Shaft residual magnetization causing bearing currents?
Shaft residual magnetization causing bearing currents?
(OP)
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....





RE: Shaft residual magnetization causing bearing currents?
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
www.gke.org
--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
RE: Shaft residual magnetization causing bearing currents?
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.
=====================================
Eng-tips forums: The best place on the web for engineering discussions.
RE: Shaft residual magnetization causing bearing currents?
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.
=====================================
Eng-tips forums: The best place on the web for engineering discussions.
RE: Shaft residual magnetization causing bearing currents?
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"
=====================================
Eng-tips forums: The best place on the web for engineering discussions.
RE: Shaft residual magnetization causing bearing currents?
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?
RE: Shaft residual magnetization causing bearing currents?
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
www.gke.org
--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
RE: Shaft residual magnetization causing bearing currents?
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
RE: Shaft residual magnetization causing bearing currents?
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