On a good motor, a megger reading between phases will show a short. If a motor has to have heat applied to get the megger reading up, then it will need a rewind eventually as the insulation is either tracking to ground or absorbing moisture. So, if you have 480 volt motor, we look for a minimum of 1.5 megohm or we won't start it.
HOW TO CHECK 3 PHASE MOTOR WINDING WITH MULTIMETER PLUS
At our plant, we consider a motor startable if it megs 1 megohm per each 1KV of running voltage plus 1KV. The only test that will find shorts between individual coils of wire in a phase is the surge test. Sometimes you have to dis-connect the motor and eliminate the leads from the MCC to the motor. If some of the windings are shorted or you have bad connections, then the resistance readings will be unacceptable. We also check the phase to phase resistance reading and are looking for no more than a 10% difference max. The megger test is checking the condition of the ground wall insulation of the motor. Most, if not all our motors only have 3 leads coming out of the connection box. Typically we megger motor just one phase of the motor to ground as the phases are all connected inside the motor. WB82- I work in a power plant of a large utility company. Depending on what you get, it tells you the condition of your ground wall insulation. You then divide the ten minute reading by the one minute reading to get a number between 1 and 10. It is like taking a 10 minute megger reading. Another test we might do is called a Polarization Index test.