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Air compressor dying, need help diagnosing *VIDEO*

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  • #16
    Re: Air compressor dying, need help diagnosing *VIDEO*

    I hope you followed what I said about unloading them!!!!!!!! Not worth getting hurt over.
    Start capacitors are always black, run's are silver.
    Motors > Motor Supplies > Capacitors > Run Capacitor,20 MFD,370 VAC,Round : Grainger Industrial Supply

    uf and mfd are interchangeable. You are exactly correct on your thinking for the mfd's on the cap's. The start capacitor has to work much harder to turn over the load. It works to about 75% of run speed and then the run cap takes over. Then you have flywheel effect, and inertia, and kinetic energy and all the other things I've forgotten to keep things moving, so the motor isn't working as hard...

    Motor rating's surprise me more and more all the time. The motor on my compressor is 21 amp at 240v, rated at 5 HP. The start capacitor on mine is 375 nominal mfd's.

    Originally posted by spills View Post
    ok, got both capacitors off, and I can only guess which one would be the run. It's too late tonight to put up pics, here's a quick description of what they look like:

    "Start" Cap: Black cylinder, maybe 2" diameter and 3-4" long, it's got a resistor running in between both contacts. Ratings are 161-193 MFD 250VAC, 50-60 Hz

    "Run" Cap: Silver cylinder, maybe 1.5" diameter, 2-3" long. Ratings are 20 UF (same as MFD, millifarads or whatever?" +/- 6%, 370 VAB 50-60 hz

    I say "Run" and "Start" mainly because I'm assuming the start cap would have a higher farad rating than the run. Any thoughts?
    Last edited by woodjames; 06-13-2009, 09:40 AM.
    James Bateman

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    • #17
      Re: Air compressor dying, need help diagnosing *VIDEO*

      thanks for the link man. I'll go by after work and pick one up on Monday.

      As far as the discharging, I unplugged the compressor and tried to discharge with a screwdriver, but nothing happened. I've heard that usually, the caps drain in about 2-3 seconds after power is shut off. Although this was stated in reference to HVAC fans, I assume maybe it's the same principle

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      • #18
        Re: Air compressor dying, need help diagnosing *VIDEO*

        No problem. As long as you shorted it you're okay. I've seen them hold for days!!! Looks like the spark from a welder when you unload them.

        Originally posted by spills View Post
        thanks for the link man. I'll go by after work and pick one up on Monday.

        As far as the discharging, I unplugged the compressor and tried to discharge with a screwdriver, but nothing happened. I've heard that usually, the caps drain in about 2-3 seconds after power is shut off. Although this was stated in reference to HVAC fans, I assume maybe it's the same principle
        James Bateman

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        • #19
          Re: Air compressor dying, need help diagnosing *VIDEO*

          so about how long does the start cap actually run the motor? A second or so?

          The more I'm thinking about it, I think this cap is the problem. My buddy was doing some sand blasting and I told him to let the compressor rest every now and then, so of course he didn't. On top of that, the pump was already overdue for an oil change. Right after the belt started squeaking, I told him to knock off and I changed the oil. And I haven't really noticed the belt squeak since, so I think I was trying to solve 2 problems without knowing it, you follow? And somehow, the run cap just happened to fail on me and I contributed it to the same problem that was making my belt squeal

          EDIT: Just for curiosity's sake, on the side of the cap it says "a combustible fluid". WOuld it hurt anything to open this thing up just to see what's inside?
          Last edited by spills; 06-13-2009, 03:03 PM.

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          • #20
            Re: Air compressor dying, need help diagnosing *VIDEO*

            Loud and clear on the 2 problems statement. Related but not - I've tried changing 2 things at once to solve plating problems, and it's always catastrophic, either it gets solved because 1 or 2 of the changes was right, or it gets worse because 1 or 2 of the changes was wrong.
            When things are running correctly the start cap's cycle is very quick, seconds, exactly. But if the run cap don't work, it'll get stuck at the point where it should normally take over. Basically you're trying to run at the top end of the start cap's cycle.
            As far as the fluid, there's usually an oil in there to keep it cool. I've never been inside a cap, take some pics! Obviously don't forget your chem glove and glasses would be a good idea also. There's no way it'll work after poppin' it open, so count on getting one at this point.


            Originally posted by spills View Post
            so about how long does the start cap actually run the motor? A second or so?

            The more I'm thinking about it, I think this cap is the problem. My buddy was doing some sand blasting and I told him to let the compressor rest every now and then, so of course he didn't. On top of that, the pump was already overdue for an oil change. Right after the belt started squeaking, I told him to knock off and I changed the oil. And I haven't really noticed the belt squeak since, so I think I was trying to solve 2 problems without knowing it, you follow? And somehow, the run cap just happened to fail on me and I contributed it to the same problem that was making my belt squeal

            EDIT: Just for curiosity's sake, on the side of the cap it says "a combustible fluid". WOuld it hurt anything to open this thing up just to see what's inside?
            James Bateman

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            • #21
              Re: Air compressor dying, need help diagnosing *VIDEO*

              yea. I mean at $10, I can afford to tear my old one apart and get a new one just to see if that's the problem. At that price, testing it really isn't worth it to me. I'll let ya know if that fixes it though

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              • #22
                Re: Air compressor dying, need help diagnosing *VIDEO*

                Got the run cap replaced, and it *may* have been the problem. I put it on, flipped the switch, and it definitely was not as slow as it was. IT still slowed down after a second or so after the start cap kicked off, but no where near where it was. I'll wait till I go out there next time and give it a shot, I only had time to test it out once today. But I'm feeling confident that was the problem, I think I'm just being anal about it

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