PERIODICAL REVERSAL OF ROTOR SUPPLY POLARITIES

[appunni]

Sir,
In one generator maintenance manual, I read that the polarity of dc supply to rotor poles through slip rings may be reversed once in a year in order to minimize wear and tear of the machine. When I asked about it to the engineer in charge of the power station, he said that that instruction was according to an oral suggestion of the commissioning engineer from M/s Alstom and he does not know the details about the same.
Can anybody describe about this? Whether this procedure is good or destructive? I request for your valuable comments. Advance thanks.
With regards,
Appunni

 

[edison123]

appunni,

In the rotor dc ckt, the negative slipring will wear out faster than the positive one due to electrochemical reaction. So, to maintain even wear of both sliprings, the polarities are swapped.

In a dc exciter, due to same reasons, a pair of brushes (one +ve and one -ve)are staggered to avoid grooving in the commutator.

 

[jbartos]

Suggestion: A suitable cathodic protection might be applied.

 

[edison123]

you can also refer thread238-67863.

 

[sngpl]

jbartos: Can you please elaborate?

 

[xabproject]

dear appunni,
i have operated and maintained 30MW turbo-generators for over 8 years and routinely changed the polarity every year after the first tripping of the unit on Rotor Earth fault - the negative brush had worn out totally and the pigtail was touching brush-holder! we had static excitation equipment panels where the polarity reversing facility was available in the downside of rotor CB - from where cables are run to the sliprings. the rotor earth fault relay connection was upstream of this tap-off to cables and did not get affected by this change in polarity to sliprings. the observations of edison123 et al seems correct - thread referred.
best regards

 

[jbartos]

Suggestion to sngpl (Electrical) Oct 17, 2003 marked ///\\jbartos: Can you please elaborate?
///The electrochemical potential might be suppressed by a reduction of the electrochemical potential. Two materials, i.e. negative material and positive material cause the electrochemical potential. By a suitable selection of these two materials, the electrochemical potential, e.g. .5VDC may be reduced to lower dc voltage.\\\

 

[4654]

I agree with edison123