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Compressor motor - high current draw

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  • Compressor motor - high current draw

    Greetings all;

    I bought a small 1 hp (real 1 ph - 13.2A, motor plate: ) twin cylinder compressor off eBay about 10 yrs ago. Seller poorly described the item and I guessed looking at the images that the motor was at least a 3/4 hp. Won the auction for just under $5 and paid the seller an additional $4 just cause I felt sorry for him. THe comp was old w/o electrical controls - instead there was a pressure switch that unloaded compressor valves when the comp reached pressure - motor ran all the time the unit was plugged in. I liked it enough to add conventional controls w/ a pressure activated relay and pressure release valve for starting.

    This machine has always needed to be on a 20A breaker. A 15A with no other activity on it would trip when the pressure reached about 70 psi in the tank. Fast forward 10 years of occasional use and now it is straining while running and trips a 20A breaker around 80 psi. Not a breaker issue as I have experienced this on 3 separate breakers in two separate locations.

    I presume the single capacitor is start only and is not used for run - correct? Motor has no issues starting. Motor has slip rings and centrifugal switch - I presume to drop the cap out of the circuit - correct? Looking at the cap, I see what might be a bulge in the top of the cap - looks to be a tar top - not sure if this is a bulge or just how the unit was made. If not, what range of capacitor is reasonable for 1 ph - since the cap size is worn off of the cap can.

    I figured on putting a smaller drive pulley on the motor - I am guessing that the drive pulley was a bit too large from the get go. With the small tanks, it builds up pressure very quickly and for how I use it a slightly slower fill rate would not be an issue. After removing the belt to remove the pulley, I could hear a dry sounding bearing so I took the motor partially apart to access the bearing (on the pulley end).
    Sticking my finger into the opposite motor shaft bearing it fees smooth and not dry. If I decide to replace it, is there anything I need to be aware of before dismantling the contacts for the slip rings? My very limited motor dismantling is limited to universal motors w/ brushes and one 3 phase; I have never dismantled a motor w/ slip rings before.

  • #2
    The cap size looks like it used to say 500.
    Location: Long Island, N.Y.


    • #3
      It does not have slip rings if it is cap start. It has washers that work a centrifugal switch when weights fly outwards sufficiently.

      Take note of the assembly order of parts and put it back the same exact way if it has to be taken apart.

      500 uF seems high but it is a 115V capable motor, so that is a possible value. Usually such capacitors are labeled with a range, such as 108-130 uF. That's an old part, and may not have been labeled the way newer ones are.

      "trips a 20A breaker" is a sort of measurement, but a real current measurement would be better if you can.

      1) take off belt. Start motor and measure current. Should be about 40% of full load current, possibly 6A.

      2) If that looks OK, then measure when the tank is pumping up. Watch carefully to see if the current suddenly jumps up and if the motor seems to slow down. That would indicate the start circuit might be cutting in again.

      If the bad bearing is the one near the pulley, as I suspect, the problem may be partly that the bearing is not turning freely. Probably it will be hot. replacing it will return the unit to at least as good as it was when you got it. But it cut out early then too.

      A smaller pulley would reduce the power needed, so long as the oiling is still good. Most rely on splash oiling, and need a certain speed to get it.
      Last edited by J Tiers; 03-13-2017, 03:09 PM.

      Keep eye on ball.
      Hashim Khan


      • #4
        J Tiers:

        I stand corrected on the slip ring comment. I had never seen a motor disassembled that had slip rings, or a centrifugal mechanism, (only read of slip rings in one of several forums I read) and made an incorrect assumption - guess I should have gone to Google Images before posting that.

        Thank you for the long detailed reply, especially: " Watch carefully to see if the current suddenly jumps up and if the motor seems to slow down. That would indicate the start circuit might be cutting in again." In it you may have pointed me to the problem even though I did not see it as significant or mention it - mainly that the plate that retains the bearing in the endbell (pulley end) had lost its retaining screws and was loose on the armature shaft. This may have allowed the armature to shift in the direction of the centrifugal switch. The motor made s struggling sound, possibly a loping rpm by the sound of it just before the breaker tripped. I was unaware of the loose plate and the noise of the plate free on the armature was only a second or two as the motor spun up (and spun down, and certainly not noticeable over the pumping of the compressor).

