REGULATION OF ALTERNATORS 311 



been used in the impedance triangle E t PE of Fig. 130. This 

 virtually assumes the demagnetizing and distortional effects of 

 the armature current to be equivalent to a fictitious reactance 

 drop capable of being treated vectorially like any other reactance 

 drop, and of which the direct effect on regulation is proportional 

 to the sine of the angle of lag a not unreasonable assump- 

 tion, though scientifically inaccurate. This gives good results 

 in machines of normal design. It is when departures are made 

 from standard practice that such approximations are liable to be 

 abused. 



Armature current 



FIG. 130. Vector diagram showing construction to obtain terminal voltage 

 for any load and power factor. 



109. Influence of Flux Distribution on Regulation. So long 

 as a sinusoidal air-gap flux distribution can be assumed both on 

 open circuit and under load conditions, the previously described 

 methods of predetermining regulation are satisfactory; but in the 

 case of new or abnormal designs of machines, correct results can 

 be obtained only by taking into account the alteration in the 

 amount of the useful flux due to cross-magnetization and the 

 changes in the e.m.f. wave-shapes due to flux distortion. An 

 attempt will be made to outline as briefly as possible a method of 

 study which, although it has been elaborated by the writer, is not 

 essentially new; indeed, it is probably used in a modified form by 

 some practical designers when aiming at a closer degree of 

 accuracy than can be expected from methods based on the usual 

 sine-wave assumptions. 



The method about to be described is based on the fact that for 

 salient-pole machines approximately correct flux-distribution 

 curves can be drawn when the width and shape of the pole shoe 

 have been decided upon ; and for high-speed generators with air 

 gap of constant length, when the disposition and windings of the 

 slots in the rotor have been determined. From these flux curves, 



