298 PRINCIPLES OF ELECTRICAL DESIGN 



expected of the vector construction, in which the loss of pressure 

 due to armature distortion was assumed to be in accordance with 

 the sine law. Fig. 122 illustrates the same condition as Fig. 

 121 except that the e.m.f. waves have been re-plotted to rec- 

 tangular coordinates. 



The phase angle between equivalent sine waves of voltage 

 and current is correctly defined as the angle of which the cosine 



real watts , ,, . . 



is the ratio r > and this is not always exactly the same 



volt-amperes 



as the angle that would be obtained by the approximate method 

 here described. If the distorted waves depart very considerably 

 from the sinusoidal form, methods of calculation involving vec- 

 tor constructions should be avoided ; but by multiplying together 

 corresponding instantaneous values of e.m.f. and current a power 

 curve can be plotted and the average watts determined. The 

 product of the virtual values of the e.m.f. and current will be 

 the denominator in the expression : 



average true power per phase winding 



COS = 7 ; : r ' 



apparent power per phase winding 



The practical application of equivalent sine-waves in prede- 

 termining the regulation of an alternating-current generator will 

 be taken up in the following chapter, and again in Chap. XV, 

 when working out a numerical example. 



