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Generator Brushes Arcing Problem

Discussion in 'Electrical Engineering' started by Rowbotth, Apr 8, 2004.

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  1. Rowbotth

    Rowbotth Guest

    We have a few GE Generators (all under 80 MW) about 30 years old, and
    we've been having more problems than normal with brushes arcing.

    We think some of the problems may have to do with CBLF deviations, but
    not all.

    Is there a source for information on this sort of problem that I could
    get access to, please?


  2. SQLit

    SQLit Guest

    Do you care to explain what CBLF is ?
  3. daestrom

    daestrom Guest

    Don't know what 'CBLF deviations' is supposed to mean.

    If this is an AC unit (the brushes are for slip-rings), then I'd check....

    Runout of the rings (if bad, check the vibration and tolerance of the shaft
    bearings). Once the problem is fixed, you'll have to 'turn' the rings to
    get them back into round. Even if round, roughness caused by contact with a
    brush holder or other rigging can cause the brush to vibrate and spark.

    Same hardness of brush as recommended by manufacturer? Extremely hard
    brushes can sometimes spark under normal operation. When replacing, have
    they been 'seated' to match the curvature of the rings? Is the face of the
    brush all one smooth and shiny surface? If there seem to be two or more
    'polished' areas, then it's shifting around. When only part of the brush
    face contacts the ring, the current density is high and can cause sparking.
    If the face has chips missing around the edges, it may be impacting the ring
    and shattering the face.

    Brush spring tension (can allow 'chatter') and bursh holder alignment. The
    holder must hold the brush consistently and not allow the brush to wobble
    back and forth. If the brush moves even slightly, then it never gets
    completely 'seated' as it shifts position. Especially if the machine was
    recently serviced.

    Color of the slip ring face Absolutely bare brass/silver is *not* good, too
    much friction. Should be a slight brown coating. But too dark can be bad
    as well. This sort of thing can be oil contamination (too dark, 'sludge'
    buildup on rings). Too shiny, look for atmosphere contaminants like some
    paint thinners and solvents in the area. They will 'reduce' the 'brass'
    surface and the bare brass does not have the best performance (higher
    friction with the brush can cause chatter).

    Do you swap the field leads periodically, or perhaps you can't do that with
    this machine?

    I could probably go on some more, but these are things that I remember
    seeing on a variety of different machines.

  4. Rowbotth

    Rowbotth Guest

    Carbon Based Life Form
  5. Rowbotth

    Rowbotth Guest

    Yeah, sure.

    Things like not cleaning up oil, and having the hydrocarbons attack the
    oxidation between sliprings and brushes, for instance.

    And most recently we're told the sparking had to do with paint fumes
    attacking the oxidation between brushes and sliprings. This time, we
    had to have the sliprings ground?

    This is the type of thing that I'm wondering about. How many more of
    these type of "cockpit errors" are going to get us?

  6. daestrom

    daestrom Guest

    Yep. Some paint fumes can 'reduce' the oxidation layer. See my other post.
    That's like asking how many ways can someone make a mistake. You can break
    them down in different categories, mechanical, chemical, electrical. Or
    maybe operation, maintenance, environment. Or 'dumb, dumber, dumberer' ;-)

    Proper seating when replacing. Proper adjustment of holder position and
    spring tension (especially after turning the rings). Too much
    vibration/chatter within the brush box because of poor fit. Allowing too
    much shaft vibration.

    Adequate humidity (too low and the layer formed on the rings will be worn
    off, leading to high brush wear rates). Atmospheric contaminants (paint
    fumes, fuel-oil, gas, or even lube oil fumes, exotic chemicals).

    Over current. Under current. Poorly bonded 'pigtails' (more a
    manufacturer's defect). Not connecting all the leads so some current flows
    through the brush-box into the brush. Routing the leads where they can
    chaff and fray. Not reversing the leads periodically to even the
    electro-chemical reactions.

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