158 ALTERNATING CURRENTS 



Algebraic addition would have given 1,364 volts, 



a __ 0-1 -IO f) Q1 S 



a = 3L8 - 31.1 = 0.7 which is negligible. 



From Fig. 154, the field current corresponding to 1,359 coil volts, or 

 2,350 volts on the saturation curve (2,350 = l,359\/3) is 



Fi = 266 amp. 



The rated current of the coils is 376 amp. Corresponding to this 

 current (Fig. 154) the field current is 64 amp. from the short-circuit test. 

 F 2 = 266 2 + 64 2 - 2 X 266 X 64 cos (90 + 31.8) 

 F 2 = 92,840 F= 305 amp. 



From the saturation curve, the terminal voltage corresponding to 305 

 amp. field current is 2,580 volts across the terminals or 1,490 coil volts. 



1 JQf) _ I QQO 



The regulation = - = 12.0 per cent. Ans. 



1,OOU 



Because of the low saturation on short-circuit, a given mmf. 

 will produce a greater increase of flux than an equal mmf. will 

 produce under operating conditions, where the iron is saturated. 

 Therefore, the emf. corresponding to a given mmf. at short- 

 circuit will be much greater than the emf. corresponding to an 

 equal mmf. taken higher up on the saturation curve. This is 

 illustrated in Fig. 157. On short-circuit, the voltage ab corre- 

 sponds to the mmf. A. The additional voltage de corresponds 

 to a mmf. be equal to A, but taken higher up on the saturation 

 curve. The voltage de is obviously much less than the voltage 

 ab. Hence, that part of the mmf. A which replaces a 

 voltage is too small under load conditions. Therefore, the no- 

 load emf. E found on the saturation curve is too low, and the regu- 

 lation as determined by this method is ordinarily less than the 

 actual regulation. For this reason this method is often called 

 the optimistic method. This is illustrated by the foregoing ex- 

 ample, when the regulation as obtained by the synchronous im- 

 pedance method is 17.4 per cent., whereas that obtained by the 

 magnetomotive force method is 12.0 per cent. 



That part of the mmf. A which actually is armature reaction is 

 too high on short-circuit, due to the favorable position of the arma- 

 ture coils with respect to the field poles. As in the syrchrcnous 

 impedance method, this factor tends to give too high a value of 

 regulation, These two sources of error tend to offset each other 



