Ac motor diagnosis request.

Discussion in 'DIY Section' started by Lager, Mar 12, 2017.

  1. Lager

    Lager Registered Member

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    I have an oil less air compressor manufactured by Campbell Hausfeld and its not that old and has only been cycled on about 12 times. But, One day, it just would not come on.
    I'm well versed in 12 volt systems since I'm an auto tech but 120 volt baffles me a little.
    There is a control unit that has cut off switches and this controls the voltage to the motor. I have 120 v to the motor white and black wires coming out of the control unit.
    If I place my DVOM set on resistance and place the leads on :
    Black and white wires going to the motor, capacitor connected= OL or open circuit
    Both black and white to motor case= OL
    From what Ive read, this tell me that there is an open circuit in the windings, and it needs a new motor.
    Then I checked the capacitor disconnected and removed for resistance.
    One way, I get 5 million ohms slowly increasing in resistance.
    If I flip the leads, I get 2.5 million ohms slowly decreasing in resistance.
    Either terminal checked to ground shows OL, meaning its not shorted to ground.
    From what Ive read, this means that the capacitor is faulty.
    Whats the chance of this happening ? None, any suggestions?
     

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  2. 613jmm

    613jmm U. S. Military Club Subscribed

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    You usually check a capacitor for capacitance, not resistance. There should be a value and a tolerance on the body of the capacitor. You need a volt meter with the ability to check capacitance, and just check with the probes as normal but make sure you discharge the capacitor first. The chances of the capacitor having gone bad on a motor that small is not very likely. They usually go bad over time.

    Do you grind stuff around the compressor? Was the motor humming when you tried to start it, or not doing anything at all? One time I thought my compressor had gone bad because it would not start. It would hum and nothing more. I took it apart to check for anything obvious and I didn't see anything out of the ordinary. I did clean a bunch of barely visible grit out of the contacts area and it started back up just fine when I put it back together. I was grinding mower blades in the vicinity and a bunch of grit found its way into the compressor motor, shorting it out.
     
  3. Lager

    Lager Registered Member

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    613jmm, thanks for your suggestions. You are correct in your comment that a capacitor should be checked for capacitance, but I don't have access to one of those meters. So what I did was charge the capacitor from a 24 volt DC power source like I was reading. Then checked it for for DC voltage, found that it accepted the voltage and then slowly drained away as it was used up by the meter. Then shorted the terminals and got a pretty strong arc or spark. looks like the capacitor is ok and the open circuit in the motor windings are causing me all the trouble. I cannot buy just the motor alone, I have to buy a complete motor compressor assembly, Gotta love this Chinese stuff. But its all my own fault, I paid cheap, I got cheap. Now I have to buy another one and hope for the best because sometimes, Chinese stuff actually works.
     
  4. jerry williams

    jerry williams Registered Member

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    Most air compressor motors have a thermal overload button somewhere on them. You can usually push this button in by thumb and reset the button. Have you seen a red or black button on the end of the motor?
     
  5. Lager

    Lager Registered Member

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    Hi Jerry, Thanks for responding. I have this motor torn down completely, no sign of a overload button anywhere. Good idea tho and might help someone else.
     

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