Diagnosing a Motor That Trips on Start Under Load
A 15 kW, 400 V, three-phase squirrel-cage motor drives a centrifugal pump. After a bearing replacement, the motor trips its overload relay about 8 seconds after start, every time. The relay is set to 32 A; nameplate full-load current is 28 A. Line voltage is balanced at 398 V. The motor starts and runs fine when uncoupled from the pump. The pump turns freely by hand. The electrician suspects the motor windings and wants to send it for rewinding. The supervisor asks for a systematic diagnosis before any rewinding is authorized, noting that the pump was also serviced at the same time and the coupling was re-aligned by an apprentice.
1. Analyse the evidence and identify the most probable cause, explaining why the motor-rewind hypothesis is weak.
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Key evidence:
- Motor runs fine uncoupled → windings, supply voltage and starting circuit are healthy.
- Trips only when coupled, after ~8 s → the fault appears when load is applied.
- Pump turns freely by hand → no mechanical seizure in the pump itself.
- Overload relay at 32 A vs FLC 28 A → setting is reasonable (about 1.14 × FLC), so the relay is not simply mis-set too low.
- Bearing replacement + coupling re-aligned by an apprentice → misalignment is a prime suspect.
A misaligned coupling imposes large radial and axial forces, raising bearing friction and the torque required to turn the pump. Current rises above the relay setting and the relay trips after its inverse-time delay (consistent with ~8 s). The rewind hypothesis is weak because a faulty winding would show up uncoupled too, and would not selectively appear only under load.
Other checks: coupling gap/parallelism, soft foot, pump impeller clearance, and whether the pump discharge valve is throttled open (overload from excessive flow).
2. Outline a step-by-step diagnostic procedure and state what measurements would confirm or exclude misalignment.
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- Verify uncoupled run - confirm current is near no-load value (a few amps) and stable. Excludes winding/supply faults.
- Check relay setting and trip curve - confirm 32 A is appropriate and the trip time matches an overload, not a short-circuit.
- Measure current with the pump coupled but discharge valve closed (or throttled) - if current is normal, the fault is hydraulic (excessive flow), not mechanical.
- Check coupling alignment - use a dial gauge or feeler/straight-edge to measure parallel and angular offset. Misalignment shows as offset beyond the maker's limit (typically a few hundredths of a mm).
- Check soft foot and bearing preload - a soft foot distorts the frame and loads the bearings.
- Measure vibration - misalignment gives characteristic 2× running-speed vibration, distinguishing it from imbalance (1×) and bearing defects (high-frequency).
Confirmation: correcting alignment and re-testing should bring running current below the relay setting. If current remains high with the valve closed and alignment correct, then inspect the pump internals (wear rings, impeller) before touching the motor.