160 



ALTERNATING CURRENTS 



X, is ordinarily determined from a no-load saturation curve and 

 a curve taken at low power-factor and rated current. Thus, in 

 Fig. 159 (6), Oa is the no-load saturation curve and db is a curve 

 taken at low power-factor and rated current. This low power- 

 factor load is ordinarily obtained by using under-excited 

 synchronous motors as the load. The rated terminal voltage 

 of the machine is cb, and when the load is thrown off, the open- 

 circuit voltage obviously becomes ca, as the field current remains 

 constant. 



The synchronous reactance X 8 , which is practically equal to the 

 synchronous impedance, is determined by dividing ab by the 



Anr.percs a, 



FIG. 160. Characteristics of an alternator at different power-factors. 



rated current of the machine, this being the current at which curve 

 db was determined. 



Y _ ac be _ ab 

 ~T~ "7 



When the value of X 8 is determined, it may be utilized in 

 finding the regulation in the same manner as described under the 

 synchronous impedance method of Pars. 68 and 69. 



If it is not possible to load the machine, the distance Od may 

 be found from a short-circuit test and the curve db determined 

 from a knowledge of machines having similar constants. 



The A. I. E. E. method gives too large a value of regulation 

 for machines having salient poles, as the armature reaction is 