        I had intended to remove the bearing on the pulley end (the one that sounded dry) and replace it since I was about to buy a smaller pulley anyway. Then last night (initially) I thought I would avoid the risk of trying to remove it (not much space for the bearing separator and the cooling fins: ) and simply remove the bearing's outer labyrinth shield and relube it - bearing felt very smooth after a relube. I reassembled the armature and end bell and then felt some looseness in the bearing that was not detectable without the extra leverage of the endbell. I should have just ignored it and reassembled the motor completely (and gotten another decade of limited use out of the bearing). Then I thought: I have the motor apart, I should do it right and replace the bearing . Instead, I have this: Now I really wished that I had left the old relubed bearing in place. I have done a minimal amount to gas welding of aluminum but I don't think that I will have any success w/ the thin fin section and the hub of the fan drawing heat into the armature. And its not like I can preheat the armature/fan asm over one of my kerosene heaters and not damage the varnish. What are the odds that I will be able to remove the fan from the armature w/o breaking off more blades. I know that I can put a flame directly on the hub of the fan to expend it a bit. Could the blades be tig welded in place (without removing from the hub)? I could take it in to work and ask a favor of one of the machinist/fabricators to weld it for me provided I properly clean all the paint dust off of the fan. How much air does this fan acutally move? If it were only one blade instead of two adjacent blades, I would ignore it and reassemble the motor - the weight of the fan blade is insignificant to the rotating mass of the armature.


        • #5
          I would NOT try to remove that fan. It may even be cast as part of the rotor bar casting, which may also form the mounting of the rotor on the shaft (the iron in the middle is not needed for 4 pole motors, as you see from the cooling holes through).

          TWO broken-off blades WILL make an imbalance, due to the large diameter of the fan. For a compressor, the vibration may not be an issue. Don't know on the TIG welding of die casting aluminum, I am rotten at aluminum welding, but I would suppose it to be quite possible, especially since the rotor has no windings, just bars of copper and aluminum (it looks like a dual cage rotor).

          The bars were put in by die casting them around the shaft and through the rotor core. Varnish would have been cooked anyway. Usually the insulation is by oxide (mill scale) on the surface of the sheets, which molten aluminum would not bother.

          You probably DO want the cooling, compressors get hard use, so if you can weld back, or substitute blades of sheet material, it might be good. That's a rather crude fan, not terribly efficient to begin with. No sense reducing it further. I cannot give an estimate of the cooling provided, but as a naive estimate, you had 6 blades, now you have 4, you could assume the cooling has been reduced by 1/3 of the total air movement.

          If welded back, get the blades within the diameter of the rotor, and get them welded solidly. The rotor will be spinning at motor speed, and you do not want them coming off and gouging up the works.

          Keep eye on ball.
          Hashim Khan


          • #6
            My first thought is that the capacitor is not causing the problem. It should only be in the circuit while starting, and your problem is happening when you get to a high pressure.
            The first thing I would look at is the wiring to the motor. Are you using an extension cord? Bad idea on such a high current motor. Is the motor cord a large gauge wire? If it is too long and too small a gauge, it will drop too much voltage.
            You are right that a smaller pulley will reduce the required power, and that may be all you need.


            • #7
              Something to know about air compressors. Virtually all manufacturers size the motors and pulleys to be overloaded shortly before it reaches shut off pressure. This maximizes the air you can get out. The only way the compressor gets to full pressure is when you are not using much air, so it soon shuts off. If you use lots of air the high pressure will not be reached so the motor will never be overloaded. So no surprise that you may trip a 15 amp breaker.


              • #8
                Thank you all for your responses.

                Gary & Gary:
                This is my portable (the older I get the heavier the compressor seems to get) construction use compressor so I am frequently using it w/ a 12 ga ext cord. Most of my houses only have 20A readily available in the kitchen or bath and the comp is frequently somewhere outside. Mostly I use it to run nail guns and staplers so the air usage is not that great, and why I am more than willing to reduce the drive pulley at the expense of CFM.

                Judging by the surface finish (in a small are of the fan where I cleaned off the paint dust), the fan appears to be a sand casting. I'll look closely but I think the fan is pressed (shrink fit?) onto the shaft (from memory, I do not recall anything else on the rotor asm looking like a sand casting. I have almost no knowledge of how electric motors (especially older ones) are constructed or dismantled. I guess I'll tape and bag the rest of the armature asm, sandblast the fan and the broken off blades and take it in to work to have someone else TIG weld it. I am somewhat confidant that with my limited alum gas welding skills, I could weld the ears back on if the casting is in fact a sand casting (welds much easier than a die casting) and if the fan were removed from the armature so I am not dealing with a massive heat sink issue. I keep telling my wife that I **NEED** a tig. She would tell to just quit breaking stuff.


