5 HP mill motor tripping after 20 minutes in summer
A 5 HP (3.7 kW), 415 V, 3-phase, 4-pole squirrel-cage motor drives a flour mill through a V-belt. Nameplate: 7.6 A, class F insulation, rated ambient 40 °C. It is started direct-on-line from a contactor with a thermal overload relay set at 8 A; there is no separate phase-failure relay. In July the motor trips after roughly 20 minutes of running and the frame is too hot to touch. A clamp meter shows R 9.2 A, Y 8.6 A, B 8.8 A. A voltmeter at the motor terminals shows 372 V between phases. The belt is tight and the mill bearing runs dry. The ventilation slots in the fan cowl are choked with flour dust. The miller wants the overload relay set higher so the motor stops tripping.
1. Using the readings, decide whether the motor is overloaded and whether the supply is contributing. Support your answer with calculations.
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Current: average = (9.2 + 8.6 + 8.8) / 3 = 8.87 A against 7.6 A nameplate, i.e. 117 % - about a 17 % overload.
Unbalance: worst deviation = 9.2 − 8.87 = 0.33 A, i.e. 3.7 %. That is inside the usual 5 % limit but not clean; it adds rotor heating and must be watched.
Voltage: 372 V against 415 V is a 10.4 % fall, well outside the ±6 % variation a motor is expected to tolerate. A motor at constant shaft load draws roughly 10–15 % more current at 10 % undervoltage because the torque per ampere falls with the square of voltage. So part of the 17 % is the supply.
Mechanical side: an over-tight belt and a dry bearing add shaft load, so the motor is genuinely doing more work.
Verdict: it is a combined overload, not a single fault. The thermal relay tripping after 20 minutes matches its inverse time-current curve and is doing its job. Class F insulation tolerates a 155 °C hot spot, but a frame too hot to touch (above about 70 °C surface) means the winding is ageing fast. Setting the relay higher would remove the only protection and lead to a burnt winding.
2. Give the diagnostic sequence you would follow, in order, at the mill.
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- Voltage first, at the terminals, no-load and on-load. If 372 V falls sharply when the mill is loaded, the fault is upstream: loose lugs, burnt contactor tips, undersized service cable, or a distribution transformer tap that needs correcting. Measure at the meter, at the DB and at the motor to find where the drop occurs.
- Check all three phases and the neutral at the contactor. Confirm no fuse is blown and no one pole is high-resistance - a poor pole produces the 3.7 % unbalance seen.
- Run the motor uncoupled. If unbalance persists with no load, suspect the winding or the supply; if it disappears, the load is at fault.
- Mechanical: slacken the belt to correct tension, grease or replace the mill bearing, check pulley alignment, clean the fan cowl and cooling fins with dry air.
- Electrical tests after the motor cools: winding resistance phase-to-phase balanced within about 2 %, insulation resistance to earth at least 0.5 MΩ minimum (practically above 5 MΩ for a healthy winding), and a check that the relay is calibrated and its settings sealed.
- Add a phase-failure/single-phasing relay - a single-phased motor will not start and will burn out on the remaining phases while the thermal relay is slow to act.
3. What should the overload relay be set to, and may the mill keep running while the faults are corrected? Explain your reasoning to the miller.
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Setting: the thermal relay is a motor-protection device and must be set at the nameplate full-load current, 7.6 A - in practice the nearest setting not exceeding about 1.05 × FLC, so 8 A is already at the top of the range and must not be raised. It protects the winding, not the production schedule.
May it run? For a short period and at reduced load only, with the fan cowl cleaned and the belt slackened, and with someone watching for smell or a hot frame. The real cure is correcting the voltage and the mechanical faults. Running flat out at 117 % current with 10 % undervoltage will destroy the winding within weeks - a rewind costs many times the lost milling hours.
How I explain it to the miller: the relay is the smoke alarm, not the fire. Turning up the setting is like removing the battery because the alarm keeps ringing. Options to present: ask the supply company to correct the tap or install a suitable voltage Corrector, replace an undersized service cable, fit a soft starter or star-delta starter to reduce starting stress, and fit a phase-failure relay. A star-delta starter will not cure an overload - it only lowers starting current.