DYNAMO DESIGN 



79 



machine, the total number of ampere-turns, on the armature is 



M! ty 



SI = -^X I c because the current I c in -= conductors on one- 

 half of the armature surface is balanced by an equal but opposite 



FIG. 24. Current distribution bi-polar armature. 



current in -~ conductors on the other half of the armature 



surface, as indicated in Fig. 24. Now, since there are two poles, 

 we may say the ampere-turns per pole are 



l ZIc 



or just half the number of ampere-conductors in a pole pitch. 

 This rule applies also to the multipolar machines. Thus, in Fig. 



Direction of Motion of Armature 

 Surface 



FIG. 25. Armature m.m.f. multipolar dynamo. 



25, the horizontal datum line may be thought of as the developed 

 surface of a four-pole dynamo armature. The brushes are so 

 placed on the commutator that they short-circuit the coils when 

 these are approximately halfway between the pole tips. It is, 

 therefore, permissible to show the brushes in this diagram as if 



