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Need the problem solvers!!!

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  • Need the problem solvers!!!

    OK guys....

    I got this great deal on a Sears portable air compressor......."Free to good home..needs repair"

    Tank appears to be in good shape. Motor runs, but I think has capacitor problems. Blew my circuit breaker after a few seconds, but motor nameplate shows 15 amp draw at 115 volts. Was on a 15 amp circuit (with tv and lights on same circuit) and might just have exceeded the breaker. Will have to try a larger circuit.

    Anyway...Motor is GE with 2 capacitors. Motor label doesn't show HP (I wonder why anyone would leave that info off plate???), but I think it is a 3 hp, because Sear shows the "almost" identical (albeit "new") unit in their catalog as a 3 hp. Motor Part #MO-9062 is MOD #5KCR49TN2265X.

    (The compressor unit doesn't seem to have a "model" number. If it does....I can't find it.)

    I tried GE's parts search and came up ZILCH.
    Someone apparently changed (at least) one of the capacitors. It is a Mallory (other is GE). GE capacitor reads 40uf/370 VAC part #97F9614. Mallory capacitor reads 400-480MFD/165 VAC. (It bothers me that the ratings are "soooo...much different. I'm guessing that "whoever" didn't think it necessary to "match" capacitors????)

    Neither capacitor had any charge (even after motor run test). GE capacitor shows constant continuity (zero on my meter). Mallory capacitor shows continuity and then drops to infinity, but this is sporatic....sometimes it does.....sometimes not. (Is this normal??)
    I don't know much about motors (or capacitors), so I'm guessing that the Mallory is bad???

    Compressor seems to be a funny (read as "cheap") type (at least from my thinking). Connecting rod is external and operates from an eccentric attached to motor shaft. Piston is an aluminum plate bolted to end of connecting rod with a "ring" sandwiched between them. Cylinder is an aluminum sleeve. Sleeve is all scarred up....probably needs replacing. Piston ring is pretty worn and (I'm sure) needs replacing too. Cylinder head is in good shape. Reed valves are in real good condition. They have a little carbon deposits where they seat on valve openings, but otherwise perfect.

    Have I got a "POS" or it this thing worth repairing??? I could buy the thing new at Sears for $200. Maybe I should put it out of its misery and bury it in the dumpster.

    Any advice will be appreciated??


  • #2
    I don't know about the "funny" compressor but can tell you about capacitor testing. I assume you are using one of the resistance or Ohm scales on the meter. You should use a mid scale, perhaps X 1000 which would give readings up to 100,000 Ohms or more.

    The resistance scales on a meter actually place a small voltage across the part being checked and then read the current that flows. So resistance can be related to that current via Ohms law I = E/R or R = E/I. The inverse relationship between current and resistance is why the top end of the scale is zero (maximum current = minimum current). Any way, when checking a capacitor after the reading has had time to stabilize, it should read as an open circuit or a large resistance. What actually happens when the meter is commected to a capacitor is that current does flow for a short time while the capacitor charges and then it stops and the reading stabilizes. So a good capacitor will show lower resistance for that short time and then will read either a very high value or infinity. How long this takes depends on the capacity of the capacitor. The higher the capacity, the longer it takes to stabilize. This time can give you a clue as to the actual value of capacity and some meters actually read it this way.

    Very small capacitors will stabilize very qwuickly and will just show a fast kick on an analog meter and perhaps only one high reading on a digital (perhaps none). Larger capacitors in the tens of micropfarad range or larger can be easily observed charging. This is an indication of a good capacitor so your Mallary is probably good. The GE is definitely bad as it should reach a high resistance faster than the Mallary.

    Another characteristic of a capacitor that this test can determine is leakage. A perfect capacitor should read infinite resistance after charging but all real ones have some leakage. The final reading will give a clue as to how high that leakage is.

    The GE is likely the original capacitor: replace it with one of the same value and voltage rating (or higher) and see what happens.

    Paul A.
    Paul A.

    Make it fit.
    You can't win and there is a penalty for trying!


    • #3
      Thanks Paul,

      I didn't remember that the meter fed current into the device being checked for resistance. I guess that explains why the Mallory capacitor showed (what I thought was) "sporatic" continuity. I'll probably replace the Mallory unit along with the GE. Especially since they show "different" MFD values, but primarily because I noticed (after original post) that the plastic case on the Mallory is cracked. Can't possibly be a "good" feature.......

      Thanks again for the info.......




      • #4
        Rodg -

        I just got done searching for some parts from Sears. They're usually pretty good about supplying parts and parts lists on line. Have you tried

        [email protected]



        • #5
          First off ohm meters are not to be used to test motor service caps,the caps are too big.

          Second the GE you say is rated at [email protected],if it has a metal case it is a run capacitor and probibly is good.The other is a starting capacitor and will hurt you if its charged and you touch the contacts,unless the case is cracked,the end is blown out or a terminal is burned off it is probibly also good.

          Third,everybody immeadiately suspects the caps on a motor as the reason for them not starting or overheating and tripping breakers,I service motors on a regular basis for customers,the ones that exhibit the behavior you describe are 90% of the time either centrifugal switches,or burnt start whindings and not the caps.Find a motor shop and bring them the caps for testing marking the wires so you can hook them back up later.

          If the caps check out,then the next step is the centifugal switch inside the back motor endbell,mark the case,remove the endbell and make sure the fly wieghts are able to move freely and clean the switch contacts with a fingernail or points file and reassemble.If that doesn't do it save the caps and pulley if there is one and pitch the motor because the start whinding is toast.

