While the motor is in an assembled/disconnected state, apply low voltage single phase power to any two of the motor leads and slowly rotate the shaft. While rotating the shaft, observe an ammeter connected to one of the motor’s phases, keeping a close eye on fluctuation of the measured current. Variations in the measured amps by as little as ±3%, could indicate an open rotor. Increased electrical noise, during this test, may also be indicative of an open rotor.
While the single phase rotation test is quite effective on detecting an open circuit, there are a couple things that an evaluator must be aware of before taking the results to the bank. The single phase rotation test does not detect minor defects in the rotor cage/casting, the motor must be in an assembled state (and preferably heated from recent operation), and the results of this test are 100% dependent on a qualified technician’s observations during the test.