Side Band Frequency on 450 KW AC Electric motor

[JLLCUR]

We collected some vibration readings on a solorun of a 450 KW 50Hz 2 pole 3 phase refurbished AC Electric Motor with a magnetic Accelerometer, and found the following spectra.
Mutiples of running speed 2996cpm up to 5x with side bands on all the multiples of 225cpm. At first we suspected cracked or broken rotor bars, however the 225cpm does not correspond to the expected PPF of 8cpm (# poles time slip frequency)
We could not find any literature which describe a similar spectrum for electric motors. Has any one experienced a similar situation?

 

[Strong]

Does the motor have rolling element (anti-friction) bearings?
Were the bearings replaced?
Was the motor mounted on a soft mount in a shop?
Was the accelerometer magnet base firm on the motor/bearing surface without rocking?

Walt

 

[JLLCUR]

Yes Walt, it has anti-friction bearings which were replaced during refurbishment.
The same signature was obtained on rigid base plate, both in the work shop as well as at site.
Several readings were taken by several people at several locations on the motor with the same result, ruling out accelerometer mounting issues.

 

[Strong]

A couple of possibilities to consider:
1) Slip of bearing inner or outer ring/race on shaft or in housing.
2) Axial movement of motor rotor. Some motor bearing have spring washer for restraint; possibly use strobe-light to see shaft motion.

Walt