Induction Motor Tripping on Overload After Rewind With a 6% Supply Voltage Unbalance
A 415 V, 50 Hz, 75 kW, 4-pole, 1480 r/min cage induction motor drives a centrifugal pump through a belt. Full-load current is 132 A and the class 10 thermal overload relay is set at 132 A. After a rewind following a winding failure, the motor trips on overload after 25 to 40 minutes at a flow it previously handled continuously. Line voltages with the motor stopped are 420 V, 423 V and 384 V, and they are essentially unchanged when it runs, so the unbalance originates upstream of the MCC. Running currents at the trip point are 118 A, 121 A and 142 A, with the highest current consistently in the same supply phase. Insulation resistance is above 500 MΩ and dc resistance measured with a 2 A Kelvin bridge is within 1% between phases. Pump curve, valve position and suction pressure are unchanged, the shaft turns freely, and the motor input measured at the MCC is 88 kW at the trip point. The frame runs noticeably hotter than before. Maintenance proposes raising the overload setting to 1.15 × FLC and recalibrating the relay.
1. Quantify the voltage unbalance and decide whether the trip is a genuine thermal overload or a relay calibration problem.
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Voltage unbalance. Average line voltage = (420 + 423 + 384)/3 = 409 V. Largest deviation from average = 409 − 384 = 25 V. Percentage unbalance = 25/409 × 100 ≈ 6.1%.
Why that matters. The standard derating guidance for induction motors stops at about 5% unbalance, where the permissible output is roughly 75% of rating. At 6.1% there is no tabulated derating factor at all - the motor is simply not suitable for rated load at that supply quality. The physical reason is the negative-sequence field, which rotates backwards relative to the rotor at almost double synchronous speed, so it sees a high effective rotor resistance and produces disproportionate rotor I²R loss for a given negative-sequence current. Rotor heating rises much faster than the voltage unbalance figure suggests.
Is the trip genuine? Yes. Average current = 127 A, so current unbalance = (142 − 127)/127 ≈ 11.8%, and the worst phase at 142 A is 108% of the 132 A setting - the relay is doing its job. Rated input at 75 kW and about 94% efficiency is ≈ 80 kW; the measured 88 kW is roughly 10% above that, which is consistent with the extra negative-sequence losses rather than with a faulty meter or relay. A hot frame and a 25–40 minute trip time on a class 10 element fit a real thermal overload.
Raising the setting to 1.15 × FLC would push the trip point beyond the winding's thermal capacity and convert an overload trip into a stator failure. That is the wrong move.
2. The rewind is also under suspicion. What single test separates a supply problem from a winding asymmetry, and what supporting evidence would you collect?
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The decisive test is a phase rotation. Swap two supply phases at the motor terminals and repeat the loaded run. If the high current stays with the same supply phase (the same physical conductor from the MCC), the motor is electrically symmetric and the supply unbalance is the whole story. If the high current moves with a particular motor winding, the rewind introduced an asymmetry.
Supporting checks:
- No-load current test on a balanced supply. A healthy machine shows a modest spread, but more than about 10% between phases at no load, with balanced volts, points to the winding. Because the site supply is itself unbalanced, this must be done from a clean source or not at all.
- Surge comparison test. Compare the waveform of each phase against the others; a turn-to-turn or coil-to-coil defect shows as a repeatable difference in wave shape and amplitude, which the 2 A dc resistance test cannot see. Note that dc resistance within 1% does not exonerate the winding: a 1% resistance difference would give roughly a 1% current difference, nowhere near the 12% observed.
- Check the rewind's parallel paths and tap connections against the original nameplate data; a misplaced parallel circuit or a wrong number of turns in one phase is a classic post-rewind defect.
- Confirm the supply is truly upstream. The open-circuit voltage at the MCC is already 384 V on one phase, which rules out cable volt drop - but a loose or corroded joint upstream (including a blown capacitor-bank fuse) can produce the same pattern under load, so re-measure open circuit after isolating and inspect the terminations thermally.
3. Set out the corrective plan, distinguishing what can be done this week from what needs a permanent fix, and state the interim operating limit.
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Immediate (this week):
- Do not raise the overload setting. Confirm the relay is calibrated and its class 10 curve matches the motor's thermal limit.
- Trace the 384 V phase upstream: thermographic scan of the MCC, cable terminations, and the transformer tap connections. A high-resistance joint would be both the cause of the unbalance and a fire risk, so it must be ruled out before blaming the utility.
- Run the phase-rotation test to clear or confirm the rewind.
- Until the supply is corrected, restrict the pump to a duty where the worst phase current stays at or below 132 A - typically 70–75% of rated output, achieved by throttling or trimming the impeller, not by raising the trip setting.
Permanent fixes, in order of preference:
- Correct the source unbalance: redistribute single-phase lighting and small loads across the board, and check the distribution transformer for an unbalanced single-phase loading pattern or a loose delta connection.
- If the utility supply is the problem, request a voltage investigation and a tap change; document the readings over a week, because 6% will not be corrected on a verbal complaint.
- Where the supply cannot be fixed, feed the motor from a VFD or an unbalanced-supply-tolerant arrangement, or re-rate to a larger frame so that the negative-sequence heating is absorbed.
- Only if the phase-rotation test proves a winding asymmetry should the rewind be rejected and redone.