DOL Starter Tripping on a Hot, Sealed Panel
A 7.5 kW, 415 V, three-phase squirrel-cage motor drives the main lathe in a fabrication shed. Its nameplate full-load current is 14.5 A. The machine is fed from a local DOL starter panel built into a sealed sheet-steel IP54 enclosure fixed to the wall beside the lathe; the shed air temperature at mid-afternoon is 44 °C. The contactor is rated 18 A, and the thermal overload relay, range 10–16 A, is a non-compensated bimetal type with its dial set at 14.5 A. The relay is mounted directly above the contactor, with no ventilation openings. After about 15 minutes of normal cutting load the relay trips; it resets and runs again for a similar period. A clamp meter shows 14.2, 14.0 and 14.3 A; line voltage at the panel is 396 V; a thermometer left inside the panel reads 58 °C. Winding insulation resistance is 40 MΩ, the belt is correctly tensioned, and the contactor tips show light pitting.
1. Why does the relay trip when the measured current is at, not above, the motor full-load current? Support your answer with a calculation.
Read the worked answer
The relay is not measuring motor temperature; it is measuring its own. A non-compensated bimetal relay is calibrated for a reference ambient, usually 40 °C. Its effective trip current falls as the panel gets hotter - roughly 1.2 % per K above 40 °C for this type.
Correction: 58 − 40 = 18 K → 18 × 1.2 = 21.6 % reduction → effective trip current ≈ 14.5 × (1 − 0.216) = 11.4 A.
The motor current is 14.2 A, so the element is carrying 14.2/11.4 = 1.25 times its hot trip value. After the element's thermal memory is used up (about 15 min), it trips. The sealed IP54 box, the contactor coil dissipating heat directly below the relay, and the 44 °C shed air create a local ambient 14 K above the relay's calibration point. The supply at 396 V (−4.6 %) also raises current slightly for the same shaft load. The 40 MΩ winding and free belt rule out a genuine motor overload: this is thermal miscoordination, not an overloaded motor.
2. State the corrective actions you would take, and explain why simply increasing the dial setting is wrong.
Read the worked answer
Do not raise the dial to stop the nuisance trip. At 44 °C ambient the motor's own permissible continuous current is already reduced (roughly 0.95 × FLC), so raising the setting would leave the winding unprotected at about 16–17 A.
Actions, in order:
- Replace the relay with an ambient-compensated thermal relay or an electronic relay with compensation up to at least 60 °C; set it to nameplate FLC 14.5 A.
- Reduce panel internal temperature: filtered louvres or a thermostat-controlled fan, separate the relay from contactor heat, avoid bunched wiring, shade the enclosure. Target internal temperature ≤ 55 °C.
- Check contactor tips and terminal torque; light pitting is acceptable, but dress or replace tips if the pole-to-pole voltage drop at load exceeds about 0.5 V.
- Confirm the enclosure's design heat dissipation is not exceeded, and that any fan/filter does not destroy the IP rating.
- Confirm the motor is suitable for 44 °C ambient and 396 V supply; if the lathe is heavily loaded, verify shaft load rather than assuming the relay is wrong.
3. How would you prove the corrected panel is safe and functional before handover?
Read the worked answer
Prove the fix by measurement, not by 'it ran for an hour':
- Insulation resistance phase–earth and phase–phase at 500 V, plus earth continuity from the enclosure to the earth bar.
- Load log: record current in all three phases for a full production shift with a logger; expect balance within 5 % and current ≤ FLC.
- Temperature log: panel internal temperature must stay below the compensated relay's limit and below the enclosure design temperature.
- Trip test: primary injection at 1.5 × setting should not trip before 1 h from cold and should trip within 2 h; at 7.2 × setting a class 10 relay should trip in roughly 4–10 s. If injection is unavailable, use the manufacturer's published curves and record the basis.
- Functional checks: start/stop, no-volt release (contactor must drop out on supply loss), phase sequence, door interlock, lamp indication.
- Documents: as-built drawing, relay setting, test sheet and temperature log signed by the tester.