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Forward works fine, reverse trips breaker in main breaker box

Bailey4315

Plastic
Joined
May 8, 2024
Location
Alabama
Leblond Regal Lathe, 10 HP motor 480 3 phase @ 19 Amps.

Forward works fine. Reverse trips the main circuit breaker, cuts all power to Lathe. Any ideas? Checked amperage, it’s fine. Overload and non reversing contacts are good. Voltage is good.

The motor is fused @ 30 Amps. Circuit breaker is also 30 Amps.

I’ve been leaning towards it being the reverse contact. There’s no excessive arcing or anything. Bad coil?

Let me know your thoughts.
 
Definitely sounds you have a short when the reverse contactor is activated. However, the problem is not necessarily in the reverse contactor itself. If the forward contactor has a stuck contact, this could happen when the reverse contactor is activated.
I'd recommend removing the power and using a ohmmeter to check the NO and NC contacts on both forward and reverse contactors. If I didn't find stuck contacts, I would then disconnect the supply from the main contacts but hook up the control/coil power and test switching into forward/reverse to see if both contactors will activate and release properly. This has really good odds of finding a short.
 
Does it trip instantly on selecting reverse, or after a few seconds/minutes?

Instantly implies a wiring or contact issue causing a dead short.

If it gets up to speed but trips some time after, it could be dragging brakes or some other overload issue.
 
Definitely sounds you have a short when the reverse contactor is activated. However, the problem is not necessarily in the reverse contactor itself. If the forward contactor has a stuck contact, this could happen when the reverse contactor is activated.
I'd recommend removing the power and using a ohmmeter to check the NO and NC contacts on both forward and reverse contactors. If I didn't find stuck contacts, I would then disconnect the supply from the main contacts but hook up the control/coil power and test switching into forward/reverse to see if both contactors will activate and release properly. This has really good odds of finding a short.
Totally agree. But need to check the main contacts first in the forward contactor.
 








 
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