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  1. #1
    Veteran Member robbyr's Avatar
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    Default help testing electric motor on compressor

    compressor quit a while back (we had a spare, so not in too big hurry to figure out what happened). I disconnected from the 220 romex. power cord has black, white, and green wire. Have a fluke 75 multimeter if any of y'all can explain to me what do. Set to ohms? anyway be specific as dont understand electricity very well.

  2. #2
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    Default Re: help testing electric motor on compressor

    Quote Originally Posted by robbyr View Post
    compressor quit a while back (we had a spare, so not in too big hurry to figure out what happened). I disconnected from the 220 romex. power cord has black, white, and green wire. Have a fluke 75 multimeter if any of y'all can explain to me what do. Set to ohms? anyway be specific as dont understand electricity very well.
    when you say "compressor quit" what does that mean? was there smoke? does the motor just sit and buzz when you plug it in? is there ANY reaction when you plug it in?

    start of basic troubleshooting for a motor...

    step 1: remove the load from the motor. do this by loosening belts etc.
    step 2: attempt to turn the rotor by hand. what happens?
    step 3: plug in motor. what happens?

    the motor for your compressor is undoubtedly a capacitor start induction type. the picture below is of a pool pump motor, but it is the same type as your compressor.

    your issue is probably the starting capacitor, and not the windings. induction motors have no defined rotational direction; absent any "push" in one direction or the other, it will just sit there and hum (and draw a lot of current). a capacitor start circuit is thus used to force a phase change in one set of windings (aka the start winding) and give the motor a direction. once up to speed, the capacitor start circuit is then disabled, usually via a mechanical centrifugal cutout. for this reason, if the capacitor is found to be bad it is sometimes a side effect of the mechanical cutout not releasing, which causes the capacitor to fail over time.

    replace the capacitor, and check the centrifugal cutout for corrosion. the cutout is typically on the end of the shaft nearest the AC wiring entry.

    at 9 o'clock in the picture below, you can see the cylindrical capacitor (condenser for you old skool folks), and to the right of it you see the centrifugal cutoff switch. it consists of two assemblies, the mechanical portion and the electrical portion.

    the mechanical bits are attached to the end of the rotor shaft and is typically made up of two small flyweights, cam'd against spring tension. the faster the motor spins, the "flatter" the assembly gets as the centrifugal force of the flyweights pulls against the spring tension. in the pic below, you can see one of the two flyweights and one of the two springs. you can also see an "arm", co-linear with the rotor shaft, that is currently riding on the white plastic switch assembly, closing the circuit. in this condition, at rest, the capacitor and start winding(s) are switched in to the circuit.

    as the rotor accelerates, the flattening out of the mechanical portion of the cutout disconnects the two overlapping copper contacts (located at 6 o'clock) and the start winding is no longer powered. motor operation continues at the design speed, typically 1725RPM (1800RPM nominal, less the slip, with a 4 pole winding) or 3450 (3600RPM, less the slip, with a 2 pole winding).

    this particular motor also has a thermal cutout, located at the 12 o'clock position in the picture below. these can also fail, which leads to a completely inoperative motor. note that the thermal cutout may have failed because the motor overheated. if you replace the thermal cutout and don't fix the reason the motor overheated in the first place, you'll be back in there again in no time.




    Wrooster

  3. #3
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    Default Re: help testing electric motor on compressor

    I'd start at the switch.

    Pop the cover off and snap a few pics.

    Do this with no power.

  4. #4
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    Default Re: help testing electric motor on compressor

    Quote Originally Posted by Willl View Post
    I'd start at the switch.
    by switch, he means the pressure cutoff switch.

  5. #5
    Veteran Member robbyr's Avatar
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    Default Re: help testing electric motor on compressor

    Brother in law took pressure switch off a few weeks ago cuz that was his best guess as to the problem. I dont know what happened or exactly when. This compressor is used to replace water well pump (another thread I'm gonna start some day to see how many out there have seen this). We had the spare compressor, so hooked it up and forgot about it for awhile. The compressor would not kick on and there was no pressure in the tank. In my Sunday afternoon tinkering around, I was wondering if you could use ohm setting on voltmeter and test resistances on the wires in the cord. My FIL was excellent at this but unfortunately I never learned it.

  6. #6
    Super Member kenmac's Avatar
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    Default Re: help testing electric motor on compressor

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  7. #7
    Elite Member wdchyd's Avatar
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    Default Re: help testing electric motor on compressor

    If it was mine, I'd start by ruling out the drive motor first by taking belt off compressor head...
    A voltohm meter will allow you to see any voltage drop if you have problem with power coming in...
    then use amprobe on the hot legs of the power in to check for high amperage draw during start-up....if it does read too high then I'd check the capacitor start switch (if it is a single phase motor)....capacitor start switch failure in motor is a fairly common problem in single-phase motors

    If it smells bad in the motor junction box then something cooked in the motor..

    if all's good with the motor then check out compressor switch (like others have said) and the compressor head itself for binding (internal damage), or maybe the unloader valve is not unloading (resulting in the unit trying to start-up with pressurized air in the compressor head
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  8. #8
    Platinum Member Pete Judd's Avatar
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    Default Re: help testing electric motor on compressor

    The motor on my compressor has a red reset button on the back end of the motor. It is just a simple push to reset circuit breaker. Might want to see if your motor has one.
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  9. #9
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    Default Re: help testing electric motor on compressor

    Quote Originally Posted by robbyr View Post
    In my Sunday afternoon tinkering around
    start of basic troubleshooting for a motor...

    step 1: remove the load from the motor. do this by loosening belts etc.
    step 2: attempt to turn the rotor by hand. what happens?
    step 3: plug in motor. what happens?
    Wrooster

  10. #10
    Veteran Member robbyr's Avatar
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    Default Re: help testing electric motor on compressor

    Ok y'all have given me things to ponder. Probably will be next sunday as it will be dark when i get home from now on. Thanks for all the replies

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