222 COMMERCIAL ELECTRICAL TESTING 



normal load for about thirty minutes before the air 

 biast is put on. Oil cooled reactances should be started 

 at overload to shorten the heat run as much as possible. 



In making heat runs on reactances designed for six- 

 phase circuits, connect the coils in series and make the 

 heat run on three-phase circuit. Reactive coils cannot be 

 tested by the motor-generator method (Hopkinson 

 method), hence, the test must be run from an alternator 

 capable of supplying full kilo volt-amperes. If an alter- 

 nator of sufficient capacity is not available, use two in 

 multiple. They can be run as generators, but after the 

 alternators have been synchronized, it is better to pull the 

 breaker of the driving motor of one of them. By proper 

 adjustment of the field current, the one running light will 

 operate as a rotary condenser. 



In measuring reactive drop, take the volts across each 

 coil, holding full load current in one leg; then hold the 

 volts across one coil constant and read the current in all 

 three legs; after which take the drop across each leg, 

 holding full load current on all legs. This test must also 

 be made at 50 per cent overload. The frequency must 

 be held constant while the drop is being taken, as the 

 reactance depends directly on the frequency. When the 

 reactive coil has reached constant temperature at normal 

 load, run for two hours at 50 per cent overload. 



When the heat run is finished take air readings and 

 test the insulation. The insulation tests consist of double 

 potential for one minute, and one and one-half potential 

 for five minutes, and high potential tests. The high 

 potential test must be applied between the winding and 

 core, between the winding and frame, and between the 

 phases. 



Polarity Test 



The polarity test is made by supplying direct current 

 to the middle phase and so connecting a voltmeter to 

 the terminals as to get a positive deflection; the drop 

 lines are then transferred to the corresponding terminals 

 of the other phases and the direction of kick on breaking 

 the current in the middle phase is noted. 



