312 PRINCIPLES OF ELECTRICAL DESIGN 



whether representing open-circuit or loaded conditions, the e.m.f . 

 waves and their form factors can be obtained, all as explained in 

 Arts. 100 and 101 of Chap. XIII, and the problem of regulation 

 may be summed up as follows : Plot the open-circuit saturation 

 curve for the complete magnetic circuit, correcting for the form 

 factor of the developed voltage if this departs appreciably from 

 the assumed value of 1.11. Now obtain the actual flux distribu- 

 tion and corresponding full-load "apparent" developed voltage for 

 a given power factor, and correct for the internal pressure losses 

 ohmic and reactive. Let E t be full-load terminal voltage obtained 

 by this method. The field excitation for air gap and teeth is 

 known for the particular condition considered, and the ampere- 

 turns required to overcome the reluctance of the remaining parts 

 of the magnetic circuit are also readily ascertained since the 

 total flux per pole (the area of full-load flux curve C) is known. 

 It is therefore merely necessary to read off the open-circuit 

 characteristic the voltage E corresponding to the ascertained 

 value of the total field excitation in order to determine the 



, . . , . , . E Et 



regulation, which is 



tit 



The actual working out of the problem is not quite so simple 

 as this statement may suggest, the chief difficulty being that a 

 knowledge of the external power-factor angle is insufficient to 

 determine the exact position of the armature m.m.f. curve rela- 

 tively to the center line of the pole. The position of this curve 

 depends upon the internal power-factor angle and also upon the 

 phase displacement of the generated electromotive force under 

 load conditions, i.e., on the degree of distortion of the resultant 

 air-gap flux which, on open circuit, was distributed symmetric- 

 ally about the center line of the pole face. The manner in which 

 the displacement of the armature m.m.f. curve may be determined 

 approximately, for any given external power factor, was ex- 

 plained in Art. 98 and illustrated by the vector diagram, Fig. 116. 



110. Outline of Procedure in Calculating Regulation from 

 Study of E.m.f. Waves. In Fig. 131 let the curve F represent the 

 distribution of magnetomotive force over the armature surface 

 tending to send flux from pole to armature on open circuit. 

 Let BD be the magnetomotive force due to armature current 

 only. If the load current be sinusoidal (an almost essential as- 

 sumption, since its exact shape cannot be predetermined), BD 

 will also be a sine curve, the maximum ordinate CD of which will 



