Squirrel Cage Induction Motor vibration

[power2020]

Hi!
Any one please explain me the cause of the induction motor vibration with the following data:
Motor specs:75kW, 2987rpm,415V,3-ph, 50hz
Vibration details: Decoupled condition (load is pump),
DE: Horizantal:4.2mm/sec, Vertical:0.9mm/sec & axial: Nil
NDE: Horizantal:4.1mm/sec, Vertical: 1.0mm/sec & Axial: Nil
The above readings noticed at 5985rpm in the spectrum.
I am grateful to your reply.

 

[electricpete]

I would say likely the horizontal vibration is being amplified by resonance.

With runnning speed 2987rpm, and vibration at 5985rpm, you need to sort out whether the exact frequency is
A - 2*line frequency
B - 2*running speed - extremely close to 2*LF when uncoupled
C - neither of the above (non-sync) (related to bearings)

Do you know the frequency bin width of the measurement? (what was Fmax and how many frequency lines)?. Also did you use a peak label feature to determine the frequency (can get much more accurate than the bin width). If the bin width is 4cpm or above I would recommend to re-take with higher resolution to see if you can rule out C.

If you can rule out C based on frequency, you could do a power-down to try to check between A and C. Even without power-down test for 2-pole motor I would suspect 2*LF vibration. First thing to check is the hold-down bolts. Carefully loosen 1 at a time and retigten while monitoring for vib. If you see vib going away you have a soft foot condition which needs to be corrected.

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