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Capacitor Start, Capacitor Run Motor

Discussion in 'Electronic Repair' started by Robowang, Oct 15, 2006.

  1. Robowang

    Robowang Guest

    I havd an air compressor that has the Subject line type of motor,
    115vac. It only has about 200 hours on it. I had left it on for a
    couple of days recently and found that my circuit breaker in my garage
    was tripped. I restet it and the compressor started(surprised me
    because I didn't remember leaving it on.). It ran about 2 seconds and
    the circuit breaker kicked, again.
    Before I tore into the compressor, I made sure it wasn't the circuit
    breaker by plugging into the utility room circuit, 23 seconds later, it
    kicked that breaker.
    The strange thing is that it sounds different than before. It did have
    a deeper, stronger sound when the motor was running, now it sounds
    kinda puny. When the motor starts and runs, it turns the compressor
    pump an actually builds a small amount of pressure before it kicks off.
    I have pulled the starting cap and the run cap to get the numbers off,
    and have found them online. However, before I buy them, I was wondering
    if this sounds more like the main winding (not the startup winding) may
    be burned out. I haven't opened the case on the motor to look, yet, but
    I wouldn't think that the winding would NORMALLY short (thus more
    current), but sometime during the few days that I had left the
    compressor on, we had a lightning hit that destoyed a dvd player and an
    audio amp.
    It could be coincidence.
    Anyway, what would cause the motor to pull excess current. Would the
    run cap being bad cause this?
    I have checked both caps with a DMM, and all I can say is that they are
    not shorted.
  2. Short answer: replace the capacitors.
  3. Ken Layton

    Ken Layton Guest

    I agree. The capacitors are cheap to replace. In general the run
    capacitor value is much less than the start capacitor value.
  4. Robowang

    Robowang Guest

    So, a bad capacitor could cause a hi current condition?
  5. Not exactly ... see my short answer. Circuit breakers have two trip times.
    A stalled motor can evoke the longer trip time because a capacitor failure
    does not get the motor turning.
  6. Robowang

    Robowang Guest

    I ordered the caps.
    However, The motor does start. It actually gets to where I would guess
    that the start winding is cut out, maybe 50-60% of it's speed. then it
    kicks the circuit breaker.
    When the compressor is first started, it is very easy to rotate the
    pump. Of course, as the pressure is built up in the tank, the
    compressor is going to work the motor harder. But it kicks out almost
    To pull that much current, the main winding would have to be near
    shorted. I have seen older motors lose the coating on the windings,
    partially shorting the windings and lose resistance due to losing wire
    length in the winding. This will cause the motor to not be as efficient
    at the same current draw. Or I have seen an open in the winding, which
    kills the motor, totally.
    Also, if the start capacitor is bad, wouldn't that just cause the motor
    not to turn at start up and just hum and jiggle? I think the start cap
    on a single phase motor is just to cause enough of an out of phase
    condition to get the rotor to start rotaing, then once it starts
    turning, the start winding (and start cap) is cut out and the main
    winding will continue the rotation in the direction the start winding
    phase pulled the rotor orignally.
    It's all smoke and mirrors. I'll get back when I get the caps. I hope
    it is just the caps, as a new 6hp peak motor is close to $200.00.
    thanx guys.
  7. Of course the motor could be bad ... but the point is that the capacitors
    are more often bad. It is simply a matter of statistics, which is what
    you'll mostly get when posting here.

    My experience with motors and capacitors is 10:1 capacitor failures versus
    winding failures. Just me ... no science ... just statistics.

    Good luck.
  8. Hi!

    As has been mentioned, your compressor's problem is likely the run
    capacitor. Even if the start cap were bad as well, the motor might still be
    able to start and get up to running speed just fine.

    I don't think you can stress the capacitor enough with a simple meter test
    to make it fail. It sounds like the run cap breaks down with stress and time
    before turning into a short or more load than the circuit can handle. With
    time it might get worse, such that your meter test would reveal a short

    I think you will find the replacement capacitors solve the problem, but I
    would still like to hear how it goes.

  9. I had almost the exact same problem about a month ago and it was just a bad
    run cap.

    - Mike
  10. Robowang

    Robowang Guest

    That makes me feel better.
    Yeah, I think if it is a cap that it's likely that it's the run cap. I
    just don't have any experience with electric motors. The only thing I
    have ever experienced was replacing a start cap in a refrigerator. It
    wouldn't start up, just hum. The motor was a low rpm version and you
    couls actually start it by spinning the shaft until it got up to speed.
    A start cap fixed it.
    I just wasn't for sure about the run cap. It's been along time since I
    had any training in that area, but it seems like I remember that the
    run cap was to help the motor make torque efficiently once it reaches
    peak speed.
    I have used a heavy duty cap checker and even it wasn't effective. The
    best way to check big cap is substitution. But you gotta have an extra.
    I know a DMM is pretty useless for a cap out of the circuit, but it's
    all I have.
    I should have the new caps by the end of the week, So I'll let everyone
    know what happens.
    Thanx for the input.
  11. Can you confirm that the start winding is actually cutting out. It
    may be that the centrifugal switch is the problem.
    Not really. All you need is a few shorted turns which then act as the
    shorted secondary of a transformer.
    One simple (in principle) test would be to remove the compressor load entirely
    and power only the run winding. Then, spin it by hand and apply power while
    it's still turning. It should come up to speed and run without tripping the
    breaker if the windings are good.

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  12. Guest

    I read all of the posts and I find it odd that no one asked about both
    the size of the breaker and if this compressor motor was plugged into
    an extension cord? If an extension cord is used, amperage drop over
    distance maybe...just a thought!
    Cheers, Steve
  13. James Sweet

    James Sweet Guest

    That would be voltage drop over the distance, and with an induction
    motor the amperage will increase to keep the power constant.
  14. Robowang

    Robowang Guest

    The breaker size is 20 amps.
    I am not using an extension cord.
    I did connect to seperate circuit with a 20 amp breaker and it tripped.
    I hope to get the caps this week, but I still haven't been notified
    that they were shipped.
    I have a decent Electronics store about 30 miles away, so if I don't
    hear from the net store today, I will cancel my order and go to the
    local store this weekend.
    Thanx guys.
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