CHAP. VIIIJ REACTION IN SYNCHRONOUS MACHINES 



155 



the real ones, in a motor they lead the real poles. Hence, in the 

 generator diagram, E t lags 90 degrees behind E n , while in the 

 motor diagram it leads E n by 90 degrees. 



In the case of a generator the problem usually is to find the 

 field excitation M / neces- 

 sary for maintaining a 

 required terminal voltage 

 e, with a given current i 

 and at a given power- 

 factor cos <. First, the 

 figure OABD is con- 

 structed, or else the 

 values of E and <' are 

 determined from eqs. 

 (75), (76), and (76a). 

 In order to find the 

 ampere-turns required on 

 the main poles it is neces- 

 sary to determine the 

 voltage E n induced by 

 them. For this purpose 

 the angle /? must first be 

 known, for 



E n = E cos /?. . (82) 



As an intermediate step, 

 it is necessary to express 

 E t through the ampere- 

 turns M t , which are the 

 cause of E t . The m.m.f. 

 M t is small as compared 

 to the total number of 

 ampere-turns on the real 

 poles; hence, the lower 

 straight part of the no- 

 load saturation curve of 

 the machine can be used 

 to express the relation 

 between M t and /:,. Ixt v be tlu voltage corresponding to 

 one ampere-turn on the lower part of th< no-load 



Flo< 41 _ Tho ^ 

 ronoUB 



diagramof a8yn . 

 h MM / pole*. 



