REGULATION OF ALTERNATORS 



313 



be displaced beyond the center line of the pole by an amount 

 depending upon the power factor of the load and the distortion 

 of the resulting air-gap flux distribution. This maximum value 

 will occur where the current in the conductors is zero, and the 

 maximum armature current will be carried by the conductor 

 displaced exactly 90 degrees (electrical space) from the point C. 

 The point B is therefore the position on the armature surface, 

 considered relatively to the poles, where the current is a maxi- 

 mum, the length AB or /3, which depends largely on the power 

 factor, being determined approximately as explained in Art. 98. 

 Add the ordinates of curves F and D to get the curve M which 

 gives the resultant magnetomotive force under the assumed 

 conditions of load. Having calculated the permeance of the 



FIG. 131. Distribution of m.m.f. over armature surface. 



magnetic circuit for various points on the armature surface, the 

 flux distribution curves A and C of Fig. 132 can be plotted. 

 The first, which represents open-circuit conditions, is plotted 

 from the m.m.f. curve F, while curve C, showing the flux distribu- 

 tion under load, is derived from the m.m.f. curve M. The 

 respective areas of these curves are a measure of the total air-gap 

 flux under the two conditions, but we cannot say that the actual 

 ampere-turns on the field will be the same in both cases, be- 

 cause the component of the total m.m.f. required to overcome 

 the reluctance of the pole-core and yoke ring has not been 

 taken into account. 



The correct solution of the problem involves the actual 

 wave shapes of the developed electromotive forces. Assuming 



