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  1. #1
    Join Date
    Jul 2006
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    Canberra
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    288

    Default Air compressor motor tripping off -- but why?

    Air compressor motor has started tripping off at the motor overload switch. I'm wondering if this sounds familiar to anyone. Motor is a 2.3kw 13A cap-start cap-run single phase, Chinese origin, mfd 2004. The compressor is just a standard portable 50L twin-cyl DIY type, Chinese cast-iron pump.

    Here's my three tests:

    1) if the drive belt is taken off, so that only the motor runs by itself, it will run fine. So it's not the caps I think. When I switch off the motor I can hear the centrifugal switch click back one second later. I haven't taken it apart to look inside.

    2) if the belt is on, so that the motor is driving the compressor *but the air release cock on the tank is left open* (i.e. running the compressor but with no pressure build up), the motor runs for about 1 to 4 minutes (varies for no discernible reason), then the breaker in the capacitor box cuts it off.

    On these "belt on, tap open" test runs, the caps are cool, motor is mildly warm to touch, there's no smell. Can't hear the centrifugal switch because the pump is still making a racket when the switch would click back (about 1 second after the power is cut).

    3) if the air release tap is closed so that pressure builds, the motor runs for about 30 seconds then cuts. The pressure is then around 300 kPa (40 psi). Mild temperatures on motor and caps as above.

    I took off the head off the pump last year to check the reed valves (seemed fine, gave them a clean) and put in new gaskets then. So I'd be surprised if it's a pump problem. Also the Condor MDR-2 pressure switch is new last year.

    So whaddya reckon?

  2. #2
    Join Date
    Sep 2005
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    Bendigo
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    60
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    Default

    Is it on the end of a long cable run or extension lead ?
    Check the voltage with the unit running to discount voltage drop.

  3. #3
    Join Date
    Jul 2006
    Location
    Canberra
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    Default

    Quote Originally Posted by droog View Post
    Is it on the end of a long cable run or extension lead ?
    Check the voltage with the unit running to discount voltage drop.
    Good point. The 15A gpo is maybe 20m cable run from the meterbox. But there's a 15A outlet directly at the meter box. I'll wheel the compressor around there tomorrow morning. And if need be, then check voltage wile running.

    Voltage drop through the cabling might be consistent with the problem, which seems to be that as load increases, the time to trip-off decreases

  4. #4
    Join Date
    Mar 2012
    Location
    australia
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    20

    Default

    Could be a thermal fuse. If it has one then the motor will cut out when the fuse gets too hot. If the fuse is on the windings inside the motor (the only place I've seen them) then it is possible the windings overheat but the motor housing only feels warm.
    The thermal fuses are pretty simple (I'm guessing here) so it may be unlikely that it is faulty. But you never know!
    Consider dismantling the motor to check the windings for discolouration. Also consider just replacing the thermal fuse if it is not too hard to get too. It may just be a case of desoldering the old fuse connections and resoldering a new fuse.
    I'm no expert by the way but I've played around with a few motors.
    Oh, if you have an infrared thermometer then maybe that can be pointed at the windings or internals while the motor is running. Then you can see the temperature of the internals and compare to the motor housing.
    Lastly, before dismantling the motor, measure the current draw of the motor and compare it to the nameplate specs. That may give some hints.

  5. #5
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    Jul 2006
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    Canberra
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    288

    Default

    Quote Originally Posted by harry wall View Post
    Could be a thermal fuse. ... Lastly, before dismantling the motor, measure the current draw of the motor and compare it to the nameplate specs.
    I'll check the voltage and the behaviour of the motor on the 15A outlet at the meter board. If that solves the immediate problem, I'll have to think about the house wiring. But if it doesn't solve the problem then I'll look at two things -- how to measure the current (I don't think I've got any way to do that, not on 240V mains), and also that thermal fuse possibility.

    ian

  6. #6
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    Mar 2012
    Location
    australia
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    20

    Default

    If you have access to the mains wiring then a clamp meter around the active feed to the motor will give you current. Otherwise maybe the 15A equivalent to this 10A type of power meter.... Mains Power Meter | Jaycar Electronics. I have the 10A version and find it very handy.

  7. #7
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    Sep 2005
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    Default

    Quote Originally Posted by Gunnaduit View Post
    I'll check the voltage and the behaviour of the motor on the 15A outlet at the meter board. If that solves the immediate problem, I'll have to think about the house wiring. But if it doesn't solve the problem then I'll look at two things -- how to measure the current (I don't think I've got any way to do that, not on 240V mains), and also that thermal fuse possibility.

    ian

    Based on your comment of what occurs “then the breaker in the capacitor box cuts it off.
    a thermal fuse is unlikely to cause this.
    I would be having a second look at the run cap.

  8. #8
    Join Date
    Oct 2007
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    Alexandra Vic
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    Default

    Have you checked the oil level in the compressor sump? Low oil would increase friction and compressor temperature, and hence load on motor. You are seeing an increase in load between the motor free running and driving an effectively unloaded compressor (to be expected), and a more substantial load increase as the compressor loads up and warms up (again to be expected). But if the oil level is low, friction increases and the compressor heats more rapidly and needs more power than previously to meet the same target.

    Have you noticed that the motor is more sluggish starting and getting up to speed, that could indicate that the start cap is becoming weak, meaning that the start winding is remaining energised longer and causing overheating in the motor windings, so the thermal margin (difference between internal temp of motor and trigger point of thermal cutout) for operation under load is reduced, making it possible for the residual heat from the start winding and the heat from operating under load combine to trip the cutout.
    I used to be an engineer, I'm not an engineer any more, but on the really good days I can remember when I was.

