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Air compressor slowing/stopping under load

Discussion in 'General Electronics Discussion' started by buster01, Nov 21, 2010.

  1. buster01


    Nov 21, 2010
    My air compressor starts fine but as the pressure builds the motor slows down an stalls-starst- slows speeds up etc.
    After removing the drive belt and starting the motor the motor runs fine (without load)

    Is this a capacitor problem or is the capacitor only used to start the motor?

    The unit was buit in 1977 so is up ther in age (like me).

    Model 919.71752 2hp 60hz 220v 15A Craftsman. Checking the power is 220v at the breaker, plug and switch on the compressor.
  2. shrtrnd


    Jan 15, 2010
    That's a pretty normal indication that your motor is at the end of it's life-span. If Sears doesn't sell a replacement motor for it these days, there are plenty of places that will have an equivalent. Try Grainger, or power tool repair shops in your area.
  3. Resqueline


    Jul 31, 2009
    I don't know this particular motor, but there are both capacitor start and capacitor run motors, so I'd try replacing the capacitor first, they do fail - with those symptoms.
  4. Lenp


    Sep 8, 2009
    Before you go too far, measure the voltage AT THE MOTOR. A bad connection causing low voltage may cause this problem,
  5. Coolbananas


    Nov 18, 2018
    I have the same problem.

    I have an 80 gallon V4 compressor with a single phase 5hp motor, it belonged to my Dad. The capacitors were cracked so I replaced both and bought a new pressure switch, blow off valves, new power cord and changed the plumbing to suit the space I want to put it in.

    I was very excited to fire it up, and when I did it would start and stop. I took the centrifugal off and cleaned it up, put it back together and fired it up. It worked like a dream until it built to around 5 pounds pressure then started to work hard. The electric motor would ramp up and fall off continuously until it popped the breaker.

    I drained what air that was in it and left the valve open and fired it up again and it kept running, but when I closed the valve, it would build to around 5 pounds and start surging again.

    After all that I did, does anyone have any thoughts as to what I should do/try that may be different than what was suggested to buster01?
  6. Bluejets


    Oct 5, 2014
    If you run it on any extension lead you will get voltage drop which is classic of your symptoms.

    As supplied the compressor would have a 15A lead at least and more than likely, at most, around 2 metres long.

    Socket outlet and the in-house cabling will need to be able to sustain the voltage under load.
    These runs would be best kept short or much larger in-house cabling installed.

    Mains cabling may not be of sufficient sizing also.

    It is of little use to measure the voltage at the switchboard breaker unless of course this is under maximum load.

    Voltage tests need to be "at the motor" and again under maximum load.
    BobK likes this.
  7. AmyRodriquez


    Nov 19, 2018
    This is normal indication
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