Unusual 30 HZ vibration on motor with VFD

[CRS1]

I recently encountered a large 6-pole induction motor driving a vaneaxial fan in cleanroom service. The motor was tested at full speed/60 hz through the VFD. The highest spectral feature was a .112 ips peak exactly at 30 Hz. (I had sufficient resolution to determine that this was not 3 X M. When the motor was operated in bypass, without the VFD, the 30 Hz vibration dropped to 0.001 ips...Everything else stayed the same.

Adjustments to the carrier frequency from 2kHz to 6 Khz made very little difference. Tweaking the controls and tightening connections, etc. seem to have reduced the vibration somewhat, but the 30 Hz peak is still the highest peak in the spectrum.

Any ideas? Suggestions?

Thanks,
Tim

 

[electricpete]

It seems you have done your homework to gather enough data to prove the vibration is a result of the VFD.

I actually haven't worked with vfd's but maybe know enough to be dangerous?

I would say there is an unbalance on the ac supply to the vfd. First step in the vfd is to full-wave rectify the 3-phase ac into dc. If one phase is low it will show up as a dip in the dc voltage twice per cycle. Then the dc voltage is chopped and filtered to produce an ac by pwm... the dc input dips to pwm unit will show up as dips in the ac waveform twice per cycle.

Any way to access the dc voltage to check?

By the way, 6 pole motor with line frequency 60hz would have a syncronous speed of 20hz. 30hz would be near 1.5*mechanical speed, not 3*mechanical speed, correct?

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[electricpete]

Whoops. I must have been confused. Twice per cycle 60hz would be 120hz, not 30hz.

Never mind. (never mind the comment about the ac input problem causing twice per cycle fluctuations)

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[CRS1]

I am an idiot...I must have been totally brain dead when I wrote in! The vibration is at 60 Hz! That is half the frequency of the usual electrical frequency vibration I see, (120 Hz/ 2 X Line freq.) The VFD was operating at 60 Hz. Sorry for the initial misinformation.

Regards,
Tim

 

[GregLocock]

Can you rewrite your entire post, correcting the frequencies throughout? Thanks

Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[CRS1]

See new posting, with corrected info.

Thanks
Tim