Diagnosing an Intermittent Equipment Fault
A conveyor motor trips its overload breaker intermittently - roughly twice per shift, always 20–40 minutes after start-up, and more often on warm afternoons. The breaker resets fine and the motor runs normally afterward. Maintenance has already: replaced the breaker, tightened all terminal connections, and confirmed line voltage is within spec at the panel. The motor nameplate reads 15 kW, 3-phase. Ambient temperature near the motor reaches 41 °C on warm days; the motor's rated ambient is 40 °C. A clamp meter shows running current at 96% of nameplate full-load amps during steady operation.
1. Given what has already been ruled out, what is the most likely cause, and why does the timing pattern support it?
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The pattern - trips 20–40 min after start, worse on warm afternoons, resets and runs fine - points to a thermal cause, not an electrical fault. Breaker, terminals, and supply voltage are already eliminated.
Key evidence: ambient reaches 41 °C vs. a 40 °C rating, and running current is 96% of FLA (near full load). A motor near full load in above-rated ambient cannot dissipate heat fast enough; internal temperature climbs over the first half-hour until the overload trips. Warm afternoons reduce cooling further, matching the observed frequency. The delayed, repeatable, temperature-correlated timing is the signature of overheating rather than a loose connection (which would be random) or a bad breaker (already replaced).
2. Propose a corrective action and a verification step, without simply raising the breaker size.
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Do not upsize the breaker - that removes the protection that is correctly detecting an overheat and risks motor damage.
Corrective options, cheapest first:
- Improve cooling/ventilation at the motor (clear blocked airflow, add fan/ducting) to bring local ambient back under 40 °C.
- Reduce load slightly - 96% FLA leaves little margin; check for drag, misalignment, or belt tension raising current.
- If ambient can't be controlled, a higher-ambient-rated or larger motor is the proper fix.
Verification: after the fix, monitor running current and motor frame temperature across several warm-afternoon shifts; confirm no trips over a defined period (e.g., two weeks including hot days) and that current/ambient are within nameplate limits. Success = trips stop because heat dropped, not because protection was weakened.