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fburgos

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Reply with quote  #1 

Hi,

Im investigating a motor with sleve bearings that has high axial vibration during uncoupled test.

I did a waterfall measurement (without tach, Fmax=1000Hz, Lines 400 for it to update fast enogh)

Right after shuting down the machine amplitud decreased 50% and the next practically disapeared.

Im leaning towards electrical faul, but how do you difirentiante from a excentric estator, lose iron or a shorted lamination?

attached the waterfall plots during shutdown and vibration trend of band from 115 to 125Hz.


Attached Images
jpeg img00007.jpg (44.67 KB, 22 views)
jpeg img00008.jpg (116.38 KB, 22 views)
jpeg img00009.jpg (43.78 KB, 22 views)
jpeg img00010.jpg (106.46 KB, 20 views)

John from PA

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Reply with quote  #2 
Probably worth reading...https://www.maintenance.org/fileSendAction/fcType/0/fcOid/399590942964504823/filePointer/399590942964856169/fodoid/399590942964856167/Analytical_Approach_to_Solving_Motor_Vibration_Problems.pdf
electricpete

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Posts: 651
Reply with quote  #3 
I agree that the coastdown suggests it's electrical in nature, not mechanical.

Here's what I would check in order:
1 - Look for resonance of the flimlsy endshield above the bearing, similar to what I showed here   https://www.maintenance.org/topic/motor-endshield-resonance (attachment from that thread attached below).  Two things that will indicate this:

2 - Do a structural axial bump test if you see highly-directional axial 2*LF vibration. The best way to do that is during coastdown so you get the effects of the oil film. 

3 - Much less likely - check current balance. That's something I suggest for 2*LF in general although not necessarily axial. 

By the way, is this an increasing trend in 2*LF axial, or has it always been that way?  
Note there are some benign explanations for 2*LF vibration (it may just be a characteristic of the motor). 




 
Attached Files
pdf Presentation1small.pdf (1.01 MB, 11 views)

fburgos

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Reply with quote  #4 
Motor Is this, 1000 hp vibration appear after bearing repair, Right now just came back from hitting it with my regular hammer (sorry no force hammer for me) and found spectrum had resonance around 400hz and higher than the other motor.

Perhaps there's something

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jpeg IMG_20200311_113353~2.jpg (257.65 KB, 12 views)

electricpete

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Posts: 651
Reply with quote  #5 
I labeled points 1 2 3 in the photo below, but 2 is the most important.  I suggest the following:
1 - Check whether the vibration is higher at location 2 than on the bearing housing axial vibration at your normal monitoring point. 
2 - Check whether pushing at location 2 is causes the bearing axial vibration at normal monitoring point to go down.
If the above are true, then you may have a resonance of the flimsy semicircular endplate above the bearing housing which is causing your bearing housing to vibrate more.  That is what we found in the thread that I tried to link above. Actually my link didn't work, try the link here instead. 
snip2.jpg 


fburgos

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Posts: 670
Reply with quote  #6 
Motor still offline, Ill ask for a cleaning and tightening that plate and test again.
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