We have 5 identical pumps running side by side all at the same speed, and I know what you're going to say... "Their vibration frequencies are modulating each other!"
Unfortunately I don't think that's case (but I could still be wrong). We've only noticed on one of the motors that there is this relatively long beat frequency. See attached. Bearing faults aren't lining up, rotor bar and stator slot frequencies aren't lining up either.
The motor is a 400HP, GE L5011VP, 460Volts, 1180 RPM.
It is driven by a VFD. Turning speed on the attached TWF and spectrum is 1080 RPM.
Could this be an electrically induced vibration?
We've already rebuilt two of the five motors and found that the ODE bearing had fluting from arcing. We spec'd the rebuilt motors to have ground rings and insulated bearings. The remaining three motors (one of which is the one with the beat freq.) do not have ground rings.
Thanks for any help!