Pump Motor Tripping: Should the Overload Relay Be Raised?
A 7.5 kW (10 HP), 415 V, three-phase, 4-pole squirrel-cage motor drives a submersible pump through a star-delta starter. Nameplate full-load current is 14.2 A and the thermal overload relay is set at 14 A. After about four minutes of running, the starter trips. The operator has already reset it twice and asks you to 'just set the relay to 18 A so it stops nuisance tripping'. Your measurements: supply voltages 412 V, 415 V, 418 V; running currents L1 15.8 A, L2 15.4 A, L3 10.1 A; insulation resistance with a 500 V megger - 0.9 MΩ from winding to earth, 80 MΩ between phases; winding resistance balanced. The pump is delivering noticeably less water than last season, and the starter's L3 contactor tips show pitting and heat discolouration.
1. Should the overload relay be raised to 18 A? Support your answer with the current data.
Read the worked answer
No. Do not touch the relay.
- Nameplate FLC is 14.2 A and the relay is set at 14 A - that is correct for this motor. The relay is doing exactly its job.
- Average current = (15.8 + 15.4 + 10.1) / 3 = 13.8 A. Deviation of the worst phase from the average = (15.8 − 13.8) / 13.8 ≈ 14.5% unbalance, far beyond the 5% a motor can tolerate without derating. At that level the negative-sequence current heats the rotor and one stator phase long before the average current reaches 18 A.
- The three supply voltages are within about 1.5% of each other, so the unbalance is not coming from the supply - it is downstream, and the pitted, discoloured L3 contactor tips are the obvious suspect. A high-resistance L3 path reduces current in L3 and forces L1 and L2 up.
- Reduced water output fits the same story: an unbalanced supply means the motor is not developing full torque.
Raising the setting to 18 A would let the winding run above its insulation class temperature, turning a cheap contactor repair into a rewound motor.
2. The megger shows 0.9 MΩ to earth but 80 MΩ between phases. What does that tell you, and what will you do about it?
Read the worked answer
Reading it.
- 80 MΩ phase-to-phase is healthy: the winding is not internally shorted or uniformly contaminated.
- 0.9 MΩ to earth is below the acceptable minimum for a 415 V machine. The working rule is at least 1 MΩ, and many utilities want 1 MΩ + 1 MΩ per kV, i.e. about 1.4 MΩ here, measured at operating temperature.
- A low reading to earth with a normal reading between phases points to moisture at the cable gland, the motor seal or the winding overhang - exactly where a submersible fails first.
Action: treat the motor as unfit until dried. Disconnect the cable and megger the cable and the motor separately to localise the moisture (gland, cable or winding). If the cable is clean, dry the winding by hot air or low-voltage heating and re-test at 500 V. If the mechanical seal is leaking, replace it - otherwise the reading will fall again on the next run. A motor should not be left in service on 0.9 MΩ simply because it still starts.
3. Give the order of work from isolation to re-commissioning, including what you would record.
Read the worked answer
- Isolate and lock off at the starter and the upstream distribution board, prove dead and tag. Submersible work carries both shock and drowning risk.
- Inspect and replace the L3 contactor (tips and coil), then check and re-torque every terminal in the starter and the motor terminal box - a loose lug is the other classic cause of this exact signature.
- Megger the cable separately from the motor; dry or replace it if low.
- Deal with the moisture at the motor: re-seal or re-gland, dry the winding, re-test at 500 V and record the value.
- Check the pump end (suction, wear rings, impeller) for the reduced output once the electrical side is clean, so that a hydraulic fault is not blamed on the motor later.
- Re-commission: start, run for at least 30 minutes, record L1/L2/L3 currents. Unbalance must be under 5% and each phase at or below 14.2 A. Only then hand over - and write the readings in the log so the next person has a baseline to compare against.