7 6 SYNCHRONOUS ALTERNATORS. [Exp. 



than it actually has ; the electromotive force method, on the 

 other hand, is pessimistic, giving the generator a poorer regula- 

 tion than the actual. 



The two methods, therefore, give the limits between which is 

 the true performance of the machine. 



9. Data. For either method, the data required are obtained 

 from the following two* runs, which are made without loading 

 the generator: 



1. An open-circuit run, giving the open-circuit voltage #o, 

 for different field currents, i. e., the no-load saturation curve, 

 obtained as in 5, Exp. 3~A. See Curve (i), Fig. i. To save 

 labor in the many subsequent calculations, it is customary to 

 use only the ascending curve. 



2. A short-circuit run, giving the short-circuit current /s, for 

 different field currents, called also a synchronous impedance 

 test, as described in the next paragraph. See Curve (2), Fig. i. 



These data enable us to ascertain the synchronous impedance 

 of the armature and hence to compute the volts impedance drop 

 for the electromotive force method ; they also enable us to ascer- 

 tain the magnetomotive force required to overcome the mag- 

 netizing effect of the armature, for the magnetomotive force 

 method. 



The hot armature resistancef is to be found by the fall-of -po- 

 tential method. 



10. Test for Short-circuit Current and Synchronous Impe- 

 dance. With the armature short-circuited through an ammeter,! 



* (Qa). Two such runs are common in testing many kinds of appa- 

 ratus; note, for example, the open-circuit and short-circuit tests for trans- 

 formers, Exp. 5-B. 



t(9b). On account of eddy currents, the resistance will be greater 

 for alternating currents than the value found by direct current. This is 

 of importance as affecting efficiency ( 15, Exp. 3-A), but is of little con- 

 sequence so far as regulation is concerned, for RI drop has only a small 

 effect at high power factors and is negligible at low power factors, as will 

 be seen later. 



J (ioa). The ammeter leads should be short and heavy; for, by the 