                • #9
                  Not knowing the air compressor. Is it a two stage or single stage if is two stage compressor you may have a check valve is not holding high pressure in the second stage and the first stage is trying to do the job so now motor is overload.


                  • #10
                    I have an Emgo portable framing compressor. POS will not start on a 15amp breaker, and is marginal on any (even short) extension cord unless 10+ gauge and 20 amp breaker.

                    My buddy is a construction GC. He always has problem with all his compressors except the "continuous run" type with load/unload valves. It's not hard or expensive to put a real unloader on the compressors, but few do.


                    • #11
                      I did the opposite - removed the continuous run by disabling the load/unload valves and adding conventional motor relay with pilot unload controls to this comp. With the original load/unload valves, I could not set the upper pressure limit over about 80 psi - Not enough to reliably run staplers and nailers.

                      Originally posted by lakeside53 View Post
                      ...always has problem with all his compressors except the "continuous run" type with load/unload valves. It's not hard or expensive to put a real unloader on the compressors, but few do.


                      • #12
                        I just ran the model number of the capacitor, 2020408 that I see in your photo. It is100mfd. with any additional info you might want about it.

                        Sent from my SM-G900V using Tapatalk


                        • #13

                          I think you made the same mistake that I made when I began to search for this cap. I found numerous values for the same "P/N". Then I realized that the number visible on the cap is a patent number. Once I realized that, the legalese "ET ALL" after the patent number made more sense.

                          Originally posted by 6PTsocket View Post
                          I just ran the model number of the capacitor, 2020408 that I see in your photo. It is100mfd. with any additional info you might want about it.

                          Sent from my SM-G900V using Tapatalk


                          • #14
                            Greetings all:

                            I have monkeyed w/ this motor for too much time the past week. First I attempted to weld the two fins back on but I did not figure out a good way to fixture the broken off fins until I managed to destroy them. Out of desperation I cut new fins out of some 0.125 thick aluminum (guessing 6061) and welded them on. Since there is no draft on my new fins (and they are also a bit smaller than the remaining original 4 fins, I left the junction where the fins joins the hub much thicker. With the CG of the fan offset from where the fan hub mounts to the rotor, I did not have a reasonable way to try and balance the fan.

                            The motor runs and air is being drawn in from the opposite end of the motor and pushed out of the fan (pullley) end. Motor, after replacing both bearings, does not run as uniformly quiet or smooth as I would expect. With the heavy rotor, I am not thinking that the fan's probable imbalance is having a major effect. I made a quick video of the motor starting up, running and coasting down but its 70 MB and not loading well onto Photobucket.

                            Before I unbox my new pulley, I need to decide if this repaired motor is a keeper. Since the capacitor leads were spliced in multiple places w/ insulation crumbling, I replaced one of the capacitor leads. As I was putting the cap back into the cap housing, the crimped on eyelet on the capacitor came off. Choice words were screamed loudly. I cleaned off the aluminum corrosion around the tubular fitting on the capacitor and tried to crimp the eyelet back on but the tubular feature (due to corrosion) is about 1/32 inch too short to yield a crimp that I think that I can rely on. The eyelet is in place at the moment and the motor run video was taken after the cap eyelet was "crimped" back on. So if I keep the motor, I feel that I need to replace the cap and yet I do not know its value. Are there any guidelines as to cap size relative to motor size (FWIW, this is a 110/220 motor)?

                            J TIERS: responding to an earlier comment you made: running the repaired motor w/o a load, it draws 10.4 amp as measured w/ my "kill-a-watt" energy consumption monitor.


                            • #15
                              I would NOT expect much more than 6.6A draw for that motor unloaded.

                              I do NOT know how a "kill-A Watt" reads current, but I'd not expect it to be off that much no matter what. Makes me wonder if the start circuit is being disconnected. It is fairly easy to re-assemble in such a way that the start switch does not work right, so you may want to verify that, or at least watch for the drop-off of current as the motor comes up to full speed.

                              An analog meter with a needle and scale is far easier to get that information from than a digital meter such as the 'Kill a watt" probably has.

                              Keep eye on ball.
                              Hashim Khan