168 ELECTRICAL MACHINERY 



mutating poles the brushes must be shifted just as much 

 in the motor as in the generator, if sparkless commutation 

 is to be obtained. 



Commutating poles, which are very important for the 

 successful operation of a generator, are even more impor- 

 tant in motor operation because the load variations are more 

 sudden and violent. Practically all modern motors 

 are fitted with commutating poles. The brushes of such 

 motors are carefully adjusted before the machine is sent out 

 from the factory and are generally held in place by a set 

 screw or clamp; the brushes on a commutating pole motor 

 must not be shifted from this position, as correctly deter- 

 mined in the factory. 



Effects of Various Conductors on Armature. The poles 

 and armature of a bipolar, commutating pole motor are 

 shown in Fig. 103, and the direction of the e.m.f. induced 

 in each conductor is indicated. If the brushes are in the 

 neutral position (shown by line AB in Fig. 103) there is 

 no demagnetizing effect produced by the armature reaction 

 and in each half of the armature winding there are a cer- 

 tain number of active conductors generating the necessary 

 counter e.m.f.; these conductors are indicated by X and 

 Y in Fig. 103. 



The conductors marked M and N add nothing to the 

 counter e.m.f. of the armature because in each half of the 

 winding the e.m.fs. generated in these conductors neu- 

 tralize each other. Considering the right-hand half of the 

 armature in Fig. 103, it is seen that under the S commutating 

 pole there are two inductors with a " negative " e.m.f. 

 and under the N commutating pole there are two inductors 

 generating a "positive" e.m.f.; hence in the right-hand 

 side of the armature the effective counter e.m.f. must all 

 be generated by the inductors marked X. The same 

 reasoning shows that the only inductors generating the 

 counter e.m.f. in the other path of the armature are those 

 marked Y. < 



