86 SYNCHRONOUS ALTERNATORS. [Exp. 



circle with B as a center; at constant excitation, EQ is constant 

 and the locus of O is the arc of a circle with A as a center. 



23. Application of Electromotive Force Method. Knowing 

 the armature resistance and synchronous reactance* obtained 

 from the short-circuit test, the electromotive force method can 

 be used for predetermining the regulation, the external charac- 

 teristic and the full-load saturation curve for any power factor. 



24. Predetermination of Regulation at Different Power 

 Factors. By method (a) of 17-20, determine the open-circuit 

 voltage EQ, corresponding to rated full-load voltage at rated full- 

 load current, for different power factors. The values of arma- 

 ture RI drop and XI drop corresponding to full-load current will 

 be constant in all the computations, R and X being taken as con- 

 stant, f Plot the values of EQ, thus obtained, with power factor 

 (or 6) as abscissae, as in Fig. 6. This is to be done for lagging 

 and for leading currents. Arrange, also, a scale as on the right 

 of Fig. 6 to show the values of EQ as per cent, of full-load 

 voltage. 



25. The curves show the increase (or decrease) in voltage 

 when full-load current is thrown off at different power factors; 

 in per cent., this gives the regulation. At power factor i.o, the 



*(23a). Synchronous reactance is practically equal to synchronous 

 impedance. . In Figs. I and 2, synchronous impedance is Z = Eo -=- Is, and 

 is more or less constant; it can be computed for the value of o or for 

 the value of Is corresponding to working conditions. 



Thus, for normal field excitation, corresponding to Eo = 627, we obtain 

 Z = 627 -f- 1 16 = 5.4 ohms; the armature current 116 amp. is, however, 

 far above normal. 



For normal full-load current, 43.4 amp., we obtain Z = 234 -j- 43.4 = 5.4 

 ohms ; in this case the field excitation is far below normal. 



It is thus seen that Z can be computed from the short-circuit test either 

 for normal field current or for normal armature current; but field and 

 armature currents can not simultaneously be normal. When Z is constant, 

 the two computations give identical results. When Z is not constant, the 

 two computations give different results; either may be used, but it is 

 justifiable to use the method which gives the smallest value for Z as 

 being least pessimistic. (See ua and 33.) 



t See 



