I am seeing this signature on a number of fans that are driven by VFDs. I have come to find that the peak around 4KHz is the VFD switching frequency, and the sidebands are spaced by 2X the VFD output frequency (which is in effect 2X the line frequency that is being fed to the motor). My initial thought is that the stator windings are vibrating, but would like to see if anyone on here has other ideas.
Problem statement: Is the vibration signature shown in the attached figures real machine vibration? Or just noise being picked up by the accelerometer? In the case that it is real vibration, is it damaging the fans?
Thanks much! Any and all insight is appreciated!