ,when the supply is cut off by opening ,,sw1 or should the field circuit be accidentally broken1 the magnet ,e will release a and the lever1 which will thereupon fly to the off stop ,o4 ,it should be noticed that when ,,sa is off1 ,a and ,f form a closed circuit with the resistance ,r and magnet ,e4 ,the inductance of ,f has consequently no chance of causing destructive sparking when the current is shut off4 ,in starting the motor1 ,,sw is first closed1 and then1 as the lever is slowly moved1 the resistance ,r1 which at first is all in circuit with ,a1 is gradually transferred from ,a to ,f4 ,the resistance of ,r is too small to affect appreciably the current in ,f1 which necessarily consists of a comparatively large number of turns of fine wire4 ,the arrangement is adopted to render the breaking of the shunt circuit unnecessary and is rendered clearer by the diagram fig4 #dif4 ,it should be noted that ,e may be provided with a short circuiting key or push if required4 ,illustration3 ,,fig4 #dif4--,simplified diagram of the connections of fig4 #die4 ,illustration3 ,,fig4 #dig4--,starter with no voltage release and overload release connected to a compound motor4 ,with a shunt motor1 the only difference in the diagram would be that the series winding ,,se would be absent1 and the armature ,a would then be connected straight across between the main terminals ,m and ,m'4 ,when switch ,,sw is closed1 the current will enter the starter at its terminal ,s1 and pass through the magnet coil m' of the overload release to the switch lever ,l1 which is shown in the off position4 ,as soon as ,l is moved up to make contact with the first contact ,s the current divides2 part going through the resistance ,r and the terminals ,s' and ,m' to the series coil ,,se 7if a compound motor7 and armature ,a2 and part through the no voltage magnet ,e to the shunt winding ,,sh4 ,as the lever ,l is moved up toward ,e1 the effect is to take ,r out of the armature circuit and put it into the shunt circuit4 ,when the iron armature a1 fixed on the switch lever1 comes against the poles of ,e1 the laminated copper brush ,c bears against the blocks ,b1 ,b1 and so affords a better path for the current than through the spindle s4 ,should the supply voltage fail1 either temporarily or permanently1 ,e will release a1 and ,l will fly off under the tension of a helical spring coiled round s4 ,if there should be an overload on the motor1 tending to pull it up and cause an excess of current to flow through the armature2 this excess current1 passing through m'1 will make it attract its armature1 so bringing two contacts together at ,k which will short circuit ,e1 and allow the switch to fly off4 ,the connections between ,e and m' are not shown in the figure1 but they are indicated at ,c in fig4 #dih1 which is a simplification of fig4 #dig1 and which should be carefully compared therewith4 ,when only the normal current is flowing1 the attraction between m' and its armature is not sufficient to pull the latter up4 ,the actual forms and arrangement of parts on the starters are well shown in some of the figures4 ,ques4 ,what attachments should be provided on a starting box8 ,ans4 ,an overload release1 and a no voltage release4 ,ques4 ,describe these devices4 ,ans4 ,the overload release is an electromagnetic circuit breaker that opens the circuit if the motor become greatly overloaded4 ,a no voltage release may consist of an electromagnet in series with the shunt field circuit2 it holds the rheostat arm in the operating position as long as current flows through the shunt field from the line4 ,if the line switch be opened or the shunt field circuit accidentally broken1 the device becomes demagnetized and releases the arm1 which returns to its starting position by the action of a spring4 ,illustration3 ,,fig4 #dih4--,simplified diagram of the connections of starter connected to compound motor as shown in fig4 #dig4 ,the general arrangement of switches1 cut outs and starting boxes should be in accordance with the requirements of the ,national ,electrical ,code as follows3 ,each motor and starting box must be protected by a cut out and controlled by a switch1 said switch plainly indicating whether 'on' or 'off4' ,the switch and rheostat must be located within sight of the motor1 except in cases where special permission to locate them elsewhere is given1 in writing by the inspection department having jurisdiction4 ,where the circuit breaking device on the motor starting rheostat discs disconnects all wires of the circuit1 this switch may be omitted4 ,overload release devices on motor starting rheostats will not be considered to take the place of the cut out required if they be inoperative during the starting of the motor4 ,the switch is necessary for entirely disconnecting the motor when not in use1 and the cut out to protect the motor from excessive currents1 or careless handling when starting4 ,an automatic circuit breaker disconnecting all wires of the circuit1 may1 however1 serve as both switch and cut out4 ,illustration3 ,,fig4 #dii4--,view showing general arrangement of a switchboard4 ,the wires are shown to illustrate the various connections1 but in actual construction these wires are connected on the back of the switchboard4 ,switchboards4--,a switchboard consists of a panel or series of panels of slate1 marble1 soapstone or brick tile erected in an electric