... I keep tripping the GFCI

[Ben Davis]

I've finished rebuilding a JET JBS-14 circa 1981-2, but I am having trouble wiring the switch. It's a 110V single phase motor. There are no markings. From the literature published around that time, I believe it to be a 5A 1 HP motor. The switch is the normal four terminal power tool switch with nothing fancy that I can see. This is so easy, that I just don't see how it can be giving me so much trouble!

From the motor I have three leads: black, red, green.
From the (replaced by someone other than me ) power cord, I have three leads: black, gold, green.

Opening up the plug on the power cord, I can see the black and gold leads are connected to the hot/neutral prongs, while the green lead is connected to the bottom, cylindrical, ground prong.

I make the following connections:
cord_black to top left terminal
cord_gold to bottom left terminal
motor_black to top right terminal
motor_red to bottom left terminal

cord_green and motor_green are connected a bolt just under the switch on the band saw stand.

When connected this way, the GFCI trips immediately. When cord_green is loose, the motor does not drip the GFCI.

What's the deal?!? I've clearly screwed this up!

Thanks so much for all the help!

[Dan Hintz]

I'm not a motor guy, but black and red always lead me to believe it's a 240V, single-phase item, not 120V...

Obviously, 240V is not necessary for such a small motor, I'm just sayin'...

[Ben Davis]

Unfortunately, there are now stamps, markings, or ID plates anywhere on the motor!

[Charles Wiggins]

I am not an electrical guy either, but I know from experience that GFCIs are a bit more prone to wearing out than a breaker. Have you tried other tools on that circuit to be sure that the problem in not in the GFCI?

[Ben Davis]

I have a water softener that is usually connected to the same outlet. It runs without the faintest hint of a problem. I did disconnect it prior to plugging in the band saw.

Again, when the ground wire from the plug is not attached, the saw runs like a top!

[Rod Sheridan]

I have a water softener that is usually connected to the same outlet. It runs without the faintest hint of a problem. I did disconnect it prior to plugging in the band saw.

Again, when the ground wire from the plug is not attached, the saw runs like a top!

That indicates that the machine has a ground fault


Time to inspect the wiring and have the motor insulation resistance measured...............Regards, Rod.

[Ben Davis]

That indicates that the machine has a ground fault


Time to inspect the wiring and have the motor insulation resistance measured...............Regards, Rod.

Rod,
Okay... can you point me in the right direction on how I go about doing this? Any books or guides that you know of?

R/
Ben

[Rod Sheridan]

Hi Ben, you can inspect the wiring visually for any cuts/abrasions on the wiring that might touch the cabinet.

Measuring the insulation resistance of the motor will require a Megohmeter, which is an ohmmeter that uses 550 volts to measure the resistance. Not a home shop device.

You can use a regular ohmeter to measure from each wire to the machine frame (Machine unplugged of course).

Any indication that isn't in the tens of megohms is a problem.

Does it trip the GFCI when not running?

Regards, Rod.

[Ben Davis]

Rod,

The GFCI does not trip when the band saw is plugged in, but not running.

To be clear, the motor is grounded to the frame. The GFCI trips only when the ground lead from the plug is grounded to the frame.

There are no insulation nicks or problems that I could see when I took the motor apart.

R/
Ben

[Ken Fitzgerald]

Ben,

A GFCI monitors the current going out to the motor and the current returning from the motor. If there is an imbalance....ie less current returning from the motor than going to it, there is obviously a path (or short) to ground and the GFCI trips.

That is why if you disconnect the green ground wire the GFCI doesn't trip. You have disconnected the path to ground for the current to flow.

What Rod is saying is to use a ohm meter to check to see if the windings of the motor are shorted to the case which would be grounded when you have the green wire connected. Also check the wires between the motor and the switch to see if one of them is grounded or has a low resistance to ground.

Something has presented a path to ground and that is why the GFCI is tripping.

[Thomas Hotchkin]

Ben I see three wires on your left switch terminals and only one wire connected to right side is this correct. Tom

I make the following connections:
cord_black to top left terminal
cord_gold to bottom left terminal
motor_black to top right terminal
motor_red to bottom left terminal

[Dan Duperron]

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.

[Peter Luch]

Not sure if this helps but............

I got a brand new jet 1640 110 and plugged it into a GFCI and it popped it every time I turned it on. Plugged it into a normal plug for our whole house vac system and it's fine.
This is a brand new machine not a used new to me machine.

Well that's what I know......

Aloha, Pete

[Ben Davis]

Ben I see three wires on your left switch terminals and only one wire connected to right side is this correct. Tom

I make the following connections:
cord_black to top left terminal
cord_gold to bottom left terminal
motor_black to top right terminal
motor_red to bottom left terminal

Thomas,
Good catch! It should read:
motor_red to bottom right terminal

Thanks for the suggestions so far everyone!

[Mark Major]

Ben; I didn't see in the your posts whether you tried it on a non GFCI circuit with the ground connected. If it works fine in a non GFCI receptacle I think what might be happening is, as Ken pointed out, that there is a very brief inbalance in the circuit (but long enough for the GFCI to pick it up) possibly due to the mechanics of the internal contacts of the switch not being quite up to snuff. I would try disconnecting the red_motor and white/gold_cord wires from the switch and wire nut the two together to bypass the switch for the neutral. As Dan pointed out, red and black in the motor usually means 220v, but in this case the red must be the neutral since the motor does in fact run wired as it is . If it doesn't work plugged into a standard recepacle with the ground connected...then forget everything I just said .