SYNCHRONOUS MACHINERY 



265 



While- the magnetic circuit of the machine is unsaturated the 

 field excitation required to produce the e.m.f . E Q is approximately 

 proportional to it, but higher up on the saturation curve the 

 excitation increases faster than the e.m.f. The relation between 

 field-exciting current and no-load e.m.f. can be obtained by refer- 

 ence to Fig. 244 which is the saturation curve of the alternator at 

 no load. 



157. Determination of the Synchronous Impedance. In order 

 to predetermine the characteristic curves of an alternator it is 

 necessary to know the relation between the field-exciting current 

 and the terminal e.m.f. at no load, given by the saturation curve 

 in Fig. 244, and also to know the components of the synchronous 



Amperes Field Current If 



FIG. 244. 



impedance of the armature. To obtain these a short-circuit test 

 is made on the alternator as follows: Short circuit the armature 

 through an ammeter and run the alternator at rated frequency 

 but with very low excitation. Gradually increase the excitation 

 until about twice full-load current flows in the armature. Plot 

 the values of short-circuit current I 9C on a base of field current //, 

 curve (2), Fig. 244. The locus will be a straight line passing 

 through the origin and can be produced beyond the range obtained 

 in the test. 



