How do you check a motor winding resistance with an ohmmeter and interpret the reading?

• A. Measure between windings and ground; compare between windings and to manufacturer specs; abnormal readings indicate opens, shorts, or grounding faults.
• B. Measure winding resistance with the power on and compare to nameplate values.
• C. Only measure resistance between each winding and the supply line.
• D. Compare resistance readings to a standard value without reference to the motor's datasheet.

Answer: (D)

The test checks winding continuity and insulation by using the ohmmeter and comparing to the motor’s specified values. With the power off and the motor isolated, measure the resistance between each pair of terminals (line-to-line) to get the three winding resistances. In a healthy three‑phase motor, these three readings should be very close to each other; a significantly higher value in one winding suggests an open or damaged winding, while a significantly lower value can indicate a shorted turn or cross-winding. Next, measure from each winding lead to ground to check insulation; a measurable to-ground reading points to an insulation fault, while a very high or infinite reading is typically acceptable insulation-wise.

Then compare your results to the motor’s datasheet or manufacturer specs, not to a generic standard value, and consider temperature effects since resistance changes with temperature. If a reading is outside the specified tolerance, the winding likely needs repair or replacement.