Rewound 300 hp Motor Running Hot With Voltage Unbalance and a 21 °C Phase Spread
A 300 hp, 460 V, 4-pole, 60 Hz TEFC motor (355 A FLA, 1.15 service factor, Class F, rated 40 °C ambient) drives a critical cooling-water pump. After a rewind by a local shop the motor draws 342 A and its three stator RTDs stabilize at 158, 141 and 137 °C; the shop's report shows 1.2 GΩ insulation resistance and a 'satisfactory' surge comparison test. MCC line voltages are A–B 472 V, B–C 478 V, C–A 461 V; line currents are 352, 318 and 305 A. A 1,200 kvar automatic capacitor bank sits on the same 480 V bus, and its controller has displayed 'stage 3 - fuse blown' for six weeks. The pump room reaches 48 °C in summer. Production wants the pump at full load through August and argues that the motor is 'only at rated current'. The maintenance supervisor argues the motor should go back to the shop for a second rewind. Your manager asks for a technical position before the 08:00 production meeting.
1. Quantify the voltage and current unbalance and estimate how much of the overheating can be attributed to the supply.
Read the worked answer
Voltage: average = (472 + 478 + 461)/3 = 470.3 V; maximum deviation = 478 − 470.3 = 7.7 V = 1.6% unbalance. That exceeds the 1% level above which NEMA MG-1 recommends derating. Using the usual rule of thumb, temperature rise increases by roughly 2 × (1.6)² ≈ 5%, with a derating of only a few percent of nameplate output.
Current: average = (352 + 318 + 305)/3 = 325 A; maximum deviation = 27 A = 8.3% current unbalance. That is about five times the voltage unbalance, inside the typical 6–10× rule, which confirms the voltage unbalance is real and not a meter artifact.
What it does not explain: the highest phase current, 352 A, is 99% of the 355 A FLA. Add the ~5% rise from unbalance and the 48 °C room (8 °C above the 40 °C rating basis, costing roughly 8 K of thermal margin), and you can account for perhaps 15–20 K of the total. That is significant but it does not explain a 158 °C reading, and it certainly does not explain the phase-to-phase spread. So: the supply is a real, correctable contributor, but it is not the whole answer.
2. The measured 21 °C spread between the three RTDs is the key observation. What does it tell you, and which tests would you demand before accepting either the shop's 'satisfactory' report or a second rewind?
Read the worked answer
A 21 °C inter-phase spread points into the machine, not into the supply. Voltage unbalance produces a negative-sequence current that principally heats the rotor and adds stray losses; the three stator windings still see nearly balanced positive-sequence current. A 1.6% supply unbalance cannot physically create a 21 °C difference between three stator windings.
Candidate causes to eliminate, in order of likelihood:
- Unequal turns or a misconnected parallel circuit (part of the winding carrying more current).
- A partially shorted turn or a high-resistance end-winding joint from the rewind.
- An RTD that is misplaced, unbonded, or out of calibration - cheap to rule out, so rule it out first.
Tests to demand, in writing:
- Phase-to-phase winding resistance corrected to a common temperature; expect less than about 2% spread. This is the single most diagnostic field test and it was not in the shop's report.
- Surge comparison with the waveforms supplied, not a pass/fail line - a slight turn-to-turn defect can pass a go/no-go test.
- RTD calibration check and confirmation of RTD placement.
- If resistance or surge is marginal, DC step-voltage or partial-discharge/tan-delta testing on the windings.
- A heat run at balanced voltage and controlled ambient after the capacitor bank is repaired, before the motor is declared fit.
A second rewind without this data risks repeating the same defect and gives you no basis for accepting the machine.
3. State your position for the 08:00 meeting: what may run, what must be fixed, and in what order.
Read the worked answer
Position: the motor is not released for continuous full load at 48 °C ambient.
Immediate actions, in order:
- Repair the capacitor bank - replace the blown stage-3 fuse and check the other stages; before re-energizing, confirm no resonance with the plant's harmonic sources, since a partially connected bank can create more problems than it solves. Re-measure bus voltage balance afterwards.
- Reduce ambient to near the 40 °C rating basis - temporary ventilation or cooling in the pump room is cheap compared with a forced outage.
- Operate at reduced load with RTD alarms and a defined trip setpoint until the winding data are in hand; do not treat the 1.15 service factor as permission to run at 158 °C.
- Order the resistance and surge data and hold the second-rewind decision until those results exist.
Communication: put this in writing with the measured values, the limit basis, the actions, owners and dates. Correct the premise that 'only at rated current' means safe - the limiting quantity is winding temperature, and the limiting ambient is 40 °C. Note also that a repeat trip or a worsening spread after the electrical fixes would be the trigger to remove the motor from service immediately.