ALTERNATOR REd'l.. \TION AND OPERATION 



147 



This assumes that the voltage E'E is always proportional to the 



armature current, which is not strictly true. The foregoing is 

 the principle of the electromotive force or jnmchronous impedance 

 method. The rational or genera! method is first to compute E'. 

 Find from the saturation curve, the field current F corresponding 

 to E'. Add A to F vcctorially to find FI, and then from the 

 saturation curve find E corresponding to the field current F\. 

 One serious objection to this method is the difficulty of de- 

 termining the armature leakage reactance X. It cannot be 

 readily measured and can only be roughly calculated. These 

 calculations and the general solution of the diagram are both 

 laborious. The determination of the regulation is very much 

 simplified if X be increased to the value X 9 , so that E is 

 found directly without knowing E'. X, is called the synchron- 

 ous reactance of the alternator. The corresponding impedance 

 Z, ( = y/R* -f X, 2 ) is called the synchronous impedance of the 

 alternator. 



The synchronous re- 

 actance is determined 

 experimentally as fol- 

 lows: The saturation 

 curve of the alternator, 

 rid Ij, is first deter- 

 mined in t he usual 

 manner and the curve 

 plotted as shown in Fig. 

 !.")!. The field is then 

 made very weak and the 

 alternator armatun 

 -Imri-circuited through 

 an ammeter. The field 

 is then nradualK heiied and a new curve of armature 



cm-rent and /, is determined. The field is increased until the 

 armature current is almoM twice its rated value. These two 

 curv ,..\vn plotted in Fin. l.'.l. 



Connd- held em-rent /'/. On open-circuit this 



field current prod:. >iort -circuit the termi- 



nal voltage ,,( the machine is pnu-t ically /ero. The vult Mire / ] 

 ' in the armature at >hort-ciivuit 1.. 



I"i.. l.'.l. Ojn'ii-circiiit :m<l sliort-rir cuit char- 

 Mi :il(-rn:itir. 



