COMPENSATORS 219 



have 50, 65 and 80 per cent taps; those for motors above 

 17 h.p. have 40, 58, 70 and 85 per cent taps. The ratios 

 obtained should agree to within 3 per cent of the above 

 values. 



In determining ratios see that both the primary and 

 secondary meters are on the same phase. In checking 

 the ratio of quarter-phase compensators, apply 100 

 volts to the lines A and A (Fig. 76) and read the voltage 

 on the taps between the motor lead B and the taps. 

 These compensators are tested single-phase. 



Magnetizing Current 



Magnetizing current is measured at normal primary 

 voltage and normal frequency. The alternator used 

 should operate at about its normal voltage. On 60 cycle 

 compensators, the magnetizing current should not 

 exceed 25 per cent, on 40 and 25 cycle compensators it 

 should not exceed 30 per cent of the full load current of 

 the motor, assuming the latter to operate at 75 per cent 

 apparent efficiency. 



On compensators that are not standard, the magnet- 

 izing current should be measured at 20 per cent above 

 the normal potential, as well as at normal. In making 

 this test, hold the volts constant across one phase and 

 read the current in all three legs; then hold the current 

 constant in one leg and read the three-phase voltage. 

 Instead of holding the current in one leg, two voltmeters 

 may be used, one to hold the volts constant and the 

 other to read the three-phase voltage. Since a high 

 magnetic density obtains in the core, see that the voltage 

 and frequency are correct, as a slight change in either 

 will change the magnetizing current considerably. 

 Quarter-phase compensators are tested as though they 

 were single-phase. 



Heat Runs 



Short circuit the leads to the motor and apply sufficient 

 voltage to the line leads to force the required current 

 through the coils for one minute. Place a thermometer 

 on the coils to obtain the temperature. Thirty minutes 

 should elapse between successive heat runs on the same 

 compensator and after each heat run the tap leads should 



