... I keep tripping the GFCI

[John Coloccia]

If the wiring is all correct the problem could be dirt. I had a motor that exhibited exactly the same behavior. On the advice of someone on one of these helpful forums I disassembled the motor and gave it a good cleaning with spray motor cleaner (available at any auto parts store, and make sure you use it outside or with excellent ventilation!). Once I got all of the packed sawdust and gunk out of the motor and reassembled it everything was fine. There was enough crud inside for a small amount of current to leak and trip the GFCI.

While you have the motor apart you can also get a good look at the windings and see if anything looks amiss - look for broken or missing insulation, any signs of something rubbing on something else, etc.

Bottom line it could be dirt and that's a cheap fix. But your GFCI sounds like it's doing it's job so it's critical that you investigate, find and fix the problem.

This is likely the problem. All you need is for it to leak a few mA to ground and you will trip the GFCI every time. The test you did disconnecting the ground from the chassis is a tell tale indication of this problem, and it's very common. It's a good idea to check for dead shorts between the windings and ground anywhere, but you'll probably find that your meter reads open, or that it may oscillate a bit if the meter is sensitive enough.

[Jason Roehl]

I have a gut feeling you have the switch wired incorrectly. Is there a model number on the switch or can you post a picture of it so we can get an idea of what it is? With a 120V system, you only need to switch the hot wire, but you could switch both the hot and neutral, that won't hurt anything. It just sounds to me like the switch might be doing something different than what you expect.

A quick test of that without a voltmeter would be to connect (just touch the wires together if you're comfortable with this sort of thing):

motor_black to cord_black
motor_red to cord_gold (possibly it's just an old, discolored white)

If that doesn't trip the GFCI, then you have the switch connected incorrectly. If it does trip the GFCI, then you have a short to ground somewhere.

[Mark Ottenheimer]

If my electric knowledge is correct I think I see your issue. You have the hot and neutral hooked up to the two pole switch for a 120v sinle phase circuit. When you switch the neutral on a circuit connected to a GFCI it will trip every time. Try this. Wire nut the neutrals together connect the ground and give it a try. Or plug it into a non GFCI outlet.
Mark