TESTING THE MOTOR
Most service technicians test a three-phase motor with an ohmmeter. The ohmmeter can be used to check the stator winding for an open condition or
a grounded condition. To test the stator winding for an open condition, check the continuity of each winding by measuring the resistance between each of the three windings as shown in Figure 15–14. The resistance of each pair of windings should be the same. To test for a grounded motor, connect one ohmmeter lead to the case of the motor, and the other lead to one of the motor leads. There should be no continuity between any of the leads and the case of the motor, Figure 15–15. The ohmmeter, how- ever, will not generally detect a shorted winding. The resistance of the stator windings of most large horsepower motors is so low that they appear under normal conditions to be a short circuit to the ohm- meter. To test for a shorted winding, some method must be used to measure the reactance of the windings instead of their resistance. If the motor will run, an ammeter can be used to measure the current draw of the motor. The current of each line should be equal and within the full-load current rating of the motor. If one line has a higher current reading than the others, it is an indication of a shorted stator winding. If it is not possible to operate the motor, an instrument that measures the actual inductance of the winding can be used. If one winding has a lower inductance than the others, it is shorted.