Shaft residual magnetization causing bearing currents?

[BigMotorGuy]

In a typical 3ph IM (in this case a 2500Hp motor), what residual magnetization levels at the shaft bearing journals would be capable of creating bearing currents? I realize this would be dependant on the bearing impedance, but we have a shaft that had a bearing failure and now has about 125 gauss in the area of the bearing journal. I assume the mechanical influence from the bearing failure created the magnetic effect, but is it prudent to demag the shaft? Any input on this would be apppreciated....

 

[Skogsgurra]

125 gauss would equal .0125 Tesla. It is around one percent of the flux density in the air gap. Not much and my guess is that you will have it back as soon as you start the motor after degaussing.
Also, residual flux in the shaft is not known to kill bearings. The induced voltage will usually not have a dangerous level nor an orientation that could cause damage to the bearing.

Is this a DOL motor? How is it coupled to the load? What kind of load is it?

Gunnar Englund

http://www.gke.org

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

 

[electricpete]

According to "Demagnetizing Motor Shafts to Prevent Bearing Damage" EASA Currents Magazine, October 2005:

There is no industry standard regarding how much magnetism is acceptable for a motor shaft. However, the general rule is that for a motor shaft, no more than 2 Gauss
should be measured in the bearing area, and not more than 8 Gauss for the rest of the shaft.

EASA is the "Electrical Apparatus Services Association"... trade organization for motor repair shops in the US and most authoritztive reference for acceptable repair practices that I know of.

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[electricpete]

I should point out, it is an article by an EASA staffer, not an EASA Standard (like EASA AR100).

It is interesting that this type of requirement does not make it into any of the repair specifications offered by EASA (AR100) or EPRI (1000897) or API (541). I'd think motor owners would have an interest in specifying acceptable magnetization levels.



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[electricpete]

http://www.gaussbusters.com/gaussbusters.html

Generally, magnetic field levels which are 2 gauss or below will suffice for most rotating machinery parts and applications. Corners, edges and other uniquely shaped parts may display slightly higher residual magnetism values, so specifying a gauss level somewhat higher is acceptable for these areas.

These guys obviously don't have standing as an objective source... they stand to make money if people conclude there are a lot of shafts with levels that are considered "high"



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[BigMotorGuy]

Thanks for the feedback. Skogsgurra, this is a DOL driven submerged pump motor, so it is a one piece motor/pump shaft. We have had some other bearing arc problems with similar designs (all 2 pole >=6kv, >1000Hp DOL motors) and are investigating all possible causes. This one unit is the only unit we tested the gauss levels, but wanted to get some feedback on this topic as the levels seemed high.

Also, does anyone have a feel for the change in bearing breakdown voltage of traditional grease filled air motor bearings versus bearings running in LNG at -160C?

 

[Skogsgurra]

Pete's low flux is news to me. There may be a reason that I do not understand, as I am working with VSDs mostly. And, at 60 Hz, a two-pole machine induces more voltage than a four-pole machine. I still do not see, and have not experienced, that a bearing current could be developped from a radial rotating field - uniform or not.

This IEEE paper could be of some interest:

Systematic Practical Approach to the Study of Bearing
Damage in a Large Oil-Ring-Lubricated
Induction Machine

by Raymond Ong, Member, IEEE, James H. Dymond, Senior Member, IEEE, Raymond D. Findlay, Fellow, IEEE,
and Barna Szabados, Senior Member, IEEE

The paper looks at different bearing current sources and mentions shaft flux, but not residual shaft flux, as a possible source.

Gunnar Englund

http://www.gke.org

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

 

[starkopete]

Hi,
Here is another paper that you might find interesting, it also looks at some causes of shaft currents. The authors spend a little time addressing "magnetic remanence" but really don't come up with much of a conclusion.

Thanks

 

[starkopete]

Hi,
I seem to recall that on a different forum, 'electripete' came up with a scenario with nonuniform shaft flux creating a varying homopolar flux. Hopefully he will jump back in here and elaborate. I remember it because I thought that his reasoning made a lot of sense.

Thanks