When a bad motor winding cannot be found by a physical inspection, many times an ohmmeter may show that there is a defective winding. Most repair shops will use a "megger" or a ground insulation tester. The megger will help to detect areas of deteriorating insulation where the windings may soon short. Most individuals do not own a megger, but probably do have an ohmmeter. While the ohmmeter may not show potential failures, it can be used to determine if the windings are good or bad.
Remove all power from the motor and isolate the circuit. Tag the circuit breaker to prevent accidental restoration of power. Open motor housing to access windings.
Clean the windings and motor interior with shop air and inspect for any damage. Check winding insulation for cracks or brittleness.
Connect the probes to the ohmmeter and set the meter to a midrange setting. Touch the two leads together and make sure that the meter indicates continuity, or zero ohms.
Consult the motor schematic or a diagram of the motor's windings and use the meter to measure the resistance of the windings. Each winding leg should have some small resistance reading. The meter should not show an "open" or a "short" when a winding leg is measured.
Use the schematic and take readings of all of the windings. Write down each measurement and make comparisons when measurements are complete.
Use the link listed below in the Resources section for a Website on how to use an ohmmeter.
Use extreme caution when working with electricity. Ensure all power has been removed from motor circuit and that the breaker has been tagged.