High vibration uncoupled motor. Broken or Cracked Rotor?

[smrodriguez4]

Hello,

All, I am looking for some guidance in trouble shooting a Horizontal 3600 RPM, 1375 HP squirrel cage motor. This motor is used to drive a centrifugal pump and was refurbished 2.5 years ago. However, due to electrical wiring problems the motor did not ran until the last couple of weeks. As you can imagine, the motor shut down on high vibration minutes after start up. A motor shop company came out and did a vibration analysis on our unit and determined that we had multiple harmonics of running speed and that we may have a clearance problem in our sleeve bearings or structural looseness. We went around to check for soft foot and couldn't find any significant distortion or movement. We then inspect the bearings; they were in good condition but we decided to have the repaired anyways. When they came back we uncoupled the motor install the repaired bearings and fired it up. This time we took vibration readings in house and the spectrum turned out to be exactly the same as before, multiple harmonics of turning speed. However, this time we decided to take high resolution data and noticed that there are some sidebands developing in the harmonics. Those side bands are really noticeable in the axial inboard direction. Specially in the 3rd and 4th harmonics.

I am starting to wonder that we may have rotor problems I am close on having management send the motor back but I wanted to run by the experts here in case I am missing something.

What do you guys think?

My reasoning behind calling it a broken or cracked rotor is behind the fact that if you notice on the axial direction reading there is a really small peak at 2.5 Hz which is exactly the distance between the harmonic and its side-bands..

 

[Strong]

The high vibration at 1xSS may be causing the harmonics (nonlinear motion). I suggest balancing rotor by two-plane method. If both ends of the rotor are accessible in the field, then balance there. Otherwise the rotor will have to be removed and balanced in the shop. If the motor rewind shop did any balancing, then I would review the paperwork and perhaps have another shop balance check/correct it. I would not be concerned with the harmonics and try to create an explanation that the rotor is cracked without first reducing the 1xSS vibration level!

Walt

 

[eduvillela]

Sidebands indicates amplitude modulation, and by the spectrums you attached here, it seems that the frequency modulating the harmonics is abtou 250 CPM, which appears with a little peak on all the spectrum. The point is to find out what problem is generating this frequency.

But by taking all into account I would suggest inspecting the rotor and the shaft looking for cracks.

 

[electricpete]

All, I am looking for some guidance in trouble shooting a Horizontal 3600 RPM, 1375 HP squirrel cage motor. This motor is used to drive a centrifugal pump and was refurbished 2.5 years ago. However, due to electrical wiring problems the motor did not ran until the last couple of weeks. As you can imagine, the motor shut down on high vibration minutes after start up.

I'm missing the logic of "as you can imagine". What kind of degradation is suspected?

Was shaft/bearing clearance checked? Was bearing/housing clearance checked? (either before or after bearing replacement)

You'r suspecting 2.5hz is pole pass frequency…
What is machine speed as near as you can tell when these sidebands were measured? (so we can compare to pole pass frequency)
Was machine coupled or uncoupled at time these sidebands were measured?
Was it a full voltage run?
(I doubt you have 2.5hz pole pass frequency uncoupled at full voltage, but if you then do the rotor would have to be in very very sad shape to have such high slip).

I agree with Walt, it doesn't seem enough info to conclude you have a rotor problem and balance is a next logical step. Good advice there.

If you are still suspecting a rotor problem, it's worth checking checking current signature under load. Also rotor problems are often it is accompanied by time dependence. Sometimes difficult to balance. Sometimes it growls.

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(2B)+(2B)' ?