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I have a 1/2hp single phase motor that I wired to a switch. I reused an old wall light switch for testing. I believe my wiring is correct as I am able to turn it on. But when I turn it off, it trips my circuit breaker on my main control panel.

How do I go about troubleshooting this problem? I am rather a novice when it comes to electrical. I know how to wire electrical outlets and switches in rooms, but that's all I have experience with.

My motor is connect to this outlet outlet but it is the breaker in my main panel main panel that gets tripped.

My switch is temporary for testing the motor enter image description here

I am in the US.

Thanks

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  • Instead of using the switch, try yanking the plug out. Does it still trip? – Harper - Reinstate Monica Jun 13 at 23:41
  • Harper, I tried your suggestion, but when I plugged it into my extension cord, the switch must be at the on position and it burned my extension and its plug together. – Quoc Vu Jun 14 at 2:39
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The root of the problem is the inductive "kick" you get when you interrupt a large electric motor. Inductors are like anti-capacitors - a capacitor resists change in voltage by flowing high current, inductors resist change in current by flowing high voltage.

So boldly interrupting power to a motor results in a high-voltage backlash of as much as 1000 volts. That sad little light switch can't handle it, and and the voltage leaps across the switch. Once it gets a path of ionized air made, lower voltage can continue to leap through it. The current can't stop; anytime it gets near stopping this causes more inductive kick which re-ionizes the air.

So it would just be the motor refusing to turn off and arcing a lot because of the cheesy switch ... except THIS motor is also defective.

The motor has insulation problems, and that voltage spike is ALSO causing insulation breakdown between hot and neutral inside the motor, causing a dead short arc! We know it's hot-neutral because the GFCI did not trip. We know it's a dead short because the plain breaker did.

This dead short goes right through the switch due to the arcing. So it gets the attention of the breaker. Good thing it worked, or you'd have a much worse problem.

Tell me again why you're "testing" this motor???

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  • Thank you for the explanation. My neighbor gave it to me and i was planning to make a disc sander with it. It's been sitting in his scrap pile for a while and he isnt sure of its condition although he barely used it over the years. – Quoc Vu Jun 14 at 0:27
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I can think of four reasons, listed from most to least likely.

The breaker may be a GFCI breaker and your motor causes temporary imbalance on ground.

The breaker may be an arc fault interrupt breaker and switching the motor causes a temporary arc.

The motor may cause a temporary surge dumping its momentum and temporarily generating breaker overload.

And, the motor may have some winding/brushes that cause temporary shorts on shutdown.

It's most likely GFCI or AFCI, which is why people ask about whether the breaker has a test button. But note that running motors have momentum, and a motor is just a generator working in reverse, so when you turn off the power, the momentum needs to go somewhere, and if there's a party between one connected phase and ground (even with significant resistance) the motor may try to dump it's generates energy there.

For a bigger motor, like your 1/2 HP, the best option is to use a motor control relay or solid state motor controller, but that's not always practical or economical.

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  • Thnaks Jon. I didnt think 1/2 hp would be a big motor. I have a drill press with the same hp and it runs fine on the same outlet. – Quoc Vu Jun 14 at 0:30
  • the photo shows regular breakers, not fault detecting breakers, a short in the motor after the switch is turned off should be undetectable, when you turn an induction motor off it just keeps on spinning until friction steals the spin. motor momentum is capacitance-like not inductance like. – Jasen Jun 15 at 13:48
  • the photo wasn't there in the original version of the question. However, the question now contains a picture of a GFCI outlet. I'm surprised the breaker would trip, and not the outlet, though! – Jon Watte Jun 16 at 4:03
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Standard breakers are inverse time thermal magnetic breakers, overloads cause heating which trips, short circuits have high current that trips a magnetic element. The magnetic element is different from the additional function of arc-fault breakers.

I think the condition of the switch is inadequate to extinguish the arc in the switch, and the arcing is causing an irregular magnetic pattern to be felt in the breaker and reacts as a dead short. It could be the magnetic detection portion of the breaker could be failing and has become overly sensitive.

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