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RustyCas

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Need some ideas.  Direct-coupled, overhung fan with a 250 hp 6-pole induction motor (across the line start).  High vibration on the motor and it feels really rough.  Drive end fan bearing is an SKF double row roller bearing, and the wheel-end bearing is a Revolvo split race roller bearing. Obviously the split bearing is noisy.  But fan bearing vibration is actually good for this fan.  The motor, no.  Below is the spectrum from the inboard end of the motor.  The outboard spectrum is similar, but lower amplitudes.  Verticals actually look quite a bit different.  None of the bearings "sound" particularly bad.  The split bearing is fairly noisy (as expected) but the motor bearings really aren't.

Wierd Vibe 1.png 



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seanddd

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Reply with quote  #2 
What type of coupling is on this application?  I had as similar looking spectrum from a cross the line starter application that had a poorly lubricated grid coupling.  Once we opened the coupling to inspect we found there was also had wear.
ivibr8

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Reply with quote  #3 
Possible misalignment?
What does the TWF show?

JimP
Danny Harvey

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Reply with quote  #4 
What's all that little stuff around 4 x running speed?

I'd be looking at the coupling, too. I'd also be running my finger around the joints and welds on the structure.

Got any history?
Walt Strong

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Reply with quote  #5 
I think bearing looseness is the possible cause of the vibrations, whether from rolling element wear or fit clearances. Adding to what Danny mentioned, I would zoom in on the frequencies at 3x and 4x and use Log amplitude scale along with frequency units (not Orders). You might see Sideband frequencies at Pole-pass frequency. If yes, then it would confirm looseness in bearing; but not an actual electrical fault. I have see this before on motor bearings, so I am not making this up!

Walt
arie mol

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Reply with quote  #6 
The haystack at 3*rpm and 4*rpm (60 .. 80 Hz) tells me there is a resonant condition present that amplifies 3rd and 4th harmonic of fundamental speed 20 Hz.
A bumptest will confirm / cancel this potential root cause for elevated vibration level at 3rd/4th.
Same for 12th!
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