The motor insulation is breaking down or a winding is shorted. The motor draws excessive current even though the mechanical load is normal.

What you'll see

Overload trips but the airend turns freely by hand. Motor current is unbalanced between phases or significantly higher than nameplate rating. The motor might smell like burnt insulation. Starting current is excessively high (almost three times what it should be).
Before you assume this is the problem

Check for voltage imbalance at the supply first -- the problem might be upstream, not in the motor. Also check that the relay itself isn't worn. See: Voltage Drop, Worn Relay.

See all causes of overload relay trips →

How to diagnose

  1. Megger test the motor insulation

    Use an insulation resistance tester (megger) at about 1000V. Measure resistance between each phase and ground, and between phases. It should be in the mega-ohms. Low readings indicate insulation breakdown.
    Result: Below 1 mega-ohm = insulation failure.

  2. Check winding resistance balance

    Measure resistance between pairs of motor terminals. All three should be approximately equal (within 5%). A significant difference indicates a shorted or open winding.
    Result: Unequal resistance = winding fault.

How to fix it

  1. Motor rewind or replacement

    A motor with failed insulation needs rewinding or replacement. Send it to a motor shop for evaluation. Rewinding is usually cheaper than replacement for larger motors.

Common mistakes

Don't assume the motor is fine just because it runs. A partially shorted winding will run but draw excessive current and eventually burn out completely. Catch it early with a megger test.

Parts & tools

Insulation resistance tester (megger). Multimeter for winding resistance. Clamp meter for current measurement.

Review safety precautions before starting →

Safety

Lock out/tag out before testing motor insulation.

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