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Air compressor motor

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  • Air compressor motor

    I have the chance to get a large air compressor for free. The present owner states the motor on the machine is damaged and a new one is needed. I have looked the compressor over and it is in great shape. The motor is a 5 hp compressor motor nothing fancy. The capacitor (sp) has been torn off the motor and that looks to be the only damage. If the motor has to be replaced can a new one be had cheap. I have done some looking around and everything seems to be going for around $300.00 for the motor. I can buy a similar new compressor for a couple of hundred more. I would like to get into this compressor as cheap as possible any help would be appreciated.

  • #2
    If you can get it for FREE, haul it home, remove the motor and take it by an electric motor shop and have it checked out. It may be that you will only have to replace the Capacitor. Also, the fellows at the shop may be able to point you to a cheap, used replacement motor if it comes to that.


    • #3
      Wouldn't you be morraly obliged to tell the present owner of your findings I E the motor is not dead just needs a new capacitor I personally couldn't sleep at night thinking I was taking advantage of a decent mans generousity.
      Please excuse my typing as I have a form of parkinsons disease


      • #4
        All a capacitor does in a normal single phase motor is provide phase shift to start the motor in the right direction. Staggers the pulse to give it direction, that is why you can change the motor direction by changing the capacitor-winding logic from 120 to neutral, and other end (5&8) ?? I can't remember at the sec.

        YOu can test the motor and points *(that open when the motor spins up to speed) by isolating the wiring if it is loose, taking the belt off, applying power as you spin the motor.. It will start in either direction and run if nothing else is wrong..

        THE compressor I had the points corroded. I polished them with a slip of paper till they were shiny once again. That was about six months ago. I bought a V4 7.5 hp compressor from and love it.. THE other one was a joke next to this behemothe. UNLOADING it liked to have hurt me thou. It weighs in about 1800lbs.

        We did more on testing a single phase motor on Adrians site in the cnc section.
        Excuse me, I farted.


        • #5
          Well got the compressor home today. The motor is shot and the head gasket needs to be replaced. I hooked up a 1 hp motor to the compresser and the pump seems to work well, with the exception of the head gasket (minor leak issue). The 1 hp motor can pump up to about 50 psi and them it starts to slow down and stall. What type of oil goes into the compressor. I need to change the old stuff out. THe compressor is a Magna Force (made my Coleman). Any idea where I can get a 5 hp motor new or even rebuilt. Over all the compressor looks pretty good.


          • #6
            Hey Gear...
            My 5hp, 2 stage motor recently threw craps. I floundered around googling and wasnt being inspired, so tried flea bay and immediately got some good prospects. Ended up ordering one from .....
            ummm...around $250? -- I think it was, -- a competitive price from those I had looked at. and they shipped it really quick, packaged well, etc, etc.

            If that compressor is a 2 stage---and with a 5hp on it, it prolly is---then youre well ahead by fixing it.
            If everything seems to be going well, you have obviously overlooked something........


            • #7
              The eaton site I listed above also has parts, motors.. real motors.. Real pumps.

              I am impressed about everything about my compressor except the paint job. It has never hardened. Compressor is starting to show rust around the edges of the fins, it is inside a building.

              I could not build one as cheap as I bought it, Seems was near 1500 or so. FOR a compressor you can sandblast with, has a continous run valve, IS quieter than any compressor I have ever been around, it is actually a ten horsepower pump turning slower on a 7.5 hp motor.

              It should outlast my need without further investment. IT Did require running a 50 amp service to it. #6 wire and 50 amp breaker.
              Excuse me, I farted.


              • #8
                David brings up some good points. Before you buy anything first determine what SCFM you need and what costs you are willing to pay for it. If the compressor can deliver an adequate SCFM with a smaller motor geared down (smaller pulley on motor or , if possible, larger pulley on pump) than go for that. The sound will drastically be reduced! This way you can use it for all the work that doesn't require the high delivery air volume - which is probably 90% of time.

                Since this is basically a "found compressor" I'd try to get it to work for me as inexpensively as possible - even if it doesn't meet all your needs. Also, if you decide it needs the 5-hp motor, be patient, and I bet you’ll find a good used 5-hp motor for a good price.
                Last edited by Mike Burdick; 11-06-2006, 03:39 PM.


                • #9
                  Never thought about changing the pump pulley. I have a large pulley at the house so that is a really good idea. One thing I see is the pulley that is on the compressor is a really heavy duty type. The pulley I have is a thinner material but larger (ID). Do you guys see a problem with the lighter weight of the replacement pulley.


                  • #10
                    Forrest Addy wrote a good bit about air compressors round here or on pm , but also i think in one of the Magazines.


                    • #11

                      Many compressor pulleys have a fan built into the spokes to blow cooling air over the compressor fins. If the old one has them, the replacement should have them or rig a separate fan.
                      North Central Arkansas


                      • #12


                        Have you examined the motor? If the windings aren't fried or damaged, it likely can be repaired. A motor shop can determine this or you may be able to find out yourself.

                        The capacitor(s) are used for STARTing (to provide phase shift on single phase motors), and if there's a second cap it would be a RUN capacitor. Some motors have the run cap as well as the starting cap.

                        Suppose your motor is a capacitor start motor (only one cap).
                        You can test the motor, less the starting cap, by powering it and manually spinning the shaft to get it going; it usually will run which ever direction you start it turning and run up to full speed. If it continues to run without getting hot, the motor is prob OK. Then you need to replace the start capacitor.

                        Jack C.


                        • #13
                          Since the pump pulley often is a special, what about the motor end?

                          I put a 3/4 HP on an old Craftsman A/C that came with a 1.5HP. I then turned the motor pulley down proportionately to compensate. It has a serpentine belt, which is easier to use with small pulleys than a V-belt.

                          I needed to run it off a 2500watt inverter.

                          Keep eye on ball.
                          Hashim Khan


                          • #14
                            I have looked at the motor and the windings appeared to have been fried. I noticed the copper windings at the rear portion of the motor were fused together by and electrical arc, and that's how I determined the motor needed to be replaced. Both of the capacitors are exposed and are hanging from the wires that attach them to the motor. The motor has a strong burnt smell also. I just took it for granted the motor needed to be replaced after seeing all the problems with it. I did not try to hook the motor up to any electrical current to confirm the motor was shot, but the way the motor looks and smells made me to believe it was a goner.....

                            I will try to forward some pictures If I can figure the system out......


                            • #15
                              The overriding all important first step is to fiigure out why the old motor burnt so the new motor doesn't do the same.Chances are there is a reason it fried.I would check the unloader valve if it has one and the check valve which it must have and also the pressure switch before going any further.A stuck valve or welded contacts in the pressure switch are all suspects.

                              Also,post if you will the motor nameplate HP,voltage,rpm,amps,shaft size and frame size.That might be a 5hp motor,and it might be "rated" at 5hp,there is a big difference.

                              As a word of caution,I have been down this road before,that might be a good compressor and it might be a barbeq grill in disguise.I wouldn't spend too much time or money on it just in case.
                              Last edited by wierdscience; 11-06-2006, 08:57 PM.
                              I just need one more tool,just one!