280 



PRINCIPLES OF ELECTRICAL DESIGN 



on the armature surface, the position of these points being 

 considered fixed relatively to the field poles. 



This has been done in Fig. 109, where the windings are shown 

 covering 60 per cent, of the armature surface, and the distance T 

 is one pole pitch. This distance is divided into 10 equal parts, 

 each corresponding to 18 electrical degrees. The thick line repre- 

 sents the armature m.m.f. when the current has reached its 

 maximum value. The armature is then supposed to move 18 

 degrees to the right of this position, and a second m.m.f. curve is 



FIG. 109. Instantaneous and average values of armature m.m.f. in single- 

 phase alternator. 



drawn, corresponding to this position of the windings. Its 

 maximum ordinate is, of course, less than in the case of the first 

 curve, because the current (which is supposed to follow the sine 

 law) now has a smaller value. This process is repeated for the 

 other positions of the coil throughout a complete cycle, and the 

 resultant m.m.f. for any point in space (i.e., relatively to the 

 poles, considered stationary) is found by averaging the ordinates 

 of the various m.m.f. curves at the point considered. In this 

 manner the curve M of Fig. 109 is obtained. It is seen to 



