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OT - electric motor issue

Discussion in 'Electronic Repair' started by Smitty Two, Apr 18, 2008.

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  1. Smitty Two

    Smitty Two Guest

    Our air compressor at work is powered by a 5hp, single phase, 230 volt
    motor. Often when the compressor tries to start again (triggered by
    falling tank pressure) the breaker trips. By rotating the motor by hand
    prior to resetting the breaker, the motor will then start. Since this
    happens between two and five times per day, we're getting tired of it.

    The nameplate says the motor draws 24 full load amps, so I had the
    electrician (who was there for another job) pull the motor off the 20
    amp breaker and put it on a 30 amp. (wire gauge sufficiency verified.)

    Still trips, and not knowing what else to do, I replaced the motor
    starting capacitors.

    That didn't fix it. So, before taking this to a motor repair shop, or
    replacing it, is there anything else an idiot could look at and possibly
    fix? TIA.
  2. James Sweet

    James Sweet Guest

    The starting amps are much higher than the full load amps, for a 5HP motor
    you're probably looking at around 100A for a split second as it starts up. I
    suspect the wire run is long, or you have some resistance somewhere which is
    delaying the motor getting up to speed long enough for the breaker to trip.
    One option is to run the circuit with heavier wire, though a likely better
    option is to install an unloader valve on the compressor which will greatly
    reduce inrush. These release the pressure on the line between the compressor
    and the valve so that the motor isn't working against the tank pressure when
    it starts up. Also if you haven't changed the oil in the compressor recently
    that wouldn't hurt.
  3. Smitty Two

    Smitty Two Guest

    That all makes sense, but the reason I think it's an internal motor
    issue is that the motor seems to have one or more spots that create hard
    shorts. Rotating the motor by hand, even through 20 degrees or so,
    enables restart without tripping the breaker. We've observed this
    repeatedly: Once the motor stops in a given position and trips the
    breaker on attempted restart, the breaker will trip repeatedly until the
    motor is rotated, and then it will always start without tripping the
    breaker. Is my logic faulty?
  4. James Sweet

    James Sweet Guest

    That's interesting, I've never seen a fault like that in an induction motor.
    It really does sound like a problem with the motor itself. If a specific
    position causes the breaker to trip I would suspect the rotor is damaged,
    there's not much to these but I suppose it's possible.
  5. David

    David Guest

    The rotor of an induction motor is filled with shorted
    turns. This is what makes it an induction motor. If some of
    those turns become open, the fault you observe will be the

  6. PeterD

    PeterD Guest

    Check your unloader valve.
  7. Smitty Two

    Smitty Two Guest

    Is that a problem that you would characterize as economically repairable
    on a 5 hp motor?
  8. James Sweet

    James Sweet Guest

    I would assume it to require a new rotor, in which case probably not, but a
    motor repair shop probably has some good used motors available. Depending on
    the nature of the fault, it might be possible to fix it, I'd at least take
    it in for an evaluation if you don't feel like opening it up yourself.
  9. I'd go with that. Although very unusual for a motor like this. The
    rotor is just a pile of steel plates with the bars of the squirrel cage
    embedded in them and welded at the ends.

    Is it always the same orientation of the rotor?

    Another possibility is that the centrifugal or other starting switch
    is somehow not closing so the starting winding is not being energized.
    I don't know how rotating the shaft a few degrees could cure this though.

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  10. Smitty Two

    Smitty Two Guest

    I don't know that.
    If I monitor an analog voltmeter on the starting caps while I cycle the
    power switch until it fails to start and the breaker trips, would that
    tell me whether it could be the centrifugal switch?
  11. Yeah, if you dont hear a 'hiss' when it shuts down it's very likely the
    unloader valve needs attention.
  12. Baron

    Baron Guest

    I have had experience with a motor with a bad squirrel cage ! It ran a
    little rough and was way down in torque. It also got a lot hotter than
    it normally did.

    I tried to solder the copper bars, two of them, where they had cracked
    at the end. A waste of time ! A new rotor cured the problem, curtesy
    of Brook Motors.
  13. Smitty Two

    Smitty Two Guest

    Status report: I've traced this to a malfunctioning centrifugal switch,
    as suggested, but still cannot account for why a small manual turn of
    the motor would "fix" that, particularly 100% of the time.

    The mechanical portion of the switch seems fine, so I have to suspect
    corroded contacts. Unfortunately, they're buried inside a metal shell
    and all but completely inaccessible. But, one half of one of the two
    contact pairs definitely shows erosion, in profile view.

    I put a call into the manufacturer today to see whether parts are still
    available, even though the motor is obsolete. They have to do some
    research and get back to me. Failing that it looks like we'll be out
    $500 or so for a new motor. That'd be a shame, given the otherwise
    perfect condition of it.
  14. James Sweet

    James Sweet Guest

    Would this work?

    They have a 1750 RPM model for about $100 more, not sure which your
    compressor uses.

    The motor you have is probably better built, if you can figure out a fix for
    the start switch.
  15. Guest

    A further note on open circuit bars on a squirrel cage motor - if you
    can measure the to current to the motor preferably with an analog
    ammeter (with a pointer) it shall oscillate at the difference between
    shaft speed and synchronous speed. That is at the slip speed. Not to
    be confused with any fluctuations at the compressor shaft speed.
  16. Smitty Two

    Smitty Two Guest

    Thanks again to all respondents. Tracking down a replacement centrifugal
    switch was a bureaucratic nightmare of epic proportion. Cost about $42
    plus shipping, so not counting time and frustration, about 1/10 the
    price of a new motor of same specifications. Compressor has been up and
    running fine for about a week now.
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