Unusual 30 HZ vibration on motor with VFD
Unusual 30 HZ vibration on motor with VFD
(OP)
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
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





RE: Unusual 30 HZ vibration on motor with 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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RE: Unusual 30 HZ vibration on motor with VFD
Never mind. (never mind the comment about the ac input problem causing twice per cycle fluctuations)
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RE: Unusual 30 HZ vibration on motor with VFD
Regards,
Tim
RE: Unusual 30 HZ vibration on motor with VFD
Cheers
Greg Locock
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RE: Unusual 30 HZ vibration on motor with VFD
Thanks
Tim