270 



ELECTRICAL ENGINEERING 



10 20 80 40 50 60 70 80 90 100 110 120 180 140 150 160 

 Amperes Exciting Current Iy 



FIG. 248. Saturation curve of an alternator. 



160. Short-circuit Currents of Alternators. If an alternator 

 is short circuited, when operating with full-field excitation, the 

 current is limited only by the synchronous impedance of the 

 armature; it is 



Vr 2 + (x l + x') 2 ' 



(258) 



and will usually be from 3 to 6 times the normal full-load current. 

 At the instant of short circuit the current will be much larger 

 than the value given by equation 258, because the component x' of 

 the synchronous reactance, which represents the effect of armature 

 reaction, does not act instantaneously to limit the current. It 

 represents a change in the flux which interlinks with the field 

 circuit of the machine and on account of the self-inductance of the 

 field winding with its large number of turns, this change of flux 

 cannot take place instantaneously but may take several seconds 

 to become complete. During this time the short-circuit current 

 is limited only by the true impedance of the armature, and is 



rr 



(259) 



It may reach 8 or 10 times full-load current. 



