I received some data today to review that has me a little stumped. The equipment is a 510 RPM Vacuum pump motor--actually there are 2 of these units. Most of the data shows some vibration at 2*LF (120 HZ), most amplitudes are in the area of .01 to .06 in/sec. What is odd is that the dominant peak is at 240 HZ, with amplitudes of up to .2 in/sec & running speed sidebands. These units are not on drives so this is not SCR related. I am taking it on faith that this is in fact electrical in nature (this is what I was told). If mechanical it is about 28 X RPM. This does not corrospond with pump vanes, stator slots, rotor bars, rotor fan fins, # of arms supporting the rotor on the shaft, Bearing defects (6316)or harmonics of same. Has anyone ever see 2 X 2*LF before?

Thanks
dj

dj,

Have you checked for a resonance in this direction? Also, loosen up one bolt at a time on the motor and monitor vibration pattern (be carefull).

Hello,

I suspect you are correct about the fault being electrical in nature.

I have seen similar vibration data. I recently had a 3phase AC motor (also driving a Vac pump) The motor began to show 2*LF vibration (100Hz in my case) with harmonics at 200Hz and 300Hz. However, unlike your problem my dominant peak was at 100Hz. Several days later the motor failed. After testing the motor, one Stator winding was found to be burnt out. I have since suspected that the 2*LF vibration was caused by a failing winding. which in turn created a magnetic imbalance causing the 100Hz etc vibration. This is something I have since proven on a test motor in the workshop.

Hope this helps
SWB