Question about motor vibrations

[yurilevi]

Hello,

There is interesting vibration phenomena during operation of the elctric motor of one of ours machines.
During the first ~4 hours of operation the radial vibrations of the motor are increasing. At some point the vibrations are getting too high and we are stop the machine for couple of minutes. After this the vibrations of the motor are stable at low level.
Did somebody meet this phenomena?

 

[electricpete]

I assume this is DOL start squirrel cage induction motor.

Would you mind sharing the horsepower, speed, orientation (H/V), beairng type (anti-friction or sleeve).

For the general symptom of vib increasing over time after start on large motor, I would suggest thermally-sensitive rotor (possibly due to rotor bar /end ring deterioration, but also possibly due to mechanical factors such as bars moving axially). We have had 2 sister motors that both experienced that symptom.... we thoroughly investigated and ended up changing out the rotors with brand new one from the OEM, which solved the problem. We still have the 2nd one in our warehouse waiting to do some analysis on it. But we did not jump to that conclusion until we had one a tone of data gathering and analysis. Roughly in order of relevance:

* - Do current signature analysis (FFT of current, looking for pole pass sidebands around line frequency).
* -What does the vib look like? Predominantly 1x or 1x with harmonics? Measured on housing or with prox probes?
* - Does high resolution spectrum reveal pole-pass sidebands?
* - Is there any pulsing characeristic to the noise/vibration at pole pass frequency
* - Uncoupled run.
* - If possible, examine variations with load and ambient temperature.
* - Power down test... does vib go away immediately
* In our case, with large horizontal sleeve bearing motor, it was easy to pull end-covers off and check bearing condition, bearing/housing clearances, airgap. These are not likely cause of symptom you described, but imo doesn't hurt to check all easy things before you reach your conclusion.

We did not have the symptom that the vibration went away when shut down and restarted. We have seen on turbine driven pump where the machine vib stayed lower after shutdown and restart... in that case the cause was lockup and unlocking of gear coupling... that machine had thrust beairng in each machine which proabably may not be representative of your motor.

Also thermal growth leading to change in misalignment is another possibility that can cause vibration change over time. This would be primarily if the machine is driving something that changes temperature dramatically while running. Also may be ruled out if vib remains uncoupled (although other causes also may remain uncoupled).

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(2B)+(2B)' ?

 

[electricpete]

Also you may want to check TIR at accesible locations of shaft looking for bow (which can change thermally). In the repair shop you can have more troubleshooting options such as checking TIR hot and cold.

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(2B)+(2B)' ?

 

[electricpete]

Also, what does the motor drive and through what type of coupling? Is the load constant? Is the voltage constant? What is the environment (temperature)... outdoors or indoors.

I'm not saying all these facts are relevant, but it helps paint a complete picture. Sometimes a seemingly small detail can become important.

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(2B)+(2B)' ?