Static excitation system and shaft current 1

[nelvex]

It is a relation between the current in the hydrogenerator shaft and static excitation system?
I know that this current can destroy the isolation bearings.
Is someone who knows more about this?

 

[desertfox]

hi nelvex

have a look at this reference:-

http://books.google.co.uk/books?id=4-Kkj53fWTIC&pg=PA662&lpg=PA662&

dq=electric+motor+bearing+
failure&source=bl&ots=95bfgpmhpD&sig=her3Fx7uaHO5ZH9A26lkLqT_Puw&hl=en&
ei=2AtUS864Bory0gSF-syrCg&sa=X&oi=book_result&ct=result&resnum=8&
ved=0CCkQ6AEwBzgU#v=onepage&q=electric%20motor%20bearing%20failure&f=false

http://www.sprint-electric.com/news.php?view=20

http://www.appliancedesign.com/Articles/Motors/BNP_GUID_9-5-2006_A_10000000000000129428

http://ecmweb.com/mag/electric_bearing_current_problems/

regards

desertfox

 

[Skogsgurra]

Hello nelvex,

Yes, many people have experienced this kind of problem. Me, for example. I visited two plants last week. One where 315 kW machines had severe bearing problems after a little less than 1 year of operation and one where small two-pole motors had problems after eight months of operation.

I have done this for 15+ years and there are many myths and simplifications. One of the more common simplifications is "You need a good ground". True as that may be in many instances, it seldom helps when you got EDM (Electric Discharge Machining - or electric erosion) in your bearings.

Another myth is that only large motors are affected. Wrong again. All motors are effected, but the coupling mechanism is different. Inductive coupling in large machines and capacitive coupling from stator windings to rotor in smaller machines.

And then, there is ground potential differences. That is where a good ground can help. Or, better, equipotential bonding.

There is a basic course on bearing currents in

http://www.gke.org/presentationer/files/EDM Physics and Reality 1.pdf

And a short version in

http://gke.org/beppe/files/Bearing currents on one page 1.pdf

Gunnar Englund

http://www.gke.org

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

 

[ScottyUK]

A problem which affects big turbo machines is windage-induced static which punctures the oil film. This is the primary reason for the shaft grounding brush on large turbo-generators, rather than diverting HF currents caused by switching which is a problem affecting motors fed from a power electronic converter. The shaft grounding brushes are similar but the problem is from a different source. Excitation system rectifiers operate at power frequency and don't generate particularly fast switching edges to give rise to destructively high voltages, although it is certainly possible to pick out commutation with a reasonably fast transducer and a half-decent 'scope.

I'm not sure if this also applies to salient pole hydro machines as they are lower speed and much larger diameter. Maybe one of the hydro guys will comment of he sees this thread.


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

 

[Compositepro]

Gunnar, very interesting information in your presentation! We make a product that uses carbon fiber to solve static electricity problem on machinery (not on motor shafts). If you ever want some carbon fiber to play with, let me know.

Carbon filaments are only 5 microns in diameter so they are effective at inducing ionization of air in the presence of an electric field. For this reason you also have to be careful with it around high voltages. The individual filaments are invisible and can be several inches long. They can cause unintended shorts. At 120 volts the filaments will flash and burn-up. At 480 volts they will create a self sustaining arc that will trip circuit breakers.

 

[esee135]

Hydro or thermal? If you take apart a vertical hydro, it's very apparent within the thrust and guide bearing assembly as to how the unit is isolated while having a single ground reference.

Mostly, units without ground brushes get them added with a static exciter upgrade (hydro) or 64F protection. I usually test isolation at 1000-2500V, which is overkill, however, it's not convenient to disassemble the guide and thrust to re-establish isolation. The iso materials are sufficient for such a voltage. The bearing isolation of a generator is a case where we break the electrical code rules.

For horizontal hydro, regular bearing isolation is maintained.

For hydro, the effect is thought to be less than thermal since we have rotational speeds significantly less than cylindrical machines, resulting in a much lower d-phi/dt of the rotor based on speed of rotation.