VFD motor bearings

Hi guys,
I wonder if any of you have come across the problem of noise in a VFD motor. I have a 306kw VFD motor that has a "screeching noise" ( it has on board SPM monitoring but the trend readings are level). Here`s the thing: It's noisy on application of power ie from 0-3000 rpm and when it off loads and idles. However, as soon as the power shuts off the noise stops instantly even though the motor is still turning.

Searching I found this and it makes sense but I wonder if it actually exists in practice.


"As soon as the motor is started with a VFD, the common-mode voltage imbalance searches for a path to ground, and a parasitic coupling of the rotor and stator windings develops. Once the oil film in the dielectric breaks down, voltage levels increase on the motor shaft, and the lowest impedance path for the current imbalance becomes the motor bearings themselves. Voltage begins to arc across the bearing at high frequencies, leading to electrical discharge machining (EDM)".
By Philip M. Gonski, P.E., Keystone Engineering Group, Inc.

Glenn1340,
Interesting problem, does this motor have insulated bearing & earthing brush on rotor.
This helps eliminate bearing problems with arcing etc.
NOrmally on 150kw & above motors.
IF it runs down with no noise, then does'nt sound like bearing problem.

MAybe talk to drive manufacturer for possibilities of noise.
Check basics like coupling & grease bearings first, but sound like drive.

Hi Ranger,
No there`s no earthing rings and it`s fitted with standard c3 bearings. I`ve checked the coupling (it`s rubber inserts so no grounding there) and the bearings are greased as per the manufacturers recommendations. I`ll try to locate an oscilloscope to check the voltage of the shaft failing that maybe a multimeter might give an indication.

http://www.culvermotor.com/images/st...rathon-sgr.pdf
http://www.designworldonline.com/how...r-duty-motors/

Hi Peter_1
good information, thank you.