Motor Vibration

[TangoCleveland]

We have a horizontal 15,000hp motor driving compressors at each end of the shaft. We get concerned when vibrations approach 0.5 in/sec. We are getting high axial vibrations (up to 1.4 in/sec) at the brush end bearing pedestal at 2x running speed; no other harmonics, and no high readings at the other pedestal. We have checked compressor and coupling alignment and checked for soft foot; no problems there. Motor runs on magnetic center with adequate clearance at thrust bearings. Axial vibrations stop when power is removed and the motor coasts down. MCE test indicates no problems with air gaps or rotor bars. Can anyone suggest where else we can look?

Larry

 

[electricpete]

You say that “axial vibrations stop when power is removed”. If they disappeared abruptly/immediately this would normally indicate an electrical problem. Less likely might indicate coupling problem which disappears when torque is removed.

Most common electrical problems occur at 2*Line frequency. Is it possible that this frequency is 2*line frequency instead of 2x running speed?

Alignment was a logical item to check with 2x running speed and high axial vibration…. Might be worth double checking thermal growth compensation et cetera. Also a check of phase might give you some ideas.

 

[TangoCleveland]

Problem appears to be solved. We did a Dynamic Spectum Data Collection with a 21/20 CSI Spectrum Analyzer. We found what appeared to be a natural frequency of the vibrating bearing pedestal. We added weights to the pedestal and axial vibrations went from 0.9 in/sec to 0.3 in/sec. We'll try to fine tune the extra weight, and watch the motor. This is an old problem, and it appears to be fixed with a $450 field service call and some scrap blind flanges.....

The problem is always obvious once you see what it is.

Larry

 

[drmayer]

electricpete, when you say that most problems occur due to a 2x excitation frequency, what do you mean by that? I'm trying to figure it out and haven't gotten anywhere.

thanks.

 

[electricpete]

drmayer - I said most common electrical problems occur at 2*Line frequency (for example in USA 2*60hz = 120hz)

For instance an airgap eccentricity (static) will cause twice line frequency vibration.




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[eemotor]

Sometimes your power supply may also cause 2x line frequency problems. Whether it's harmonics or line unbalance as the cause, we've seen 120Hz (US) at high levels before. Often going on motor-generator set seems to reduce the 120Hz level.