Motor Circuit Protection Tripping on Start
A 15 hp, 460 V three-phase conveyor motor trips its branch circuit protector about one start in five. The motor nameplate shows 21 A full load. The starter has a thermal overload relay set at 21 A and a magnetic contactor. The branch protection is a 30 A inverse-time breaker. The conveyor is heavily loaded at start because product accumulates on the belt overnight. The maintenance tech has already replaced the motor once and the breaker once, and the trip persists. A clamp meter shows starting current near 130 A for about four seconds and running current of 20 A.
1. Diagnose the likely cause of the trips and evaluate whether the 30 A breaker is correctly selected.
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The running current of 20 A is below the 21 A nameplate, so the motor is not overloaded in steady state and the overload relay is not the likely trip. The 130 A start for four seconds is roughly six times full load, which is normal for a direct-on-line start but long enough to trip a 30 A inverse-time breaker whose magnetic or thermal element responds to that duration. The breaker may be undersized for the starting duty. Motor branch short-circuit protection is permitted to be sized above the conductor ampacity to ride through starting, with the overload relay providing running protection. The tech should check the breaker's trip curve and consider a higher-rated inverse-time device or a motor-rated protector within the code allowance, while confirming the conductors and controller are protected.
2. The tech proposes raising the overload relay setting to stop the trips. Assess that proposal and list the non-electrical fixes worth pursuing.
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Raising the overload setting is the wrong fix. The overload relay protects the motor from sustained overcurrent, and running current is already at nameplate, so increasing the setting removes protection without addressing the start. The real issue is starting torque demand. Non-electrical fixes include clearing the accumulated product before start, reducing belt loading, improving belt tension and alignment, and checking for a mechanical bind or failed bearing. Electrically, a reduced-voltage or soft start, or a variable-frequency drive, would cut inrush. The tech should verify the overload is correctly classed for the motor and confirm the trip is the breaker, not the overload, by observing which device trips.