One of the three phases is missing or significantly lower than the others. The remaining phases carry more current and the overload trips.

What you'll see

The motor hums loudly, vibrates excessively, or runs hot. The overload trips. Measuring current on all three phases shows one is significantly lower (or zero) while the others are high. This can happen suddenly (blown fuse, broken connection) or intermittently (loose contact).
Before you assume this is the problem

If all three voltages are balanced and correct, the problem is elsewhere. Check the motor windings and the relay.

See all causes of overload relay trips →

How to diagnose

  1. Measure all three phase voltages

    Check voltage between each pair of phases at the motor terminals: L1-L2, L2-L3, L1-L3. All three should be equal (within 2%). If one pair is significantly lower, a phase is compromised.
    Result: One phase lower or missing = supply fault.

  2. Measure current on all three phases

    Use a clamp meter on each phase conductor. All three should be roughly equal. If one is zero and the other two are elevated, you've lost a phase.
    Result: One phase zero or very low = lost phase.

How to fix it

  1. Restore the missing phase

    Check fuses, breakers, and connections on the missing phase. Trace from the motor back to the distribution board. Fix the broken connection, replace the blown fuse, or reset the tripped breaker.

Common mistakes

Don't confuse a phase loss with a motor winding fault. A phase loss starts at the supply; a winding fault is inside the motor. Measure at the incoming terminals first to determine which.

Parts & tools

Multimeter. Clamp meter. Replacement fuse if needed.

Review safety precautions before starting →

Safety

Running a three-phase motor on two phases will destroy it quickly. Don't keep resetting and restarting.

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