There are some points I want explanation to it:

1-The brushes wear rate increase for brushes numbers (1, 2 & 3).

2-The brushes wear rate fixed for brushes numbers (4, 5, 6, 7, 8 & 9).

3-The brushes wear rate decrease for brush number (10).

4-The vibration bearing no.10 direction (x) increased& increased during MVAR increased from 375 MVAR to 400 MVAR .

5-At load 627 mw (field current, field voltage. field temperature) don't change.

Please advise me because I am afraid that may be a problem inside generator or brushes collector

Thanks,

 

There are a number of things you should check on your brush gear/slip rings:

a. check that brushes are free to move in their holders, i.e., there are no sticky points in the travel.

b. check the tension of the brush collector springs as seen by the brush. Basically try to pull the brush off the slip ring with a spring balance or some other instrument to measure pulling force. Note the force as soon as the brush just looses contact with the slip ring. Do not pull the spring as the force or force direction you would be applying would not be the same as that seen by the brush

c. with the generator running measure the vibration of each brush using a portable vibration meter and a piece of rigid insulation rod. Again make sure you measure the brushes' vibration and not the springs'. Excessive vibration on some brushes may be signs of eccentric wear on the slip rings.

d. check the surface roughness of the slip rings. Brushes don't like coming in contact with too smooth slip rings, as in such cases their surface tends to glaze. Make sure you have the roughness indicated by the brush manufacturer.

e. are the brushes operating at their optimal temperature. Typically this should be around 70-80C, but again check the specification of your brushes. If possible adjust cooling air flow to get the desired temperature.

f. if possible try to measure the current passing through each brush. You will find some strange current sharing on the brushes, but if it is excessive, i.e. one brush practically no current and the other handling more than its design, then the other points in my reply need proper checking.

g. when was the last time that the slip rings were machined/ground with the generator at nominal speed? This is tied to (c) above.

h. finally, but this is a severe case, has the clearance on the generator bearing next to the slip rings increased, or was an increase in vibration observed on this generator bearing? Excessive shaft vibration may be pushing the brush collectors beyond their capability to keep the brushes tracking the slip rings. But as I said, this is an extreme case.

 

Sounds like field shorts. What did it turn out to be?