Borehole Motor Tripping on Overload
A 15 kW, 400 V, 50 Hz, three-phase squirrel-cage motor drives a borehole pump through a star–delta starter. The nameplate full-load current is 28.5 A. After about eight minutes the thermal overload relay, set at 28 A, trips. Line currents measured with a clamp meter are 29 A, 30 A and 41 A. Supply voltages are 398 V, 401 V and 396 V. Ambient temperature at the motor is 42 °C, and the motor is totally enclosed fan-cooled with a dirty cooling fan cover. Insulation resistance at 500 V is 2 MΩ to earth. Winding resistance measured between phases is 4% higher on one phase than the other two. The terminal box shows discoloration on one phase and a loose lug. The pump has been running without maintenance for 14 months. The motor is class F insulated and the overload relay is located in the starter, 15 m from the motor.
1. Diagnose the likely cause or causes of the trip and explain which measurements support each cause.
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The current imbalance is the main clue: (41 − 29)/29 × 100 = 41% above the lowest phase, which is far too high for a healthy motor. The voltage imbalance is small: average = (398 + 401 + 396)/3 = 398.3 V; maximum deviation = 2.7 V; 2.7/398.3 × 100 = 0.67%. A 0.67% supply imbalance does not explain a 41% current imbalance. The discoloured terminal and loose lug on one phase point to a high-resistance connection. That raises local heating, distorts phase currents and can damage the winding. The dirty fan cover and 42 °C ambient reduce the motor’s cooling and its continuous current capability, contributing to the thermal trip. The 2 MΩ insulation resistance is above the 1 MΩ minimum but low enough to require investigation, and the 4% winding resistance imbalance is at the limit; compare it with the manufacturer’s tolerance. Do not simply reset the overload.
2. State the tests you would perform before restarting, including safe isolation and motor circuit tests.
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Before restart: isolate, lock off, prove dead and secure the starter. Inspect the terminal box and remote starter; replace the discoloured lug and damaged insulation, clean and torque all terminals to the manufacturer’s setting. Measure winding resistance phase-to-phase with a micro-ohmmeter and compare all three; repeat insulation resistance to earth and between windings. Check supply voltage and phase sequence at the starter, then run the motor uncoupled if the pump allows, or at least at no-load, and record current on all phases. Verify the overload relay setting and calibration, and check the pump for mechanical binding or a blocked impeller. Only after the imbalance and thermal causes are corrected should the motor be returned to service under observation.
3. Decide whether the overload setting is correct, calculate the current imbalance, and recommend corrective action.
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The overload is set at 28 A, slightly below the 28.5 A nameplate FLC, so the setting alone does not explain a trip at normal load; it may even be too low for a healthy motor at 42 °C. However, the 41 A phase current is about 44% above nameplate FLC, so the relay is responding to a real overcurrent. Current imbalance = (41 − 29)/29 × 100 = 41.4%, which is unacceptable and points to the loose lug, terminal damage or a winding problem. Corrective action: repair the loose lug and discoloured terminal, clean the cooling fan and cover, re-test, and re-set the relay to the nameplate FLC or to the manufacturer’s derated value for 42 °C. If the winding resistance imbalance remains above the manufacturer’s tolerance, or insulation resistance stays low after drying, send the motor for repair or replacement. Also check the pump load; a mechanical fault can cause overload without an electrical fault. Record before-and-after currents, insulation resistance and the final relay setting.