314 



PRINCIPLES OF ELECTRICAL DESIGN 



all the flux in the air gap to be cut by the armature conductors, 

 the wave shapes of the "apparent" developed e.m.fs. can be 

 drawn, and their form factors calculated as explained in Arts. 100 

 and 101. The terminal voltage which must be known before 

 the regulation can be calculated is most readily obtained by 

 using vector diagrams; but this involves the substitution of 

 " equivalent sine curves " for the irregular waves. The maximum 

 value of a so-called equivalent sine wave is \/2 times the r.m.s. 

 value of the irregular wave; but its time phase relatively to any 

 defined instantaneous value of the irregular wave is not so easily 

 determined. The approximate amount of this phase displace- 

 ment can be obtained from the irregular curve when plotted to 



Fia. 132. Flux distribution, (C) under load, and (A ) when load is 



thrown off. 



polar coordinates as explained in Art. 102, Chap. XIII; but a 

 method to be preferred, although more tedious, consists in ob- 

 taining the average value of the true power and making the dis- 

 placement between electromotive force and current vectors equal 



to cos"" 1 ( ) . The current wave (assumed to be 



\apparent power/ 



a sine curve) from which the m.m.f. curve BD of Fig. 131 is de- 

 rived would have its maximum value at the point B, displaced 

 |8 electrical degrees beyond the center, A, of the pole. The 

 actual full-load e.m.f. wave can also be drawn in the correct 

 position relatively to the center line of the pole; and, by multiply- 

 ing the corresponding instantaneous values of electromotive 

 force and current, the power curve can be drawn and the average 



