96 



ELECTRICAL MACHINERY 



tator for clearness of representation. They really bear 

 on the outside surface of the commutator. 



A single coil does not form a closed winding and would 

 never be used. If another coil is wound on the armature in 

 the same direction as the first, and if the coils are con- 

 nected as in Fig. 57, we have a two-coil, closed circuit 

 winding. The direction of the induced e.m.f. is marked on 

 both coils by arrows and it is seen that the e.m.f. of both 

 coils acts in the same direction as far as the outside circuit, 

 is concerned, but that the two e.m.f s. oppose one another 

 n the local circuit made up of the two coils only. In this 



FIG. 57. Two-coil Armature and two-part Commutator. 



two-circuit winding the e.m,f. of the generator is evidently the 

 same as the e.m.f. per path, but the current capacity of the 

 machine is equal to twice the current capacity per path. The 

 e.m.f. wave form of the armature shown in Fig. 57 would 

 be exactly the same as that shown in Fig. 55, which was 

 for a single coil armature. 



Necessity of Many Coils. The e.m.f. wave of Fig. 55 

 is not suited for ordinary purposes of lighting, running 

 motors, etc. A uniform, non-pulsating e.m.f. is desired 

 and this is the purpose of making an armature with many 

 coils and many commutator bars. Consider a four-coil, 

 closed circuit armature with a four-part commutator, 



