ALTERNATING GENERATORS AND SYNCHRONOUS MOTORS. 69 
experimental determination of the short circuit current 
is effected by exciting the field with a certain current and 
then measuring the current which flows in the armature 
circuit, when the terminals are connected together by 
-means of copper bars having practically no resistance. 
For this test two measuring instruments are required, an 
ammeter in the field circuit and one in any one phase of 
the armature. 
The above test can be made with ease and certainty, and 
with three or four readings, carefully taken, the short 
circuit characteristic can be drawn. Fig. 34 gives the short 
circuit test of a 235 KVA. three-phase generator, running 
at a speed of 429 r. p.m. and rated for a normal current 
at 26 amperes. 
118. Shape of Curve.—dAs seen from the test data the short 
circuit characteristic is nearly a straight line, deviating 
from its course for very high excitations only. 
It is necessary to keep in mind that the short cireuit current 
lags 90 degrees behind the E. M. F. of the armature, 
because the ohmie resistance of the closed circuit is exceed- 
ingly small compared with the inductance. 
119. Effect of Armature Ampere Turns.—The effect of 
the armature ampere turns being directly opposed to that 
of the field coils, the resultant magnetization will be just 
enough to produce the necessary small E. M. F. to over- 
come the ohmic resistance of the circuit and the attendant 
leakage field. Consequently it is obvious, that the short 
circuit current is a measure of the inductance and leakage 
effects of the armature which influence the inherent 
regulation. 
120. Ratio of Armature and Field Ampere Turns.— 
If there were no leakage at all then the short-cireuited 
ampere turns would be necessarily equal to that of the 
