How do you perform an insulation resistance test (megger) on a motor winding, and how do you interpret the results?

How do you perform an insulation resistance test (megger) on a motor winding, and how do you interpret the results?

• A. Disconnect power, apply insulation tester to windings and ground, measure resistance at the specified voltage and time; higher resistance indicates good insulation, low resistance indicates deterioration or shorts.
• B. Energize the winding and measure current while applying a megger to the line.
• C. Use megger to measure the inductance of the winding.
• D. Use megger between windings to measure phase-to-phase continuity.

Answer: (A)

Insulation resistance testing with a megohmmeter is about charging the insulation with a high DC voltage and measuring how much leakage occurs, which reveals the condition of the insulation between conductors and to ground. To do it properly, first ensure the motor is completely de-energized and isolated, remove any stored charges, and follow safety procedures. Then connect the megohmmeter so the test voltage is applied between the windings and ground (earth), and, if the procedure calls for it, between the windings themselves. Apply the specified test voltage for the required duration and read the insulation resistance in megohms. Interpreting the reading: a high insulation resistance value indicates the insulation is in good condition and there’s minimal leakage. A low resistance suggests the insulation has deteriorated or is compromised—potential moisture, contamination, or breakdown leading to leakage or possible shorts. Temperature affects readings, so compare against the baseline values at the same temperature or use trend data from previous tests, rather than relying on a single number. After the test, discharge the winding safely before handling. This approach is the standard method because the megger measures insulation integrity, not winding continuity or inductance. Energizing the winding while testing, measuring inductance with a megger, or using the megger to assess phase-to-phase continuity do not provide reliable insulation health information and are not appropriate uses of the instrument.

Insulation resistance testing with a megohmmeter is about charging the insulation with a high DC voltage and measuring how much leakage occurs, which reveals the condition of the insulation between conductors and to ground. To do it properly, first ensure the motor is completely de-energized and isolated, remove any stored charges, and follow safety procedures. Then connect the megohmmeter so the test voltage is applied between the windings and ground (earth), and, if the procedure calls for it, between the windings themselves. Apply the specified test voltage for the required duration and read the insulation resistance in megohms.

Interpreting the reading: a high insulation resistance value indicates the insulation is in good condition and there’s minimal leakage. A low resistance suggests the insulation has deteriorated or is compromised—potential moisture, contamination, or breakdown leading to leakage or possible shorts. Temperature affects readings, so compare against the baseline values at the same temperature or use trend data from previous tests, rather than relying on a single number. After the test, discharge the winding safely before handling.

This approach is the standard method because the megger measures insulation integrity, not winding continuity or inductance. Energizing the winding while testing, measuring inductance with a megger, or using the megger to assess phase-to-phase continuity do not provide reliable insulation health information and are not appropriate uses of the instrument.