Residual Magnetism in turbo-rotor 1

[aolalde]

I have a 3600 rpm turbo Generator 15.6 Mega-Watts.
The field rotor assembly is 175” long and the rotor core has 28” dia and 75”length.
The owner has reported problems in the babbitted bearings due to parasitic currents and the rotor sows signs of residual magnetism.
Do you have any suggestion to demagnetize that rotor?
Has someone any experience of parasitic currents due to residual magnetism?
Will the field excitation restore such a residual magnetism again?
Thanks in advance for any help or comments.

A. Olalde

 

[ScottyUK]

Hi A. Olade,

I'm more familiar with bigger sets, but the presence of residual magnetism in the rotor doesn't seem unreasonable. The DC field always has the same polarity, and thus the magnetic field will also have the same polarity. In the absence of any deliberate reversal of the field to degauss the rotor, there will be some small residual magnetic field after the field current is de-energised. I don't think this will contribute to a bearing problem, as the magnetic field present when the field current is energised will be of the same polarity and several orders of magnitude larger than the residual magnetic field.

The bearing problem may be due to poor grounding of the shaft, leading to electrostatic discharge in the bearing. Earthing brushes aren't the easiest things to keep working at peak efficiency, and can tend to get neglected by operating companies. Is the brush on this set in good condition?

The other possibility is that the bearing has circulating currents passing through the closed loop formed by the rotor, the bedplate, and a bearing at either end. Examine the bearing pedestals to check that any insulation between the pedestal and the bedplate is not breached.

Hope this helps.




-----------------------------------

Do it right, or don't do it at all.

 

[aolalde]

Thanks ScottyUK for your valuable comments.

I will investigate the shaft grounding brushes condition and bearing insulation, I will let you know ASAP what I will find.

 

[aviat]

Generators I deal with have residual voltage, but it is very low, less than a volt. A build up of carbon may be causing problems. Vacuuming and cleaning out generator on a regular basis could help. Also seating brushes when installed may help.
The site below shows an aircraft engine having similar problems.

http://www.tdatacorp.com/SAIB/CE-03-24.htm

 

[jbartos]

Suggestion: Visit

http://www.bently.com/articles/1Q01snyder.asp

for:
1.
Electrostatic Discharge in Rotating Machinery
2.
Also, potentially there may be common-mode voltages and currents due to higher harmonics caused by nonlinear loads, which will be damaging the bearings.

 

[ScottyUK]

A. Olalde,

An interesting link for shaft earthing brushes:

http://www.sohreturbo.com/

I've never used their products - never even touched one - but they look well-engineered and I suspect are one of the better designs out there.



-----------------------------------

Do it right, or don't do it at all.

 

[aolalde]

ScottyUK:

I saw the shaft earthing brushes link. Certainly those brushes seem well engineered and specially designed to succeed under contamination. I did not realize that capacitive coupling and high frequency could develop currents trough the bearings in spite of the pedestal insulation.
Thanks for your help.