  9. #9
    Join Date
    Nov 2016
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    Gulfview Heights, Adelaide
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    83

    Default

    I would check the voltage at the motor first. If that is ok, then I would replace the capacitor.
    On single phase motors, I have found capacitors to be a common problem, especially as they get older.

  10. #10
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    Jul 2006
    Location
    Canberra
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    Default

    Quote Originally Posted by malb View Post
    Have you checked the oil level in the compressor sump? ... Have you noticed that the motor is more sluggish starting and getting up to speed ...
    Good diagnostics. But: the oil level is ok -- I'd refilled it and cleaned the sight glass last year -- so I can see what's going on there. And checked yesterday. No, I don't notice the motor to be any more sluggish than when it runs with the drive belt off, i.e. than when it runs at no load.

  11. #11
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    Jul 2006
    Location
    Canberra
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    Default

    Quote Originally Posted by paul33 View Post
    I would check the voltage at the motor first. If that is ok, then I would replace the capacitor.
    On single phase motors, I have found capacitors to be a common problem, especially as they get older.
    So I wheeled the unit around to the meterbox which has a 15A outlet. This is 5 metres from the street mains pillarbox (to explain: most of Canberra has underground power, no overhead wires). I measured the line voltage at the 15A outlet as 240V. I also measured the voltage at the motor terminals as the compressor was running (and pumping at pressure). It wavered from 239V to 240V, then the motor cut out again after about 30 seconds running. (I couldn't see the start voltage, because I can't get the test probes wedged by themselves against the terminals). From what I observed, you'd have to say the voltage is pretty well ideal, better than many houses would have. Can't be a voltage drop problem.

    Yeah so the caps *might* be the problem. I'd only come across motor caps totally failing, as in, motor no longer starts. Do they have a flaky-under-load kind of failure mode? Anyhow, I've found a test procedure for them: "Electric Motor Capacitor Test Procedures" https://inspectapedia.com/electric/M...itor_Tests.php, so I'll check it out later today. I've got replacement caps lined up for purchase if it looks like the caps are, after all, the source of the problem.

    Ian

  12. #12
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    Jul 2006
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    Athelstone, SA 5076
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    4,255

    Default

    Quote Originally Posted by Gunnaduit View Post
    So I wheeled the unit around to the meterbox which has a 15A outlet. This is 5 metres from the street mains pillarbox (to explain: most of Canberra has underground power, no overhead wires). I measured the line voltage at the 15A outlet as 240V. I also measured the voltage at the motor terminals as the compressor was running (and pumping at pressure). It wavered from 239V to 240V, then the motor cut out again after about 30 seconds running. (I couldn't see the start voltage, because I can't get the test probes wedged by themselves against the terminals). From what I observed, you'd have to say the voltage is pretty well ideal, better than many houses would have. Can't be a voltage drop problem.

    Yeah so the caps *might* be the problem. I'd only come across motor caps totally failing, as in, motor no longer starts. Do they have a flaky-under-load kind of failure mode? Anyhow, I've found a test procedure for them: "Electric Motor Capacitor Test Procedures" https://inspectapedia.com/electric/M...itor_Tests.php, so I'll check it out later today. I've got replacement caps lined up for purchase if it looks like the caps are, after all, the source of the problem.

    Ian
    always buy a meter that can check capacitors...save a lot of messing around.

  13. #13
    Join Date
    Sep 2012
    Location
    York, North Yorkshire UK
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    Default

    Hi Ian, Guys,

    Yeah so the caps *might* be the problem. I'd only come across motor caps totally failing, as in, motor no longer starts. Do they have a flaky-under-load kind of failure mode? Anyhow, I've found a test procedure for them: "Electric Motor Capacitor Test Procedures" https://inspectapedia.com/electric/M...itor_Tests.php, so I'll check it out later today.
    It depends upon the type of internal construction of the capacitor ! In older capacitors, the insulation medium is usually waxed paper ! These tend to give very little trouble, often lasting for many many years. Others use a insulating liquid soaked paper for insulating the capacitor plates from each other, more modern types don't even have the paper in them. This type are known as electrolytic capacitors.

    In these the liquid eventually boils off and the capacitance value reduces over time, this boiling off of the insulating medium determines the life of this type of capacitor. Unfortunately most modern capacitors use an electrolyte simply because its cheaper than waxed paper.

    Also much higher capacitances can be put into smaller canisters, where a waxed paper capacitor may be, say 1.5 inches in diameter and 2.5 inches long and have a 25 mfd value, the same size can could have a capacitance value of 75 mfd or more. Waxed paper capacitors will also withstand higher operating currents than electrolytic ones.

    HTH:
    Best Regards:
    Baron J.

  14. #14
    Join Date
    Apr 2021
    Location
    Adelaide
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    Default

    Quote Originally Posted by BaronJ View Post
    Others use a insulating liquid soaked paper for insulating the capacitor plates from each other, more modern types don't even have the paper in them. This type are known as electrolytic capacitors.
    There have been a large number of oil soaked paper capacitors and oil filled capacitors that are not "electrolytic capacitors" but they do dry out and sometimes leak over time. At one time the oil was a PCB.

    These were often used for ballast and electric motor applications as well as many other high voltage and high current applications.

    Jack

  15. #15
    Join Date
    Oct 2011
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    One thing I would check is the inlet air filter. If it is clogged, the motor is having to work harder to draw air in.

    Michael

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