I have a question, and after doing a ton of research, I think I may have it narrowed down a bit. If you could give me any insight, I’d really appreciate it.
We had a 900 hp motor in for repair. Vibration in the shop looked good. Zero sidebands and only 3 harmonics, each declining in amplitude.

The motor gets installed on a compressor, runs fine for the first hour, then jumps 15 amps.
I took vibration readings under full load, and I have sidebands everywhere. I have limited information on the compressor specs, aside from a “guess” as to the bull and pinion teeth.

These Sidebands appear at operating speed of the motor and it’s harmonics. One note, the sidebands are more prevalent on the low pressure side of the compressor in the horiz, vertical, and axial directions. They also are found on the high pressure side as well. The sidebands are found even in the upper frequency ranges as well.

On the motor side, the largest amplitude is at the 5th harmonic (.129in/sec). At 1x, the amplitude is only .03in/sec.

The motor has 72 slots, 58 rotor bars, 4-pole 4160v. I’ve looked at RBPF and stator slot frequencies, and nothing is there. My slip is 37.5, so pole pass frequency would be 150cpm, right? The sidebands are 142.5 and these show up everywhere on the machine. Would these sidebands show up on the compressor side of the unit if it were a true rotor bar issue? Or am I looking at a gear mesh issue, with limited data?

Can anyone point me in the right direction?

Sounds like rotor bar problem in the motor. High amps is another indication. The motor may also run hot.

I agree, smells like a broken rotor bar. Verify with el. current measurement and please post the data if possible.

With a broken rotor bar the torque developed by the motor fluctuates causing torsional vibration which in turn could be sensed in radial direction not only on the motor but everywhere else in the power train even if machines are sitting on separate bases.

David

As you know the motor's slots and bars ,you can easyly calculate the motor fault frequency.
Compareing whth the main frequency,I can help you to calculate the fault frequency if you need.