How do you measure resistance across AC motor windings?

• A. De-energize and lockout; disconnect motor; set multimeter to the lowest Ω range; measure between T1-T2, T2-T3, T1-T3; readings equal and very low; measure from each terminal to ground should be OL.
• B. Power the motor and measure current.
• C. Measure T1-T2 only while running.
• D. Use a megohm meter on insulation while energized.

Answer: (A)

Measuring winding resistance is about checking continuity and symmetry of the motor windings after removing all power. You want to verify that each winding is intact and that there’s no leakage to the frame.

To do this safely, de-energize and lock out the motor, disconnect it from the circuit, and discharge any stored energy. Set the multimeter to the lowest resistance (ohms) range. Measure between each pair of line terminals: T1-T2, T2-T3, and T1-T3. The three readings should be approximately equal and represent a small resistance value (the exact number depends on motor size). Equal, low readings indicate all windings are intact and similar in resistance.

Also test from each terminal to ground (the frame); the readings should be OL (no continuity to ground), which shows the windings are insulated from the frame. If a measurement to ground shows a path, or if one winding pair reads very differently from the others or shows open, that points to a fault such as an open winding, a short between turns, or a ground fault.

Powering the motor and measuring current isn’t a resistance test, and measuring only one pair while the motor is running won’t reliably reveal winding integrity. A megohm meter is used for insulation resistance with the circuit de-energized, not for measuring winding DC resistance while energized.