242 



ALTERNATORS. 



can be varied and ultimately short-circuited should be put 

 in series with the switch. 



If a magnetisation curve at no load for the same speed 

 has been previously taken, it will be unnecessary to take 

 voltmeter readings in this experiment, since these readings 

 form a no-load magnetisation curve. 



In such a case, the voltage corresponding to each 

 value of the excitation should be taken from the previqusly 

 plotted magnetisation curve and entered in the column 

 " Volts on Open Circuit " after correction for difference in 

 speed, if necessary. 



Results should be entered in tabular form as indicated 

 below. 



DETERMINATION OF SHORT-CIRCUIT CHARACTERISTIC. 



Alternator No Type 



Normal output . . . .amps volts at ... .revs, per min. 



Frequency Normal excitation .... amps. 



The readings should be varied so that the maximum 

 armature current is somewhat greater than the normal full- 

 load current. 



. A curve comparing exciting current and short-circuit 

 armature current should be plotted from the readings. For 

 moderate values of the excitation this will be found to be 

 practically a straight line, but bends upwards rather sharply 

 if continued for higher excitations. 



Fig. 115 shows the results of a test carried out in the 

 manner just described on a 6-kw. 4-pole Schuckert alternator 

 with rotating field, the speed being 1,200 revs, per minute, 

 and the normal maximum current 35 amps. The curve 

 is a straight line cutting the vertical axis at a point correspond- 

 ing to 1-3 amps. This shows that the residual magnetism is 

 sufficient to send a current of 1-3 amps, through the short- 

 circuited armature. 



