SYNCHRONOUS GENERATORS 293 



short circuit, the generator being short circuited at the terminals 

 of the armature winding. Comparing the currents for phases 

 A and C, it is noticed that the latter gives an approximately sym- 

 metrical relation of the current crests with respect to the zero- 

 axis, while in the former case the wave is displaced so that the 

 maximum peak of the initial current is nearly double that of phase 

 A, the actual ratio for the average machine being about 1.8. In 

 calculating the instantaneous short-circuit current which may occur 

 under the worst conditions, an unsymmetrical current wave should, 



Field Current 



A 



FIG. 163. Oscillogram of Three-phase Alternator Short Circuit. 



therefore, be considered as well as the fact that the short circuit 

 may occur when the generator is excited for full load, which would 

 mean a still further increase of say 10 per cent in the flux and in 

 the short-circuit current. Thus, for a generator, with 20 per cent 



100 

 reactance, the maximum peak would be - - or five times the normal 



mean effective current times V2 times 2. 



The sustained short-circuit current is, as previously stated, 

 limited by the synchronous impedance, or less exactly, the syn- 

 chronous reactance, of the generator, and, neglecting saturation, 

 it is directly proportional to the field exciting current. Although 



