Table Saw Motor trips GFI

Any ideas what could be wrong with it? How to test it? Ect.

The switch is kinda interesting. The motor has a cord but no switch of it's own. The table is homemade and it has a heavy power cord coming from it with a light switche that controls a receptical that the motor plugs into.

Ok.
Table unplugged from GFI.
Saw plugged into table.
Switch on table off.
Plug table in and GFI trips.
Strange.

Ok.
Unplug saw from table.
Plug table into GFI.
Everything is fine. Switch on or off.

Continued by plugging saw into table with switch off.
GFI trips.

Confirmed Hot is the switched wire. Didn't think GFI would care about neurtal. Strange.

I moved the motor to a work bench for further testing. Different GFI receptical at that bench. No switches now involved just plugging it in or unplugging it. And well you know the story.

I will check continuity but as I was holding it on the bench barehanded to prevent startup torque from flipping it onto the floor I don't expect to find the case connected to either hot or neutral.

I disassembled it late last night. There is a LOT of dust and junk in it. I am going to blow it out but I'm skeptical if that will help. I'll post back after my examination today. Oh, I'd kinda think I'd smell something burnt if the field coils were fried. Hopefully I'll find an answer but I'll settle for it working again mysteriously.

Dusty

Capacitors store energy, not "electricity". A capacitor in the circuit will not cause a difference between the current going into a load and the current going out.

Kirchoff's Current Law states that the sum of the current going into a node must be zero. The motor is a node. If the current on the hot side is different than the current on the neutral, there must be another path for the remaining current. Which is pretty much the definition of a ground fault.

If there is any type of "growl" when the motor is running, it could be a bad bushing/bearing or a shorted coil; then it's cheaper to buy a new motor than repair it.

Once your confirm that you have leakage in the motor, start looking for the source. It might be something as simple as sawdust in the motor. Hopefully it's not the motor windings, as you would have to replace the motor.

Note that if the outlet is dedicated for the saw, code does not require a GFI.

Mark's first parragraph has given you the best way to check for nussience tripping. I like using a analog meter myself with a 20megohm scale or more.

With the meter set too 20megohm or higher, I ground one test lead to the motor case then hold the other test lead between my fingers while touching the motor case with the opposite hand. This should give me a meter reading of high resistance telling me I have a good enough ground for winding testing. No meter reading ?, then check the frame ground test lead and repeat.

I then test each motor lead, common, start, run. A slight reading on any motor lead says the winding insulation is starting to break down or contains alot of moisture.

In a open air cooled motor, I would then tend to dry out the winding before condeming the motor. Bake the motor in a oven at 300� for 24hrs as this will boil off any water content. Retest the motor leads again.

If this is indeed your problem, just remember that August is generaly the highest humidity month that we have.

T_Bone

Nowadays, there are other electronic AC leakage potential testers on the market.

An electric motor shop should be able to check it out for, the heck of it is, repairing the motor likely would cost more than replacing it.

That being said, high starting current motors can cause freaky things with GFCI's, but the way it sounds it DID work OK before you overheated it, and DOES trip another GFCI, so you have pretty much answered your own question... the motor is "toast".