Slip Ring Wear

[mrcoal]

We have a turbine with the field slip rings that are showing alot of wear/grooving. We change polarity on the ring after stoneing the bad ring, the wear followed the polarity and is now grooving/wearing the other one. Its a GE exciter, direct connected 125KW at 250 volts at 450 RPM. I am wondering why or what I should look at to find out why this is following the polarity. Thanks in Advance

 

[byrdj]

I would recomend you review the GEI/GEK in your generator instruction book. the rings wear by a combination mechanacal and electrical, and the electrical should cause the greater. I can't recall which way the polarity goes for the most metal removal from the ring, something about the current direction arcs. A normal ring wear should be 0.001"/1000hours. wear greater than 0.005"/1000hours indicates a problem.

Since you are noticing differential ring wear, the mechanical abrassive wear is proably not the problem but due to the electrical. Once the ring surface deteroates, the contact is reduced and the minute arcing gets even worse, thus causing even greater wear.

the rings should be resurfaced to restore the contact surface finish

 

[edison123]

The negative slipring will wear out faster due to electrochemical action. Hence, the polarity switch after every few thousand hours to ensure even wear on both sliprings. The normal life of sliprings is the same as the generator winding i.e. about 20 years. If yours is wearing out at much faster rate, check the brush grade and the spring pressure. If the sliprings have porosity, that too will make them spotted with intermittent internal arcing leading to further wear-out.