Hi, I read about changing the po

[ErdoanZ]

Hi,

I read about changing the polarity of exciter of generator once for year because one pole will wear out faster than the other one so while we change the polarity the poles will wear out approximately equal.
I have two question about this process:
1. If I change the polarity so cable that is minus pole (-) put on bus-bar of (+) and vise-versa, should I change other thing like measurement cables etc. or is not any problem with that?
2. Is any other positive ( or negative) aspect about changing the polarity?

 

[edison123]

In a rotating field with sliprings, you interchange the + and - leads once in a year or two to ensure an even wearing out of the sliprings. Otherwise, the negative polarity slipring will wear out faster.

Muthu

http://www.edison.co.in

 

[waross]

Interesting. Where did you read that?
Did it include instructions on how to deal with a "worn out pole"?
eg: Rest, a good diet and a plane ticket back to Poland?
When this is done there should be a rubber bucket on hand to catch the voltage drop.
Most generators depend on a small amount of residual magnetism to start generating or "bootstrap".
When the exciter polarity is reversed, the current induced by the residual magnetism is the wrong polarity and the generator often will not generate.
No problem, just re-flash the exciter field.
I have seen a number of failed AVRs due to a mistake when trying to re-flash the field.


Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[waross]

Hi Edison: Large versus small.
The small sets all went to brushless exciters decades ago. There used to be standby "take-outs" on the market a couple of decades ago.
These were typically old set from telephone exchanges with many years but few running hours.
These originally were equipped with brushes but were all converted to brushless exciters before being resold.
On the small sets, as well as replacing a few AVRs that the last guy had destroyed while trying to flash the field, I was called out to a a couple of machines that wouldn't generate after an AVR was changed because the polarity was reversed. Correct the polarity and there was still enough residual magnetism to bootstrap the generator.
Sliprings may wear out but exciter poles, never.
In a large set with PMG, residual magnetism is not an issue.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[edison123]

Bill

In AC generators, big or small, changing the field polarity does not affect either the residual magnetism or the stator's capacity to build up voltage. Polarity interchange in brushed field are done all the time (for slipring wear-out rate) without any issues.

Muthu

http://www.edison.co.in

 

[waross]

Big versus small Muthu.
The question was to do with exciter poles, not slip rings, and slip rings haven't been used by the major manufacturers on small sets for decades in North America. (Small, less than about 2 MVA)
PMG excitation is becoming more common but self excited sets won't build up voltage if the polarity is reversed.
I have seen this when the excitation has been disconnected and the polarity lost. Mostly when I was the second person to work on the set.
On a self excited set, the main field leads are not accessible.
The only exciter leads that are accessible are the leads from the AVR to the brushless exciter. If the leads from the AVR to the brushless exciter are reversed on a self excited set, the voltage will not build up. Not a guess, not from a book, actual field experience.
It's early in the morning here and I hadn't had my coffee yet and I answered the question as asked.
Thank you for giving the OP the answer that he was looking for, Muthu.
I imagine that the year in question is 8760 running hours (24/7) and not sitting on standby for a year with only a few hundred running hours.


Bill
--------------------
"Why not the best?"
Jimmy Carter