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  • Compressor problem

    I have an old used compressor, Sanborn 5HP 30 Gal. tank, model ??????.

    It sat around unused for a long time and I have it now. I have only used it a small amount and at first it was fine but yesterday it developed a problem. The Electric motor will start (hums) but the compressor doesn't turn over and It trips the breaker. Now if I reach under the guard I can easily pull the belt and it moves freely, If I try running it again, after I pull the belt, it usually starts working. The compressor works well and quickly builds up pressure. There is oil in the sump.

    Can a compressor stop at top/bottom dead centre and would that be the problem (doubtful with an electric motor). If so why would it do it so much.

    Is there another reason that it would work his way.

    Right now I have to run the pressure up and then turn it off so it won't stall and trip the breaker when the pressure drops and manually start it to run up the pressure when needed. More of an inconvenience then anything so I would like to fix it if possible.
    The shortest distance between two points is a circle of infinite diameter.

    Bluewater Model Engineering Society at https://sites.google.com/site/bluewatermes/

    Southwestern Ontario. Canada

  • #2
    Hums but doesn't start? Sounds like a failed start capacitor to me...

    -js
    There are no stupid questions. But there are lots of stupid answers. This is the internet.

    Location: SF Bay Area

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    • #3
      Failed start cap or failed centrifugal switch.
      I seldom do anything within the scope of logical reason and calculated cost/benefit, etc- I'm following my passion-

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      • #4
        Originally posted by loose nut View Post

        Right now I have to run the pressure up and then turn it off so it won't stall and trip the breaker when the pressure drops and manually start it to run up the pressure when needed. More of an inconvenience then anything so I would like to fix it if possible.
        If you bleed all the air off and try to start it, what happens?

        I just need one more tool,just one!

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        • #5
          ^ could be an open check valve if the tank is under pressure while you try and start it. Most likely a start cap
          https://www.flickr.com/photos/csprecision

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          • #6
            If it hums but the belt moves freely, as the OP says is the case, then I would be replacing the start capacitor.

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            • #7
              Thanks guys, now I just have to find one.
              The shortest distance between two points is a circle of infinite diameter.

              Bluewater Model Engineering Society at https://sites.google.com/site/bluewatermes/

              Southwestern Ontario. Canada

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              • #8
                Well no joy in Mudville tonight.

                I changed the cap but it didn't fix anything. The cap was starting to leak a bit so it would have failed later but that isn't any help now. I suppose it is the clutch that is buggered but I don't have any experience with them.

                &^%)(*@#$%^[email protected]$^ and then some!
                The shortest distance between two points is a circle of infinite diameter.

                Bluewater Model Engineering Society at https://sites.google.com/site/bluewatermes/

                Southwestern Ontario. Canada

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                • #9
                  What exactly does the clutch do?

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                  • #10
                    My first thought was the start cap as well, but rereading the original post, I think I may have misunderstood. It sounds like once the compressor is shut down when the tank is full, the motor will hum and not spin when the unit calls for air once the pressure drops. OP mentioned nudging the belt and then it runs, but I assumed that was while the motor was still stalled and humming. I think he may be saying that he cuts the power, moves the belt a bit, then turns the motor on and then it will start and run.

                    I think that's a different issue. Does the pressure switch shut the motor down once the tank is at pressure? Typically that same pressure switch is what actuates the unloader valve the releases the pressure on the compressor head so that the next start is not under load. Is the switch actuating the unloader valve? Do you hear the short hiss of air being released when it shuts off?

                    Perhaps it is not actuating or is clogged with junk? The check valve mentioned in another reply is a possibility as well, allowing tank pressure to leak back intot he compressor head when the motor is off.

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                    • #11
                      I had the exact same issue on a smaller compressor. Grandpa "fixed" it by cutting a hole in the belt guard to roll the pulley. Dad fixed it right. I will ask him what he did tomorrow.
                      21" Royersford Excelsior CamelBack Drillpress Restoration
                      1943 Sidney 16x54 Lathe Restoration

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                      • #12
                        A couple of questions for clarity.... First, when you say 'clutch' so you mean clutch or do you mean start switch or something else? I have never seen a 5 Hp compressor with a true 'clutch'. Second, does the compressor EVER fail to start when the tank has 0 pressure or is the fail only when the tank has pressure? If it fails with 0 pressure and with a new capacitor it is now either a start switch or wiring issue. Most likely a switch. If it is only with pressure in the tank, then it is most likely an unload valve issue. The unload valve is often in the pressure switch and they do get clogged with oil and dirt. A good cleaning, with a solvent, can fix the problem.
                        Robin

                        Happily working on my second million Gave up on the first

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                        • #13
                          Centrifugal clutch or switch that is used to energize the start winding of the electric motor. Sorry for the confusion.

                          I figure the "switch" is stuck in the run position so the motor just hums but I can start the motor by giving the belt a pull and switching the compressor on why the motor is still revolving. The compressor, itself, works just fine, just a motor problem. I can use the compressor but I have to shut it off when the tank is full and then manually restart it when the tank runs down otherwise the motor won't start and it trips the breaker. It is a pain in the butt but still usable. I am a little gun shy about tearing a motor apart. As an electrician I would make a good plumber!
                          The shortest distance between two points is a circle of infinite diameter.

                          Bluewater Model Engineering Society at https://sites.google.com/site/bluewatermes/

                          Southwestern Ontario. Canada

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                          • #14
                            Check this out: How to Troubleshoot a Motor and Adjust Centrifugal Switches - Bing video
                            “I know lots of people who are educated far beyond their intelligence”

                            Lewis Grizzard

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                            • #15
                              Yep, time to pull the back cover off the motor and check the start switch. You'll likely find a bad connection or a burned contact. If a contact, try lightly sanding the surfaces to restore decent contact.

                              Attached Files

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