Motor Question

[otlski]

Megger your motor windings
If you don’t have a megger, do you have a known good Inverter duty motor you can temporarily swap in?

No megger
No other motor of known or unknown goodness!

 

[otlski]

I think the suggestions above are great.

I would think about using 240V

Personally, I would bolt it up and see where things are. Inverter driven motors have a tendency towards leakage current from the usual sources, plus of course the stray current induced in the rotor. I've seen larger motors with grounding brushes to keep the stray voltage out of the bearings.

I would be inclined though not to suspect the motor. Rather, I would suspect the VFD for not producing a clean (enough) sine wave. I think it is good practice to put input and output snubbers on VFDs as a general precaution, but especially if you are not using an inverter rated motor or 120V power. A couple of appropriately sized ferrite toroids and you are in business, or you can buy a reactor. I saw someone once used some black pipe elbows and unions to make a square that he wrapped his incoming power line around. He'd measured the inductance and figured out how many wraps he needed to get it to 60Hz resonance. Pretty slick, I thought. More generally you can buy EMI suppressors to put on the input side. E.g.
View attachment 3531260

https://www.amazon.com/Uxcell-CW4L2-20A-S-Noise-Suppressor-Filter/dp/B016EJ5DU2

If you can, I would try to use 240VAC as input so the VFD only has to create one phase. That is the nature of a VFD. Using two hots will cut down on the (harmonic) noise generated.

All the best,

Peter

Thanks for the detailed reply Peter.

No option to go to 240 VAC.
The motor is VFD rated.
I will move forward on some of your suggestions.

 

[DarkBlack]

I would think about using 240V

Rather, I would suspect the VFD for not producing a clean (enough) sine wave

If you can, I would try to use 240VAC as input so the VFD only has to create one phase. That is the nature of a VFD.

Sorry to say, but you don’t seem to understand how VFD’s work.
Not trying to be sarcastic or criticizing, but….

1) Using 240v as an input voltage won’t help motor frame voltage.
The first stage of a VFD converts AC, to a DC bus voltage.

2) VFD’s don’t, and shouldn’t produce a clean sine wave. A clean sine wave would overheat a motor at reduced speeds. A VFD is designed to produce a modified sine wave which greatly increases the area under the curve, at the initiation of each waveform, during non-stated rpm frequencies, to promote torque.

3) Again, from #1, the VFD has to synthesize 3 phases from a dc bus voltage. It has nothing to do with, if the front is fed L-N, or L-L.

 

[ponytug]

Sorry to say, but you don’t seem to understand how VFD’s work.
Not trying to be sarcastic or criticizing, but….

1) Using 240v as an input voltage won’t help motor frame voltage.
The first stage of a VFD converts AC, to a DC bus voltage.

2) VFD’s don’t, and shouldn’t produce a clean sine wave. A clean sine wave would overheat a motor at reduced speeds. A VFD is designed to produce a modified sine wave which greatly increases the area under the curve, at the initiation of each waveform, during non-stated rpm frequencies, to promote torque.

3) Again, from #1, the VFD has to synthesize 3 phases from a dc bus voltage. It has nothing to do with, if the front is fed L-N, or L-L.

I do know some things, but I wouldn't claim to be the expert.

The reason that I suggested considering 240 over 120 is not about the voltage, it is about improving the quality of the DC on the internal bus, which has been shown many times to affect the quality of the output power. I know of case where folks have added additional capacitance to the DC bus to help stabilize it for particular applications that were experiencing harmonic issues.

Having two phases instead of one can help reduce harmonics in the output. "Can help". I suggested it because the OP was having an issue. Some VFDs are better than others and some have better specifications, and some units better builds. Do I think 120 vs 240V is a panacea? No. Could it help a VFD that appears to be having issues? Maybe. Hence the suggestion.

Just trying to help.

All the best,

Peter

 

[otlski]

First, success. I tested by providing a direct ground to the motor frame and everything worked fine. The voltage on the motor frame went from 90 VAC ungrounded to 30 mVAC with a ground.

For posterity's sake...
To get there, I snuck up on it. I replaced the 3 ohm inline-ground resistor with a 1.5 ohm and the voltage across it was cut in half. So the current was consistent at 50 mA. I did the same thing with a 0.7 ohm resistor and the voltage drop halved again. At that point I went for the direct connection via clip leads which probably are not too much different than a very low ohm resistor anyway.

Along the way, I noticed that my original 90 VAC at 0 rpm (no ground), went down to 45 VAC when at 1725 rpm (no ground).

Per Cal's diagnostic step, I did have a jumper setting to double the output frequency, so I did. The voltage started out at 90 VAC at 0 rpm, went to 45 VAC at 1725 rpm, and dropped to 30(ish) VAC at 3400 rpm.

I put the frequency jumper back to X1 after the experiment.

I think I am pretty much set. Thanks for the help guys!