          As far as determining true motor hp,multiply the amps times the volts and devide by 746,then multiply by the motors effeciency which in your case should be about 85%.

          [This message has been edited by wierdscience (edited 06-06-2004).]
          I just need one more tool,just one!


          • #6
            15A is NOT 3hp. More like 2hp. I wouldn't run it on anything less than a 20A circuit. You have what is called an Oiless Compressor. Loud, I bet. Good thing you got it for free. My uncle got one and had to replace the rod twice. The third time, he scraped it and gave me the tank. Nice tank though.


            • #7
              No problem checking large caps, use 1 or 10k scale. (VOM)
              Observing polarity, needle heads for zero, then back towards infinity.
              Or,briefly hook up correctly, then revsrse leads, meter will go to zero, stay there awhile. (cap out of circuit)
              Sort of a go-no go test.


              • #8
                if this is "portable" how long was the line you ran it with?

                Had similar problem, realized that I had a voltage drop problem. The motor pulled enough current that the line voltage dropped, which reduced power and didn't let it pull out of "start" mode.

                Your problem with capacitors, if it has a cap problem, is with the larger starting cap. But be sure the line drop isn't a problem (don't use an extension cord) before suspecting other things.

                Start switch is a good possibility. If they lied about motor ratings (and they did), they probably under-rated the start switch.

                Keep eye on ball.
                Hashim Khan


                • #9
                  Sorry I didn't get back sooner guys, had to go help a brother install a hot tub. Man....are they heavy. Even without water...

                  Herb...I tried "partsdirect", but haven't gotten an answer yet. I didn't have a model# for the the compressor, so my part search went nowhere.....Had to e-mail them.

                  Wierd....the GE cap does have a metal case and the Mallory has a small crack in the side of the plastic case. I wouldn't have known the difference between the "start" and "run" capacitors if you hadn't said that, but I "did" mark the wires and fortunately "knew" enough to not touch the contacts.

                  CCWKen......I was only guessing at the HP, but I think your right. Shouldn't be more than about 2, but I've heard Sears tends to overrate their motors (for whatever reason).

                  PSD Ken.....yes, that seems exactly what the Mallory cap did. The GE cap, however, just swings over to zero and stays there.

                  J Tiers.......I have to admit.....I "did" use an extension cord. It was about 12 ft long, but I believe it is at least a 14 ga wire. I thought that it might be enough. Maybe not........Just to be sure, I'll take it out of the equation when I get the compressor put back together and see if that helps. I have to (at least) replace the cracked capacitor and find a new cylinder and piston ring before I hook it back up to the juice.

                  Right now, I'm trying to find out (approx.) what all of this is going cost and decide if the thing is worth saving. Sometimes "free" isn't as economical as we would like. I have a 2 year old "free" beagle that I have about $1000 tied up in. If the wife wasn't so fond of it (and me of the wife), they'd both be out of the house. Then I'd have more room for machinery.....HE, HE.

                  Thanks for the help gang.....I'll let you know what I find out.....




                  • #10
                    I'm confused about the nameplate amperage draw. Devices designed to operate on a 15 amp 117 volt circuit are not allowed to draw a continous current over 12.5 amps. If it is stated as a 15 amp draw then it must be run on a 20 amp circuit.
                    Free software for calculating bolt circles and similar: Click Here


                    • #11
                      Some companies, not naming names, have used a rating which is peak power draw at stall. An almost, but not quite, useless rating. That's the only way a vacuum cleaner can be rated at 6.25 horse, and still come with a plug that fits the standard 110 v socket. Just something to impress the consumer. Wow, man, 6 horse! That must really suck!
                      Anyway, Yours is probably a 1.5 horse, could be a 2 horse. It sounds like someone else had a problem with it not starting, and did some work on it. It's entirely possible that the motor didn't get enough power to it to start, especially under load, and that caused problems. I have a compressor that doesn't like being powered through a 25 ft extension cord, and it's not a high power job, I think it's a half horse. Sometimes it won't turn when starting against the tank pressure. I usually have it set at 75 lbs, with dropout at 90 lbs. It's a very quiet model, and the only indication I get is a humming. I can't tell if it's running, or not. (all self-enclosed) But if this one has a problem on an extension cord, a higher powered one will definitely not like that, and might be totally ok on a short run from the power panel.
                      I seldom do anything within the scope of logical reason and calculated cost/benefit, etc- I'm following my passion-


                      • #12
                        quote *That must really suck..

                        Yes, any dishonesty sucks in advertising.

                        I have a 7hp campbell hausfield that puts out 1.5 times what my 2hp did.



                        • #13
                          Alright time for a little logic,the start cap if cracked is probibly bad,but the next question is was the cap bad before you started it up or did the cap blow as the result of the overload?

                          The centrifugal switch could be the real culprit,you may want to check it out good before you try the new cap.Could save you $10.00
                          I just need one more tool,just one!


                          • #14
                            There's a multitude of people on the list with better electrical skills than mine, but what I'm wondering about and havn't yet heard anybody address is wether or not the compressor is worth the effort given your description of the mechanicals and your estimated cost to replace it. In other words: even if the motor was running perfectly would this be a usable compressor? I've not had good luck getting salvaged compressors to put out much presure and in the end have replaced them.



                            • #15
                              If meter reads zero, then cap. is shorted.