PROCEDURE IN DESIGN OF D.C. GENERATOR 221 



open circuit, the m.m.f. between pole face and armature core, 

 (i.e., bottom of slots) must exceed this value. Referring again 

 to Fig. 82, it will be seen that, for a density of 7,840 gausses under 

 the pole face, the m.m.f. to overcome tooth reluctance amounts 

 to a little over 200 ampere-turns. The ordinates of the dotted 

 m.m.f. curve of Fig. 83 may therefore be increased throughout 

 in the proper proportion, the maximum addition being 200 

 ampere-turns. This corrected curve may now be used to plot 

 with the aid of the magnetization curves of Fig. 82 the actual 

 distribution of flux over the armature surface, when the effect 



FIG. 84. Curves of flux distribution over armature surface. 



of tooth saturation is taken into account. This has been done 

 in Fig. 84, where the curve marked A is similar to the flux curve 

 of Fig. 79 except in so far as its shape may be modified by tooth 

 saturation. 



The procedure above described for obtaining the actual flux 

 distribution curve is logical and correct; but for practical pur- 

 poses it is usually permissible to assume that the curve A of 

 Fig. 79 shows the actual flux distribution, the slight modifica- 

 tion brought about by variable degrees of tooth saturation being 

 neglected. It is then a simple matter to plot the required open 

 circuit field m.m.f. curve directly by taking from Fig. 82 the 



