Pump Motor Tripping After Rewind in a Tea Factory
A 7.5 kW, 415 V, three-phase, 4-pole squirrel-cage pump motor at a tea factory was rewound last month. Since then the star-delta starter trips on overload after about four minutes of running, every time. The pump delivers normal discharge pressure, the suction strainer is clean, and there is no abnormal vibration or noise. Line currents measured at the starter input are 14.2 A (R), 12.8 A (Y) and 13.5 A (B); the nameplate full-load current is 13.8 A. Line voltages are 418 V, 395 V and 408 V. Insulation resistance to earth is 80 MΩ at 500 V. The overload relay is mounted inside the delta contactor and is set at 7.5 A. The factory electrician proposes to raise the setting to 11 A and, if that fails, to replace the motor. You are the trainee asked to give a written opinion.
1. Is 7.5 A the correct setting for a relay in that position? Calculate the correct value and explain what the current readings imply.
Read the worked answer
A relay in the delta contactor carries only the winding (phase) current, which is line current ÷ √3.
Correct setting = 13.8 / 1.732 = 7.97 A, i.e. about 8.0 A (manufacturers often print this as 0.58 × FLC for star-delta).
Measured line current averages 13.5 A, so the relay actually sees 13.5 / 1.732 = 7.8 A - just above its 7.5 A setting. A small sustained excess of this kind takes a few minutes to operate a thermal overload relay, which exactly matches the observed four-minute trip with everything else healthy.
Implication: the relay is set roughly 6% below the correct value, so the trip itself is a setting error, not proof of a motor fault. The insulation resistance of 80 MΩ is good, so there is no earth fault. The correct first action is to set the relay to 8.0 A and retest; only then does the question of genuine overload arise.
2. Assume the relay is corrected to 8.0 A. Using the voltages given, identify the most probable remaining cause of a trip and support it with a calculation.
Read the worked answer
Voltage unbalance: average = (418 + 395 + 408)/3 = 407 V. Maximum deviation = 407 − 395 = 12 V. Percentage unbalance = 12 / 407 × 100 = 2.9% (just under the 3% mark).
Consequence: at about 3% voltage unbalance the negative-sequence current in the rotor is high, and standard derating guidance (NEMA MG-1 style) allows the motor roughly 88% of its nameplate output before the hottest winding reaches its rated temperature rise.
Load demanded: 13.5 / 13.8 = 98% of nameplate. Once the motor's capacity is derated to ~88%, a pump demanding 98% of nameplate is a genuine overload of roughly 10% - enough to trip a correctly set relay after several minutes, but only after the setting error is removed.
Conclusion: the corrected relay will restore protection but may still trip at full flow. The root cause is the unbalanced supply (uneven single-phase loading on the factory transformer, or a high-resistance joint, fuse clip or contactor pole on the Y phase), which must be found and corrected, or the pump load reduced by throttling or impeller trim. The rewind itself remains a secondary suspect (wrong wire gauge or fewer turns raises current) and should be checked against the original data if the supply is proven clean.
3. Evaluate the electrician's two proposals - raising the relay to 11 A, then replacing the motor - and give your recommendation.
Read the worked answer
Raising to 11 A is unsafe. A relay in the delta leg set at 11 A allows a line current of about 11 × 1.732 ≈ 19 A, roughly 138% of nameplate, before it trips. The motor would run hot and fail within weeks, likely taking the rewind warranty with it; the protection would exist on paper only.
Replacing the motor is wasteful. The measured currents, good insulation and normal mechanical behaviour point to supply and setting, not to a failed machine. A new motor on the same unbalanced supply would be derated in exactly the same way and fail the same way - the fault would be repeated at full cost.
Recommendation, in order:
- Set the relay to 8.0 A and record the setting.
- Measure voltage and current unbalance at the MCC and at the transformer; inspect the Y-phase fuse clips, joints and contactor pole for heat discolouration or resistance.
- Ask for the rewind data (turns, wire gauge, connections) and compare with the nameplate.
- Log current and winding temperature (or casing temperature) for a week.
- Only if a clean, balanced supply still produces a trip should the pump duty point be examined or the motor replaced.