FLUX DISTRIBUTION 



127 



length as the distance from the center of the pole increases, the 

 case of a salient pole alternator has been considered. The flux 

 lines and equipotential surfaces for an alternator with shaped 

 poles are shown in Figs. 42 and 43. The object of shaping the 

 poles by gradually 'increasing the air gap from the center out- 

 ward is to obtain over the pole pitch a distribution of flux which 



FIG. 42. Flux lines in air gap of alternator. (One pole acting alone.) 



FIG. 43. Flux lines in air gap of alternator. (Effect of neighboring poles.) 



shall approximate to a sine curve. The data of the machine 

 under consideration are as follows: 



Pole pitch 22 cm. 



Pole arc = 14.3 cm. 



Equivalent air gap at center of pole face d e = 1 cm. 



Air gap at other points on armature surface = 



COS 



where 



6 is the angle (in electrical degrees) between the center of the 

 pole and the point considered. 



In Fig. 44 the curve marked "permeance" has been plotted 

 from Fig. 42. Its shape indicates the flux distribution over the 

 armature surface on the assumption that the effect of neighbor- 



