Bearing Fluting - recomendations, causes

[whitedog1]

Our customer has a 250 hp 460v drive running a 250 hp motor driving a fan at approx 63-68hz. After 6 months, pulled the motor and noticed fluting on the ODE bearing. The mill has had other motors on drives with fluting problems.
For this mill, on large hp motors (above 200 hp), should the standard be insulated ODE bearing and shaft grounding brush? Could there be a issue with the electronic grid in the mill? Could running above 60 hz cause the fluting? Is there any way to predict fluting?
Thanks for your comments.

 

[gsjhand]

We had same issue with one of our motors. Fluting is a common problem in large motors if the motor is fed from VFD. This happens due to grounding current passing through the bearings. Solution for this problem is either to have insulated bearings or to have grounding brush installed on inner race of bearing. More details can be found on NEMA APPLICATION GUIDE FOR AC ADJUSTABLE SPEED DRIVE SYSTEMS (Application_Guide_AC_Adjustable_2007_watermarked[1])

 

[DickDV]

This is almost certainly due to shaft currents from the drive output pulses.

There are lots of different proposals around for fixing the problem but, for me, the only economical, almost foolproof method is a shaft grounding ring.

These are made by Parker Corp (the hydraulics people) and Aegis, a division of ITW in Mechanics Falls Maine. Both are good products and are simple to install.

 

[gsjhand]

Here is the site address for downloading Application Guide

http://www.nema.org/stds/acadjustable.cfm

 

[jraef]

The Aegis product has been discussed in here several times if you want to use the keyword search function. I highly recommend them.

 

[gsjhand]

Dick DV: Can we do sizing calculation for these brushes? Last year we sent out our 750 HP DC motor for grounding brush installation. The only reason for sending it out was that we were not sure about sizing of brush. I was thinking that installation should be simple but decided not to take a chance

 

[Skogsgurra]

Grounding brushes are quite effective in most applications. The general rule is: Larger is better. There are a few issues with insulating film being created on the shaft and/or oil or grease on the shaft. A discussion whether to use hard or soft brush qualities or carbon-silver brushes is going on in the profession. Some use a combination where a hard keeps shaft clean and the soft or silver-carbon brush does the "electric job".

The AEGIS ring usually does both the cleaning and the conducting work. Wear is also usually not a problem.



Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[oftenlost]

Attached find an interesting study on VFDs and bearing currents

http://www.rema.uk.com/pdfs/Report No 2.pdf