Hi guys,

I was recently called in by a client to check an ID Fan motor having repeated bearing failures. The motor is a 4-pole motor driven thru a VFD.

Demodulated spectra do not show any defect frequencies, however severe random high frequency vibration is present. Looks a lot like the fuzz you get when fluting occurs in VFD driven motors. Some typical marks too are seen on the bearing race. Another thing I noticed was that the client was using regular bearings instead of C3 clearance bearings.

The question is: Is the root cause improper bearing selection or shaft current flowing thru the bearings? The marks appear to be a fluting pattern. However, the bearing housing insulation is intact!

Also, normally fluting occurs on the uninsulated DE side bearing. Here, the marks are on the NDE side. The DE bearing too is failing, but the marks there are quite different (looks like improper seating).

I have attached pictures of the NDE bearing & would truly appreciate your views about them.

Thanks,

Aditya

Picture.doc (522 Kb, 38 downloads)

In my opinion it looks like a event with much higher current like welding or so but I can´t say I have seen that many isolated mounted bearings with current damages. So the other question is if this motor run at so high temperature so C3 bearings are required? Are you really sure the isolation is OK if it´s the version where aluminium oxide or similar are used on the bearing house mount, it´s real hard to keep intact.
Next thing I would look at is how fast the bearing damage is, VFD killing the bearing can be repeatedly extremely fast, down to a few weeks and certainly within 3 years.
Normally when I find bearings that for some reason has to tight clearence I find a wear ditch around where it has been running at high load, sometimes off center axially, then that is a reason it´s been tight. Maybe different reasons for tight clearences leave different marks. Olov