I bought a tool grinder with a 1/4-HP GE motor which is probably 50 years old. The motor runs on 115 volts at 1725 RPM. A thin belt connects it to a pulley on the shaft that holds the grinder wheel.

The grinder's bearings are bad. They provide a lot of resistance to rotation. I am in the process of replacing them. The resistance makes it harder for the motor to spin up.

When I turn the grinder on, it trips the GFCI in my shop. The GFCI is in a wall socket, not a breaker. If I remove the belt so there is no load on the motor when I turn it on, the GFCI does not trip and the motor runs.

What's the best approach to fixing this thing?

I have read that I can't test the windings properly without a megohmmeter. Is that true? I don't have one.

I was thinking a capacitor might be leaking current, but I haven't been able to find any capacitors. There is no capacitor on the outside of the casing. The motor has special rubber mounts that will have to be removed before I can disassemble it completely, and I am reluctant to do it unless I have to, because there is zero possibility that I can get replacements if I tear the mounts.

Thanks for any help you can provide.

 

I’d wait until after you replace the bearings. The bad bearings may cause an increased current draw which is tripping the GFCI. You may be fixing a problem that doesn’t exist.

 

Thanks. I hope you're right. Not that these particular bearings are fun to replace.

 

The GFI is most likely tripping due to the high load, you could temporarily remove the earth ground to the motor and carefully test, but I doubt it is a ground problem.
Obviously your first move would be to replace the bearings!
I see an oil cup above the bearings, are they sleeve cintered bronze?
Max.

 

I agree that it's obvious the bearings have to be fixed first. I needed a gear puller and some other tools in order to do it. The puller will be here today, and then I have to machine a one-off wrench for one of the nuts. I also have to make two thin nuts to immobilize the shaft. I'm hoping I can get the removal done this weekend. Then I have to wait for the new bearings to get here.

I have people telling me the excessive load can't make the GFCI trip. I figured I would get more authoritative information here. I don't know much about GFCI's. My understanding is that the sense current imbalances, so I was thinking along the same lines as djsfantasi. More current, more imbalance. It sounds like maybe I was right.

 

The "GFC" in the GFCI stand for ground fault current.

Meaning that a large enough current is leaking from the motor's windings into its frame to exceed the ground wire's maximum current threshold.

Your motor is quite old. There might be some insulation somewhere deep in the motor's bowels, which is failing.

EDIT; since the motor does not have starting capacitors, it may be resistance starting type. In this type of motors, the auxiliary start winding is made of very thin wire, to purposedly have high resistance. This L/R constant causes the necessary phase shift for the start winding to have an angled field which will produce the startup torque.

Needless to say, this type of winding heats very much, although it runs intermittently. It might be that, after all of this years of service, the insulation has finally degraded.
Since the start winding is cut out by the centrifugal switch, that would explain the behavior at startup.

 

I have people telling me the excessive load can't make the GFCI trip. I figured I would get more authoritative information here. I don't know much about GFCI's. My understanding is that the sense current imbalances, so I was thinking along the same lines as djsfantasi. More current, more imbalance. It sounds like maybe I was right.

Typically a GFCI senses a imbalance between L and N such as when ground leakage occurs, this would not normally occur with just O/L, but it is possible that the O/L feature is also built in to the GFCI you have.
A GFCI It does not measure ground leakage, but imbalance of the L & N.
Max.

 

Good chance you’ll have to go away from a gfci. I’ve run into this a couple of times where motor circuit noise creates leakage beyond the gfi trip point.

 

The problem is that the whole shop is on GFCI outlets.

 

it is possible that the O/L feature is also built in to the GFCI you have

That was my thought too.

A GFCI It does not measure ground leakage, but imbalance of the L & N

There are some GFCIs that also look at ground leakage.

 

There are some GFCIs that also look at ground leakage.

Really? Do they do this in addition to the normal method of comparing line vs. neutral current, or instead of that method? Can you give any examples of products that do this, or documentation on how they do it? I'm intrigued.

 

I was sure I had seen internal schematics of GFCIs that had an extra core on the safety ground to detect currents on that line. But l'll be darned if I can find any now. I may have been mistaken.

 

I was sure I had seen internal schematics of GFCIs that had an extra core on the safety ground to detect currents on that line. But l'll be darned if I can find any now. I may have been mistaken.

Thanks for looking. I'm no expert and I'm certainly not claiming they don't exist - this is just the first time I've heard of them.

 

Based on how a GFCI Breaker works if the motor trips it on start my best guess is when starting under load, for reasons I can't explain, there is leakage current. There can be inductive or capacitive leakage current and it only need exceed 6 mA which considering the full load current on a motor you describe is about 5 ~ 6 amps the 6 mA is nothing but enough to cause a nuisance trip. Since the fault likely rest with the motor and not the GFCI there really isn't much you can do about it. Hopefully with a reduced start load (new bearings) things will return to normal.

Ron

 

Use an ohmmeter to check the resistance between the plug hot or neutral terminal to the motor chassis or safety ground.

 

It may possibly be that with the high loading that the belt is slipping just a bit and reating enough of a static charge to trip the GFCI. Some of them are very sensitive. You said that it did not trip with the belt removed, and so that may be part of the cause. Probably the grinder bearings are sleeve bearings and so a very temporary fix could be giving them a shot of 90 weight oil. And possibly grounding the grinder fram might help. And if you are able to blow the dirt and dust out of the moroe housing, especially the area around the starter switch, that also may help.
And really, if the power switch is not attached to the motor then you do not have a serious shock hazard from disconnecting the green wire ground from the motor. Just always unplug it before working on it.

 

Based on how a GFCI Breaker works if the motor trips it on start my best guess is when starting under load, for reasons I can't explain, there is leakage current. There can be inductive or capacitive leakage current and it only need exceed 6 mA which considering the full load current on a motor you describe is about 5 ~ 6 amps the 6 mA is nothing but enough to cause a nuisance trip. Since the fault likely rest with the motor and not the GFCI there really isn't much you can do about it. Hopefully with a reduced start load (new bearings) things will return to normal.

Ron

The normal function is to detect imbalance of current between N & L conductor, but I wouldn't be surprised if they also have the feature of tripping when over the rated current, even when both conductor currents are equal.
Max.

 

The normal function is to detect imbalance of current between N & L conductor, but I wouldn't be surprised if they also have the feature of tripping when over the rated current, even when both conductor currents are equal.
Max.

I actually looked into that and could not find one which would act as an over current breaker. . Maybe some which can do both functions exist but I couldn't find one. I did find warnings telling me a GFCI is not a circuit breaker though.

Ron

 

Thanks for all the help. I am working on the bearings, and I will put all this advice to use.

 

I hate to make things more confusing, but if this grinder's problems were simple, I wouldn't be here, so here I go.

The motor is working now, even under a load. I did a couple of minor things to the grinder, but basically, there is no explanation. It works with the belt on, and I even turned it on while applying pressure to the pulley to simulate bad bearings, and the motor still started.

I have no idea at all what's going on.

The bearing on one end got hot, so I know it has to be replaced, but the grinder is usable at the moment. Still working on getting the spindle open. I have not checked the motor's speed yet to see if the bearing is slowing it down.

On a positive note, I learned that this motor is not rare at all. It turns out 1/4-HP 1725-RPM motors with rubber mounts are common. Unfortunately, a TEFC version costs over twice what an open-frame job does. It it continues running, I'll just use it until it acts up again.