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Growly, slow three phase motor, but resistance of windings is consistent


Hot Rolled
Sep 30, 2010
Guinea-Bissau, West Africa
I purchased a wood jointer that has an integral 3ph motor. I would rather have had a pulley system, so that I could just swap the (possibly) bad motor out easily, but this was what was in my price range.

The guy I bought it from had a 220v RPC at his place, but he had bought the jointer from someone else and never hooked it up. It was wired 220v, but it looked like a single phase plug. However, opening up the manual contactor, it looks like someone tried to run it from 220v single phase. The plug was a single phase plug, and inside the contactor L2 and T2 were tied together. I split them back out the way they should be and changed the connector to a proper 3 phase one. However, turning it on the motor growled and spun up slowly, like a motor sounds when a phase drops out. I didn't run it for more than a few seconds, fearing the worst.

I brought it back home and tested the windings expecting to find a shorted one, however, the resistance between the three legs was consistent, I can't remember whether 3.5 or 6 ohms, but it was the same from L1-L2, L2-3, L3-1. I then rewired the motor for 460, which is how I need it once I get it to Africa and the voltage I have available to test it here. When I did that, the resistances stayed consistent still, 3.5 or 6 ohms, whichever the low voltage wasn't.

So, since getting it home, after it didn't run right on the 220v RPC, I haven't plugged it back in. Is there anything else I can check before plugging it in? If it's not smoked yet, I don't want to ruin it. I assumed it would be bad, if someone tried on single phase, but maybe they were smart enough to unplug it immediately.

What else can make a 3ph motor spin up slowly and growl as if a phase has dropped out or is weak, even when resistances are equal? Could it have smoked the contactor? I have continuity between the L1-T1, L2-T2, L3-T3 and no continuity where it shouldn't be when I press the button with power removed. I might suspect the RPC, but he was running the rest of his shop from it, apparently. The motor does spin freely by hand.

What else should I test before plugging it in for a test on 460 now that I've rewired it?

I'm in Michigan right now, not Africa.

Thanks in advance, I appreciate the breadth of expertise on PM! Sorry for a woodworking question, but I know you guys have lots of knowledge on three phase motors!
Possible someone has the coils mixed up.....new machine to me where someone had mis labeled the wires out of the motor.
ran on 208 with very unbalanced amp draw across the legs and would not spin up on 480.
Took the time to "find" each coil and two wires were labeled wrong, corrected and motor ran just fine.

If you have 480 you have 208 also.......I would use 208 to test with as 480 is a lot less forgiving
You could have continuity but lousy contacts that cant hold amperage.....contacts burnt?
Bad wiring, loose connections.
Before I would possibly smoke something it would be worth the time to take apart and inspect everything........
Check each phase to ground/motor case also. A path there can do exactly what you describe. A regular VOM might not be up to the task if it can't measure high enough resistance.
Thanks. L1/2/3 to chassis reads open with my DMM. Contactor is making the correct connections from L1 to T1 etc.

I have 9.8 ohms L1 to 2 to 3.

T1 to T4/7 is 3 ohms, T1 to T5/8 or 6/9 is 7 ohms

T2 to T5/8 is 3, to T4/7 and T6/9 is 7

T3 to T6/9 is 3, to the other pairs 7.

The contacts all look ok, but of course looks can be deceiving!
Have yopu run a high voltage insulation and continuity test with what we call a megger ? (like this https://www.screwfix.com/p/megger-dc-insulation-continuity-tester-500v/2159p)
When you do go every which way - winding to winding, winding to earth etc etc, ........IM(some say outadated) E they will pick up faults multimeters won't.

Yep, that's what I was referring to also when I mentioned the VOM might not be up to the task. Some can measure into the higher megohm range, some can't. My Fluke can measure to 50 MΩ for instance, which is probably considered midrange. There is also a tool called a "growler" that can test for shorts between turns but I think those are more often used on higher amperage motors.
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Take the cover off the RPC and get to know what is in there. Take a picture.
If you prefer a bunch of questions and possible answers then keep the cover in place.
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L2 and T2 were jumpered together?

Put a clamp meter on each leg and verify currents. That will tell you instantly if you've got an open circuit somewhere causing the motor to single-phase. You can get a nice Klein at Home Depot for $70.

Test it on commercial three phase power if possible. RPCs are notorious for causing weird starting.

Check wiring diagram against the actual motor continuity. If an import machine, possible they slapped a wye diagram on a delta motor or something equally stupid like that.
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Are you certain that the power is getting from the source to all the windings?

If it was wired up single phase, it may have other odd wiring as well. Check it from power cord to motor terminals.

If someone messed up the connections to the various coils, it might measure perfect, because the coils are similar enough. But it would not work right.

I'd make sure the coils are actually what you think they are.
I thought of that possible capacitor also. It would be in series, though, and he's getting good resistance measurements.

If he checks all the wiring, he'll find the capacitor if it is in there.
It's a 60/70's era Boyce Crane American-made jointer. It's definitely a three phase motor. I'm away from the seller's RPC now and my next test will be on commercial three phase, I just wanted to know what else I could test before trying it.

The bearings feel fine, as I said, it spins freely.

I have good continuity between the inbound line and T1/2/3 through the contractor into the motor.

I appreciate all of the help. I don't have a meggar, though they look interesting. If the motor is burnt up, I'll cut it up to turn it into a pulley system, so it's not the end of the world, I just wanted to test all I reasonably could before plugging it in recklessly.

I'm starting to suspect the seller's RPC. Maybe all of his tools start up slowly because the RPC has a weak generated leg or something, and not being used to three phase, he didn't know a 3ph motor should snap to speed right away.

If there isn't any more I can do with a DMM, I'll take my clamp ammeter with me and plug it in at commercial 3ph to see what happens.

A cheap megger is a very worthwhile tool. It will remove all uncertainty if the insulation is fucked or not. Very simple to use and no substitute.

VOM does not do the same thing at all.
A jointer is a reasonably high inertia load. It won't "snap to full speed".

I suppose that if the seller has a weak RPC, the torque may not be enough to accelerate it reasonably. It may either be very slow to accelerate, or simply fail to do so.

During the "non-acceleration", it might well be growly and sound like it is laboring, because it actually is.
A small transformer can be used as a cheap megger. Pull 440 off the hi side and test just as a real megger. If the winding is shorted to ground, current will be limited by the transformer.

It won't give you any readings, but the degree of snap and transformer hum will identify any shorted coils.

Are there any numbers on the motor leads?
Start cap on a 3 phase motor eh?
its on a rotary phase converter, so yes.... one of the circuits either is missing or has a dead capacitor on one of the legs to run and generate power on the RPC. Could be a run cap also on the rpc that is dead, I havent tested the voltages incoming, or out of the rpc, I havent tested the caps in it, nor am i there to see if its even wired up correctly and has continuity through the contactors in the RPC or the wood thing.
The run caps for the RPC are not essential, it works without them.

However, one possible issue (you may not care, since it is not your RPC) is a bad contact combined with a GOOD capacitor. Then the capacitor is all that supplies current to that leg.

That will supply lowish current, at an odd phase shift, and could easily cause a slow and growly motor.

You can do a quick test....if access is available. Remove the drive belt to the cutterhead, rope start the motor, and turn it on with single phase. If it runs on that, and also runs if you change connections to use the one you left out the first time, the motor is very likely good.

People do, as you probably know, rope start motors used as idlers in crude RPCs, so it works OK, and rope starting would start the jointer motor also.

Then you can check the cutterhead for any issues. You already know how to do that.