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Need help troubleshooting an electric compressor motor

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  • BBL

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    Feb 8, 2021
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    I have a big rolling Craftsman tank with an electric compressor on top.
    The motor has a run capacitor and a start capacitor which is controlled by a centrifugal switch. When the motor spins up, the centrifugal switch interrupts the start circuit.
    Initial problem was that the compressor would not spin up and would trip a breaker. Closer inspection revealed that the centrifugal switch housing was cracked and not allowing the centrifugal switch to engage the start capacitor.
    I fixed the housing so now the centrifugal switch works as expected. It is closed and the starter circuit is engaged when motor is stopped. Once the motor spins up, the switch properly interrupts the start circuit. Verified.
    But the motor still had a problem starting. I ordered a start capacitor because it was cheap enough to replace and the old one was made in the 1990s.

    After replacing the start capacitor, the motor still has trouble starting.
    I verified that:
    1. without the start cap, the motor has hard time spinning up but the run cap helps it and it eventually starts, slowly though.
    2. with the start cap, the motor starts great when there is no air in the tank.
    3. with the start cap, the motor won't spin up if there is more than 40 psi of pressure in the tank

    I tested the old start cap and the new one as well. Both provide great capacity, I can charge them with DC and they will hold the charge for days. They both will light a small bulb for a second or so. Of course I realize that's not a proper way to measure capacitance but at least it's a test that the cap is not blown.

    I would love to measure the start assist windings on the motor but I do not have manufacturer's specs anyway.

    What do our resident electricians think of this scenario? What do you suspect? What else would you test and how?
    Thank you in advance for any useful advice.
    Target Sports
     

    hooper

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    Jan 19, 2020
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    No expert by any means, but when it starts (on an empty tank) does it completely pressurize? If so I would look to see if it has an unload pressure switch or circuit (not sure how to explain it), but usually the compressor unloads after it completes a cycle (you should hear a burst of air pressure) so when it has to start it is not starting under pressure. At least mine does.
     

    Axxe55

    Retiretgtshit stirrer
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    Dec 15, 2019
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    Lost in East Texas Elhart Texas
    The easiest solution would be to just replace the electric motor with a new one. Very few places even rebuild electric motors anymore, and if you did find one of the few places that still does, it would probably cost way more than a new more modern electric motor.

    Check out Grainger's Industrial Supply.

     

    Eastexasrick

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    Naples TX.
    As said before, check the unloader valve. The motor should start against -0- pressure regardless of what is in the tank
     

    Eastexasrick

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    As Axxe said Grainger will have exactly what you need. If you don't mind a cheap knock off Northern and Harbor freight have imports that will run 3000 hours. Granger will have Baldor, they run 10,000. easy. Ulness you are running a shop, or paint complete vehicles a knock off will last most of a lifetime.
     

    Lonesome Dove

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    Need to know a few details.
    110-220 amp
    Hard wired or corded
    HP
    Single or Twin cylinders
    Pully sizes
    Oil capacity
    Air tank capacity
    Auto start or manual
    Ambient air pressure where located
    Yada yada yada
    I may have left out the unknown and obvious.
     

    Axxe55

    Retiretgtshit stirrer
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    Dec 15, 2019
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    Lost in East Texas Elhart Texas
    As Axxe said Grainger will have exactly what you need. If you don't mind a cheap knock off Northern and Harbor freight have imports that will run 3000 hours. Granger will have Baldor, they run 10,000. easy. Ulness you are running a shop, or paint complete vehicles a knock off will last most of a lifetime.
    Exactly. If for comercial use, I'd invest in a comercial or industrial grade motor, but for a home shop and occasional use, a lesser grade motor would probably suffice.

    Also might be a good time to upgrade to a larger HP motor as well.
     

    Eastexasrick

    Isn't it pretty to think so.
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    Jul 2, 2022
    3,501
    96
    Naples TX.
    Need to know a few details.
    110-220 amp
    Hard wired or corded
    HP
    Single or Twin cylinders
    Pully sizes
    Oil capacity
    Air tank capacity
    Auto start or manual
    Ambient air pressure where located
    Yada yada yada
    I may have left out the unknown and obvious.
    Vertical or V configuration Single stage or series, altitude, mean failure rate, NTP vs STP the square root of chickenshit, ......
     

    BBL

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    Feb 8, 2021
    1,739
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    TX
    Thanks, everybody, for the useful advice.
    To clarify more, it is a single-phase 110V "6hp" single-cylinder compressor with wheels and a 15A cord.
    No pullies, this is a direct drive compressor. The cylinder is vertical, compressor motor is horizontal with a crank and counterbalance weight.
    Altitude is pretty much flatlands, below 1,000' MSL but density altitude could be around 3,000' in this heat. :)
    The compressor is used in an air-conditioned workshop but if the A/C is off, I might be running it at 100F.
    Oil-less system. Air tank capacity is "big" (just google "6hp" Craftsman compressor). Automatic start with an on-off switch and pressure gauge.

    Answer to Hooper's question: yes, when the motor starts (below 40psi), it can pump the tank up all the way to full 150psi.

    Before replacing the motor, how about I look at the unloader? Whenever the motor stops, you can hear a sharp hiss from the on-off switch housing so I assumed the unloader is okay. I can loosen the air line from the compressor head after I shut it off and no air comes out. So again, I assumed that the unloader was good and focused on the motor instead. Anything I am missing?
     
    Every Day Man
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