148 



ELECTRICAL ENGINEERING 



in fields of opposite polarity in order that the electromotive forces 

 generated in them may act in the same direction. One side of 

 a coil is placed in the top of a slot and the other side in the bottom 

 of a slot in a similar position under the next pole. 



According to the way in which the end connections are brought 

 out to the commutator bars and the coils are connected together, 

 drum windings are divided into two classes, multiple or lap wind- 

 ings, as illustrated by coils a and b in Fig. 112 and the windings in 

 Figs. 113 to 115, and series or two-circuit windings, as illustrated 

 by the coils c and d in Fig. 112 and the windings in Figs. 116 and 

 117. 



FIG. 112. Armature coils. 



95. Multiple-drum Windings. In the multiple winding the 

 two terminals of a coil are connected to adjacent commutator bars. 

 Fig. 113 represents a multiple winding for a six-pole machine with 

 72 conductors and 36 slots. The sides of a coil are placed in slots 

 1 and 7 and the terminals are connected to bars 1 and 2. The 

 same winding is shown in Fig. 114 and the directions of the cur- 

 rents are shown by arrowheads. The brushes are placed on the 

 no-load neutral points and therefore directly under the centres of 

 the poles and as many sets of brushes are required as there are 

 poles. 



Tracing through the winding from a positive to a negative brush 

 only one sixth of the conductors are taken and there are therefore 



