292 



PRINCIPLES OF ELECTRICAL DESIGN 



phase displacement of one-third of a pole pitch (i.e. 60 electrical 

 degrees). 



Whatever may be the number and spacing of the slots on the 

 armature surface, the actual e.m.f. wave shapes can always be 

 plotted in the manner indicated in Fig. 117 where the full line 

 curve may be any one of the flux curves previously obtained, 

 the ordinates of which are a measure of the flux density in the 

 air space near the surface of the armature. Draw A and B 

 properly spaced to represent the slots, e.g., two in this illustra- 

 tion, of one phase of the armature winding. At the instant when 

 the center line C of this phase-winding occupies the position 

 shown in Fig. 117, the conductors in slot A are moving in a 

 field of density A A', while the conductors in slot B are moving 



FIG. 117. Method of deriving e.m.f. wave from curve of air-gap flux 



distribution. 



in a field of density BB f . These conductors are all in series, and 

 the instantaneous value of the voltage per phase winding will be 



B a NTrDl a Z 



60 X 10 8 

 where B a average value of flux density, in gausses; 



(107) 



AA' + BE' . ... 



~ in this instance. 



N 

 D 



la 



z 



revolutions per minute. 



armature diameter in centimeters. 



armature length (axial) in centimeters. 



total number of conductors in series per phase. 



This value of e is plotted as CC' to a suitable scale, and the process 

 is repeated for other positions of the armature slots, thus pro- 

 ducing the dotted curve 7, representing the electromotive force 



