PARTS OF A DYNAMO-ELECTRIC MACHINE 65 



Substituting in this equation the proper values for the 

 various lengths, areas, and permeabilities, and knowing 

 required flux, $, the number of ampere-turns which 

 1.1 be necessary is found at once by solving for NI. 

 Knowing NI we can use as many turns as we like pro- 

 viding the proper current is used to make the product 

 A T 7 the required amount. 



Determination of Ampere-turns Necessary. This method 

 of determining the size of the field coil is almost never used 



FIG. 34. Magnetic Circuit of a Small Old-style, Bipolar Machine. 

 Field coils shown by hatched area. 



because a much simpler method is available. Let us 

 consider a bipolar field frame of the shape given in Fig. 34. 

 The magnetic circuit consists of the yoke, two poles, two 

 air gaps, and the armature core all in series. The field is to 

 be excited by two coils shown in section at A and B. These 

 two coils give m.m.fs. which act in the same direction on the 

 magnetic circuit. We will figure the ampere-turns required 

 for the whole circuit as though only one coil was to be used; 



