136 



PRINCIPLES OF ELECTRICAL DESIGN 



and the armature m.m.f. tending to modify the flux due to the 

 field poles alone is 



ZI e (b - a) 



0.4 TT 



pr 



between the points d and d'. 



In order that a curve may be plotted on the same basis as the 

 resultant field m.m.f. curve, it is necessary to know the armature 

 m.m.f. per pole at all points. Let the distance between the 

 points d and d f be equal to the pitch r\ then the effect of the arma- 

 ture m.m.f. at d f upon the pole S will be exactly the same as 



__ 



N 



FIG. 52. Magnetizing effect of armature inductors. 



the effect of the armature m.m.f. at d upon the pole N, and 

 the m.m.f. per pole at the point d may be expressed as 



0.47rZJ c (b -a) 



(Armature m.m.f.)<f 



(65) 



Its maximum value which always occurs in the zone of com- 

 mutation is 



Armature m.m.f. per pole = 



(66) 



which is formula (48) of Art. 20 (p. 80) expressed in gilberts 

 instead of ampere turns. 



The combination of this armature m.m.f. with the m.m.f. due 

 to the field coils only, as represented by Fig. 50, is carried out 

 graphically in Fig. 53. The curves F and A represent field and 

 armature ampere-turns (or m.m.f. in gilberts, if preferred). 

 These are the two components of a resultant m.m.f. curve, R, 

 the ordinates of which are a measure of the tendency to send flux 

 between any point on the armature surface and the pole shoe N. 

 The actual value of the flux density at the various armature 

 points under load conditions can therefore be obtained by using 



