3 phase motor testing

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minilathe22

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I have a 3HP 3 phase motor, currently configured for 380v voltage only, so only 3 wires. I can see inside there is a star point if I wanted to carefully unpick the wires and convert it to 240v operation.

My concern is that while cleaning out some dust, a fair bit of varnish had visibly flaked off. There is 90M ohms between all of the coils and ground, which I believe is good, but when I measure between each of the 3 wires the resistence is 8 ohms between two of them but one set is 13 ohms.

I am currently using a VFD which is too small, the motor seems to run fine without any load applied. With a load applied the VFD is too small to power it.

Is this difference in resistance between phases a problem? I want to consider whether I have a solid motor before upgrading to a larger VFD. Any other tests I can do with a multimeter to determine the health of the motor?
 
I'm assuming that this motor is a standard UK star-wound 3 phase motor?
Not a single phase motor configured for an inverter........ i.e. no capacitor?
Who made it?
How old is it?

If it really is 3 ph, depending on the age, many motors would have been nominally rated from 380 to 450 volts, intending to run on the previous delivery voltage in the UK of 415 v.
However, the EU voltages and frequencies were harmonised some years ago, so that the 3 ph supply voltage voltage nominally became 400v.
In reality, the tolerances in the new hamonised standards allowed the supplies to remain largely the same; any changes would have resulted in significant current increases throughout the grid, so the voltages remained much the same.
All this is a long way round of saying that your motor voltage is probably well within range, BUT there should be a name plate to be precise.

I don't like the idea of dust and varnish flaking off; that suggests some age to the motor, overheating at some time and a compromised insulation value. You'll have to make your own decision on that.
It may or may not be life-terminal for the motor.

3 ph star-wound motor on an inverter? Too small? Imbalanced load? Overheating that may have caused the varnish failure..... you need an expert to clap eyes and hands on that to be sure.

In theory, with a high-volt megger insulation test, I'd say that anything over 1 m Ohm is considered 'legal' for use, but we'd be looking at close to infinity in a healthy motor. 90 meg seems good enough, but...... small mutimeters will give you an idea, but they are not powerful enough to do a meaningful insulation test.

Finally, If a 3 ph motor is intended to start direct-on-line, and it's star-point is buried in the cluster of windings, it is never intended to be disturbed. In this case you should have exactly the same value between each pair of terminals. An imbalance here on a 3 ph would suggest to me a dud motor. (Expect the measured resistance to change with running temperature). The resistance patterns you've quoted, though, are what I'd expect to see on a single phase induction motor...... which makes the 380 v part seems strange....

Good luck
 
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To add to the advice above - it may be worth taking to a motor rewind shop for their assessment. They can also give this a proper IR test at a decent voltage which you can't so with a normal multimeter. The coil resistance readings are a little odd as they are normally open circuit (high resistance) or a shorted winding meaning the resistance would be less than a good winding.
 
It's a 3 phase motor. It's meant to be symmetrical. It should be balanced between all 3 phases - I would say within a few percent.
What you describe doesn't sound right and if you have been blowing dust / dirt whatever out of it, then it presumably isn't a fully sealed unit.
Small AC motors aren't that expensive, so for your safety and peace of mind just recycle it at the local dump.
You can't do a meaningful insulation resistance test with a normal multimeter.
 
Its a very old motor. Weighs about 50kg. Marked 400V (not 380V as I said earlier, misremembered). Dynamo and Crypto Lancashire.

It would be a shame to scrap it, I am in two minds whether to have it rewound and fit new bearings. Then I could request 400V/240V wiring.

As Sideways says,the easiest route is to get a new motor and then I can choose the spec to meet the VFD I have. I would also get a totally enclosed one to avoid dust- this one sucks air through the casing. It will be on a wood lathe so guranteed dusty environment.
 

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A venerable beast, indeed. Probably as old as I am.

To throw another spanner in the works.....
That dust looks lethal. What was it previously used for?

Basically, what I'm saying is that unless you know definitely, absolutely otherwise for sure....... I'd assume that it may contain asbestos, picked up in a long career - until you know that it doesn't, don't disturb it.
If you take that to a reputable repairer, that is what they should assume and will probably charge for.

Not worth any risk.

.
 
It was on a metal spinning lathe, see Hazdent Machine Tools - big wood lathe project! for the full story. I think its just general dust build up over a long time, rather than a specific material.

A newer (and smaller!) motor would also allow me to move it closer to the wall, at the moment the large motor and control box means it has to stick out alot. I think this is the trouble free solution.
 
To get the readings you state and assuming one winding only at fault. the legs would have to be 6.5, 6.5 and 1.5 ohms which is a severe imbalance and surprised the VFD did not throw that out as an error.
Unless you can get mates rates, a rewind will be loads more expensive than a replacement motor. So unless there are mechanical or restoration authenticity reasons to re-wind then I'd go down the new motor route sticking with 3 phase if variable speed or braking is a requirement, otherwise a single phase one.
 
sounds like its toasted then! It was in a metalworking factory so would have been worked hard. Perhaps I can turn the casing into a table base or a planter.
 
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