Problem with 3-ph motor & VFD on my mill

I was getting a electrical tingle from my mill, so I took the motor off to check it. It had a "leak" of very high resistance so grounding it would stop the tingle. I put it back on and rewired with a ground. When I tried it, the motor ran strangely - what I would call a pulsing rotation.

My lathe also has a 3-ph motor and VFD so I tried using them to see if the mill's motor or VFD was causing the problem. I used the mill's VFD to drive the lathe motor and the lathe's VFD to drive the mill motor.

With the lathe VFD driving the mill motor, the motor had the same pulsing rotation. Ah-ha - the motor is the problem. But wait, when I drove the lathe motor with the mill VFD, it ran very slowly - maybe 100 rpm instead of 2000. Very confusing.

I checked the motor's windings for continuity & there were none open (it's a 2 voltage motor & has 6 windings).

Any idea of what might be wrong with the motor? (Never mind the strange slow-running with the other combination.)

Bob

Reply to
Bob Engelhardt
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Does the VFD drive like before if not - maybe you blew a phase with a bad Mill motor and then it the VFD can't drive like before.

I would make sure the lathe and mill are not connected. Is the mill motor messing with the lathe as it is checked ? Maybe disconnect the mill and try the lathe with the VFD and see it it is fast. If not, maybe you blew a phase and need to detect waveforms / voltages on each phase of the output of the VFD.

The mill motor likely has a ground now somewhere in the winding and causing this issue. Take off the ground and see if the motor runs ok. The winding is causing odd voltages to appear on the 3 phase.

The motor likely needs re-winding or replacing.

Mart> I was getting a electrical tingle from my mill, so I took the motor off

Reply to
Martin Eastburn

Bob, I am not really very conversant on this, but as I understand, VFDs produce "chopped" sine waves, and due to inductance and capacitance of wires the VFD produces barely noticeable (for the motor's mechanical behavior) voltage spikes due to fast changes of square wave voltage dv/dt, that break through the insulation, what is what you feel. That is damaging to the motor insulation as it degrades it, and is unsafe in many ways.

You can read this to begin with.

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I was getting a electrical tingle from my mill, so I took the motor off

Reply to
Ignoramus14838

Thanks, Martin & Iggy.

The lathe VFD runs the lathe motor and a spare 3-ph motor just fine, so it isn't the problem. The mill is floating and has no connection to anything but the VFD.

The problem-motor winding resistances are all good. Their resistances to the case are all in excess of the DMM's range (megaohms). As are the winding-winding resistances.

As Iggy pointed out, the VFD output is PWM to simulate a sine wave. The base voltage is 300vdc+- (240vac peak). I agree that the leak is probably a matter of this 300v breaching the winding insulation, to the case.

I suspect that the erratic rotation is a matter of the breaching being a winding-winding short, because the motor was not grounded in the alternate-VFD test.

I'm going to _carefully_ try putting 300v between windings & see if I get any shorting. Stay tuned.

Bob

(BTW - I'm leaving the funny behavior of the mill VFD with a good motor for later - it's a separate issue.)

Reply to
Bob Engelhardt

This megger applies up to 1000VDC to detect high voltage breakdown.

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The meter movement has opposing voltage and current windings which make the resistance reading (V/I ratio) valid at less than full centrifugal-clutch-limited cranking speed, and voltage.

You can roughly measure breakdown voltage with it by connecting a DVM in parallel and watching for the megohm reading to dip below 10 as you increase the cranking speed. How rough the voltage reading is depends on how constant you can hold the cranking speed. The reading is fairly stable when measuring the breakdown voltage of a Zener diode or a rectifier.

Don't use a cheap HF meter to read voltage, their input is only 1 megohm. They are OK to monitor the current, which is tens to hundreds of microamps, up to 1 mA into a short. A 90 megohm divide-by-10 resistor in series with the voltmeter will let the generated voltage go higher.

The crank pivot can benefit from a drop of light oil, which also lubes the concentric speed-up gear shaft.

It's hand-powered so you will stop if you get shocked.

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-jsw

Reply to
Jim Wilkins

Greetings Bob, With a VFD you can get much higher voltage spikes than the base voltage. The insulation in your motor should be good for at least 600 volts. So if there is any shorting at 300 volts you know the motor is hosed but if there isn't that still doesn't mean the insulation is good. Eric

Reply to
etpm

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