Motor Keeps Tripping Its Overload After a Rewind
A 15 kW, 400 V, three-phase, four-pole induction motor driving a conveyor is returned from a rewind. On restart it runs for about 12 minutes, then the thermal overload relay trips. The relay is set to the motor nameplate full-load current of 28 A. A clamp meter reads 27 A, 27 A and 26 A on the three phases. The motor body feels hotter than the adjacent identical unit, and the current is steady rather than fluctuating. The maintenance fitter wants to increase the relay setting to 32 A to stop the trips.
1. Should the relay setting be increased? Justify your answer with reference to the readings.
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No. The measured line currents (27, 27, 26 A) are balanced and below the 28 A nameplate value, so the motor is not overloaded electrically and the relay is doing its job. Raising the setting to 32 A would allow the motor to run beyond its rated current and would remove protection from a machine that is already overheating. That is a fire and winding-failure risk.
Balanced currents below FLC with excessive temperature point away from an electrical overload and towards a thermal or mechanical cause: blocked cooling fan or dirty cowl, incorrect fan direction, ambient temperature above rating, rewind with lower efficiency or wrong conductor cross-section, bearing friction, or conveyor mechanical drag. The steady (not fluctuating) current argues against a mechanical jam that varies with load, so cooling and rewind quality are the leading suspects. Keep the relay at 28 A (or per the motor's actual service factor) and investigate the heat source.
2. Describe a logical fault-finding sequence to identify why the motor overheats, and state what evidence would confirm a rewind defect.
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Sequence:
- Confirm supply: phase-to-phase voltages balanced within about 1%, frequency correct.
- Confirm load: disconnect the conveyor coupling and run the motor uncoupled. If it still overheats, the fault is in the motor; if it runs cool, the load is the problem.
- Check cooling: fan intact, cowl clear, airflow unobstructed, ambient measured at the motor.
- Measure winding resistance of all three phases with a low-resistance ohmmeter, corrected to a common temperature. A rewind defect often shows as one phase with higher resistance or a mismatch between phases.
- Insulation resistance and, if available, surge comparison test to detect turn-to-turn faults.
- Compare no-load current with the sister unit; a significantly higher no-load current suggests incorrect winding turns or core damage.
Confirmation of a rewind defect: phase resistance imbalance beyond a small percentage, no-load current clearly higher than the identical motor, or a failed surge test. Document readings and refer back to the rewinder.