plant for the purpose of mounting in a convenient group the instruments for controlling and distributing the current and safeguarding the system4 ,switchboards may be divided according to operation into two classes3 #a4 ,direct control2 #b4 ,remote control4 ,a direct control switchboard has all its apparatus mounted directly on the board and controlled by hand1 while in the remote control type1 the main current carrying parts are at some distance from the operating board1 the control being effected by mechanical devices or by electric motors or solenoids4 ,when the control system of a plant is very extensive1 it sometimes occupies a separate building known as the switch house4 ,ques4 ,what may be said with respect to the material for switchboards8 ,ans4 ,in order to avoid danger of fire from short circuits1 the panel should be made of some non-combustible material1 such as marble1 slate1 glass plates or earthenware tiles4 ,if slate be used1 care should be taken to have it free from conducting veins1 or it should be marbleized1 that is1 subjected to a treatment that will fill up the pores of the veins and thus prevent the absorption of moisture4 ,wood is seldom used1 except in cases where the switches1 fuse blocks1 wire supports1 etc41 are all mounted on porcelain or other incombustible material4 ,ques4 ,how should the instruments and connections be arranged on a switchboard8 ,ans4 ,they should be arranged so as to provide the shortest possible path for the current1 and preferably always in the same direction1 that is1 from right to left or from top to bottom1 the connecting wires being brought in on one side and out on the other1 and the crossing of wires avoided as far as possible4 ,all wires and current carrying parts should be kept far enough apart at all points to prevent accidental contact or the jumping across of the current where there is a great difference of voltage4 ,such wires should be also kept at a sufficient distance from screw heads1 metal brackets1 gas pipes1 water pipes1 and other conducting bodies1 in order to prevent accidental grounds or short circuits4 ,all instruments and switches should be placed so as to be conveniently accessible for observation and operation1 and sufficiently out of reach of accidental contact by persons2 otherwise they should be protected by some form of insulating shield4 ,illustration3 ,,fig4 #ejj4--,small switchboard suitable for two dynamos2 view showing ammeters and voltmeters1 switches1 circuit breakers1 etc4 ,ques4 ,what type of switch is used on switchboards8 ,ans4 ,the knife switch4 ,ques4 ,describe a small switchboard4 ,ans4 ,fig4 #ejj shows one suitable for two dynamos4 ,at the top is a voltmeter and two ammeters4 ,immediately below is a row of feeder switches serving to connect and disconnect the various feeders with and from the bus bars which are mounted behind the board4 ,below are two rheostat handwheels1 and two large switches connecting the dynamos with the bus bars4 ,,vs is a voltmeter switch connecting the voltmeter with various parts of the system4 ,below the voltmeter switch is a double throw switch to transfer the bus bars from connection with the dynamo switches to one with some other source of current such as a street circuit1 in the event of a breakdown4 ,at the bottom are two circuit breakers4 ,illustration3 ,,fig4 #eja4--;,diagram showing various connections of voltmeter switch of the small switchboard shown in fig4 #ejj4 ,ques4 ,describe the voltmeter switch4 ,ans4 ,fig4 #eja shows the connections1 from which it can be seen that the voltmeter can be connected with the terminals of either dynamo or with the bus bars1 or with either a central or remote part in the lamp circuits4 ,under ordinary conditions it remains connected to the circuit at the central point of distribution4 ,when one dynamo is already in circuit1 however1 and it becomes necessary to connect up the other one1 the voltage of the latter must be the same as that at the bus bars4 ,accordingly1 connections are provided to the voltmeter switch such that the attendant can compare the voltages at the dynamo terminals and bus bars before closing the dynamo switch4 ,all the positive connections are on one side of the circle swept by the switch and all the negative connections on the other side4 ,illustration3 ,,fig4 #ejb4--,roller-,smith1 single pole1 plain overload circuit breaker4 ,as its name indicates1 the function of the plain overload circuit breaker is to automatically interrupt the circuit in which it is placed when the flow of current through it exceeds the predetermined limit for which the apparatus is set4 ,it is the most common of all of the types and is utilized for the protection of dynamos and motors and all other electrical apparatus which1 by reason of the conditions of operation1 may become subject to loads in excess of the normal4 ,the single pole type may be used separately for the protection of a single wire of a given circuit or grouped to protect the two or more wires of one circuit1 becoming in the latter case the so called independent arm multipole apparatus4 ,the action of this type of circuit breaker is fully explained in fig4 #dia4 ,,hawkins ,,practical ,,library ,,of ,,electricity ,,in ,,handy ,,pocket ,,form ,,price #a ,,each ,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 ,note3 ,inconsistent spelling and hyphenation are as in the original4