Brushless excitation 2

[electrolitic]

Even new synchronous machines, specially Rotary Compensators, still use brushes/rings for field winding excitation current instead of brushless system. Probably the possibility of field polarity reversion is the reason.
When/why is this reversion necessary? Thanks.

 

[waross]

You can't use field forcing to rapidly reduce the field strength on loss of load with brushless systems.

Bill
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"Why not the best?"
Jimmy Carter

 

[electrolitic]

So, considering the rotary compensator, the field forcing can provide a faster response and a better regulation performance when field weakening becomes necessary. Am I right? Thanks waross.

 

[waross]

I believe so but wait for some replies from those with more experience with large machines. I haven't been around field forcing for about 40 years. Some of our other posters will have more knowledge on current practices.

Bill
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"Why not the best?"
Jimmy Carter

 

[ScottyUK]

A slipring machine can have a faster response because one of the R-L time constants is eliminated in the rotating exciter, and because the slipring machine can, when provided with a four-quadrant rectifier fed from a suitable supply, actively suppress the field by regenerating into the source.

 

[electrolitic]

waross and SkottyUK.thanks. Simple and direct as your answers.

 

[Skogsgurra]

I don't even think that you need a 4Q supply. That would be if you need to reverse excitation current. But in this case, all you need is a counter-EMF, which a simple thyristor rectifier can create. But you can not have a free-wheeling diode. Or a half-controlled bridge.

Gunnar Englund

http://www.gke.org

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Half full - Half empty? I don't mind. It's what in it that counts.

 

[FreddyNurk]

Basler might have a paper on it. I remember sitting through one of their presentations at one stage on migration from a brushless excitation system to a slipring one. The summary of the whole presentation was effectively what ScottyUK said though.

EDMS Australia

 

[ScottyUK]

Freddy,

A regenerating generator rotor looks a lot like a current source, so Skogs is right - a fully-controlled bridge will meet the requirement for two quadrant operation. Four quadrant would actually reverse the field polarity, which doesn't happen in practice.