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28th Nov 2022, 05:43 PM #1
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?
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28th Nov 2022, 06:19 PM #2
Is it on the end of a long cable run or extension lead ?
Check the voltage with the unit running to discount voltage drop.
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28th Nov 2022, 06:29 PM #3
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
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28th Nov 2022, 07:13 PM #4Novice
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- Mar 2012
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- australia
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- 20
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.
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28th Nov 2022, 09:00 PM #5
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
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28th Nov 2022, 09:39 PM #6Novice
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- Mar 2012
- Location
- australia
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- 20
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.
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28th Nov 2022, 10:01 PM #7
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28th Nov 2022, 10:16 PM #8
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.
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29th Nov 2022, 08:04 AM #9Member
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- Nov 2016
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- Gulfview Heights, Adelaide
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- 83
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.
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29th Nov 2022, 09:58 AM #10
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.
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29th Nov 2022, 10:20 AM #11
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
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29th Nov 2022, 10:58 AM #12Most Valued Member
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- Jul 2006
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- Athelstone, SA 5076
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29th Nov 2022, 08:16 PM #13
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.
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.
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30th Nov 2022, 11:48 AM #14Senior Member
- Join Date
- Apr 2021
- Location
- Adelaide
- Posts
- 205
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
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30th Nov 2022, 05:55 PM #15Philomath in training
- Join Date
- Oct 2011
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- Norwood-ish, Adelaide
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- 59
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- 6,540
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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