266 



ELECTRICAL ENGINEERING 



At any value of field current oa, the no-load voltage is ab and 

 the short-circuit current is ac. Since the terminal e.m.f. is zero 

 the e.m.f. ab must represent the synchronous impedance drop due 

 to the current ac. The synchronous impedance is, therefore, 



_ EQ _ ab 



ZQ 7 * 



Isc ac 



The ordinates of curve (3), Fig. 244, are obtained as the quotients 

 of the corresponding ordinates of curve (1) and curve (2). The 



50 100 150 



Amperes Field Current 



FIG. 245. Characteristic curves of a 2500-kv.a., 5000-volt, 

 three-phase alternator. 



ordinates of curve (3), therefore, represent the synchronous im- 

 pedance of the armature. It is not constant but is high when the 

 magnetic circuit of the alternator is unsaturated and decreases 

 as the excitation is increased and the magnetic circuit becomes 

 saturated. 



If the power required to operate the machine is measured and 

 the excitation loss, iron losses and friction losses are subtracted, 



