Intermittent Vibration in motor connect to VFD

[CheEng123]

I have a VFD connected to a small motor (5 HP). The motor operates fine when its not connected to the VFD, but when it operates connected to the VFD (and with no load) it intermittently vibrates and it has a frame-to-ground voltage of about 50 Volts. What could be the problem, and how do I fix it?

 

[dpc]

Does this only occur at certain speeds? Is the motor properly grounded? How are you measuring this 50 V?



"An 'expert' is someone who has made every possible mistake in a very narrow field of study." -- Edward Teller

 

[CheEng123]

dpc, it occurs at all operating speeds from the minimal (500rpm) to the maximum (1700rpm). The motor is well grounded, and I'm measuring voltage from the frame-toground with a clapm meter.

 

[DickDV]

Now, something doesn't quite make sense here. If the motor frame is actually well-grounded, the voltage from the frame to ground has to be zero. Further, you wouldn't measure voltage with a clamp-on meter.

Please describe more fully what and how you are measuring.

 

[dpc]

Any chance of connecting this VFD to a different motor as a test?

As DickDV, I'm suspicious of your 50V reading if the motor is well grounded.



"An 'expert' is someone who has made every possible mistake in a very narrow field of study." -- Edward Teller

 

[CheEng123]

I'm measuring the voltage interrupting the ground wire that goes from the motor frame to the grounding circuit, and connecting the one probe of the ac voltmeter to the frame and the other to the grounding circuit.

I also tested other motors and I keep having the same issue.

 

[davidbeach]

You ungrounded the frame and expect it to be zero volts to ground? Why? Once ungrounded you could measure most anything. Reconnect the ground and measure the current, that might actually mean something.

 

[DickDV]

For testing purposes only, change the VFD carrier frequency to the highest value it is capable of and check for vibration again.

If it goes away, I would attribute it to some minor resonance in the motor and not worry about it any further. If it does not go away, I would consider it a mechanical issue. First thing would be to check the dynamic balance of the rotor.

50V on an ungrounded motor frame is no concern at all. I'm surprised it isn't more.

 

[ozmosis]

On motors unloaded, there is a phenomena called 'beat' that may be attributed to the vibration. Beat is a form of hunting at certain frequencies where there is, I think, a mismatch of frequencies. It is often more associated with lower cost motors too and if the mounting is going to resonate this further, you will see/feel it more.
I suspect the 50V measured could be a different issue to the vibration.