generator rotor shaft current 2

[zkz]

hello,
What is the permissible value of generator shaft current? I read somewhere else that the permissible value of shaft current shall be less than 1 ampere.
I am expecting your valuable comments.
Regards,
appunni

 

[Skogsgurra]

Hydro-electric? Steam? DG?

Hydro-electric usually has shaft current monitor that can be set in the 100 - 1000 mA range. So that seems to be acceptable values in such macines. Don't know about the others.

Gunnar Englund

http://www.gke.org

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

 

[wolf39]

A shaft current alarm is indicating that arcing is present between shaft and bearing surfaces, in special cases also in couplings and gears. Arcing does not only destroy the bearing surfaces by creating craters but also deteriorates the lubricant over time and is damaging the shaft as well. Shaft currents therefore have to be avoided at all costs, especially for machines with ball bearings.

Shaft voltages of up to 50 volts are quite common in hydrogenerators. The oil film between shaft and bearing cannot be regarded as a safe barrier between these components. It therefore is imperative to insulate one of the generator bearings. A generator should be taken off line as soon as the shaft current monitor indicates an insulation breakdown. The low resistance of the shaft current circuit can be responsible for currents of 100 amps and more, far beyond the figures a shaft current monitor can be set.

I've never heard of a permissible limit figure for shaft currents.

Wolf

http://www.hydropower-consult.com

 

[zkz]

Thanks for valuable replies. Mr. Gunnar says about 100mA to 1A while Mr.Wolf says about 100A.
I hope more explanations will be there about this matter.
regards,
appunni

 

[ScottyUK]

I would be concerned to see 1A on a large turbo-alternator running on white metalled bearings. Even a few hundred mA isn't good, mainly because whatever is allowing that low current to flow will only get worse if not corrected.


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If we learn from our mistakes I'm getting a great education!

 

[wolf39]

Where did I say that up to 100 amps are PERMISSIBLE?

ScottyUK is right with his comment. Even the arcing of a few hundred milliamps does degrade the bearing oil and is affecting the bearing and shaft surfaces, it only takes more time until the bearing in question fails. This can result in an outage of weeks if not months in case the shaft is damaged. The outage of a 100 megawatt generator, for instance, can lead to a daily loss of revenue of up to 100,000 US$ and more. The correction of a faulty bearing insulation can possibly be done in one day.

Wolf

http://www.hydropower-consult.com

 

[zkz]

Thanks for explanations.
regards,
appunni