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Thread: Air compressor blowing circuit breaker

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  1. #1
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    Air compressor blowing circuit breaker

    I have a slow leak somewhere in my air line plumbing (which I possibly should not have installed in the walls...), This causes my air compressor to cycle maybe once or twice a day to refill the circular network of air lines.

    Every few days the compressor throws the 20 amp dedicated 120V circuit breaker. Compressor is an IR Garage Mate, 2HP, 20 gallon, about 2 years old. Air filters are clean, pump oil is maybe 2mm above the 'fill bubble', i.e. a tad overfilled.

    Any ideas why the breaker is getting thrown?

    Thanks,
    Mark McFarlane

  2. #2
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    IR offers these possibilities for "Motor overload trips or draws excessive current", I guess I should start by checking the belt tension:

    1. Lubricant viscosity too high.
    2. Improper line voltage.
    3. Wiring or electric service panel too small.
    4. Poor contact on motor terminals or starter connections.
    5. Improper starter overload heaters.
    6. Poor power regulation (unbalanced line).
    7. Drive belts too tight or misaligned.
    8. Compressor valves leaky, broken, carbonized or loose.
    9. Cylinder(s) or piston(s) scratched, worn or scored.
    10.Connecting rod, piston pin or crankpin bearings worn or
    scored.
    11.Defective ball bearings on crankshaft or motor shaft.
    12.Leaking check valve or check valve seat blown out.
    13.Ambient temperature too low.
    14.Bad motor.
    Mark McFarlane

  3. #3
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    From the outside looking in & understanding it is the panel/sub-panel feed circuit breaker that is tripping and not the motor mounter protection:
    - The first place I would start looking is at the compressor pump head's pressure release valve (unloader) (between the pump head & tank check valve that is connected to the pressure switch) adjustment. It reads as if the pump's head pressure is not always releasing and the motor is trying to start the pump with pressure on the head.
    - Next I would try cleaning the contractors in the pressure switch.
    - If not either of those do the trick, then I would check to insure that all the wiring connections are clean & tight.
    Last edited by Rob Charles; 11-14-2019 at 10:57 AM.

  4. #4
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    A 2hp compressor may be drawing 20 amps, and more on startup. Available voltage will have a normal variation from the grid and more in your building due to other loads. You might need to increase the available amperage or get a breaker that blows slower. First try replacing it with a new one of the same capacity just in case it is defective.

  5. #5
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    What other loads are on the same circuit? Maybe a bunch of lights, a heater, maybe the table saw is on the same circuit and running, etc. The startup current might be just enough on top of other loads. If it is a dedicated circuit just for the compressor then many other potential issues.
    Last edited by Pat Barry; 11-14-2019 at 8:15 AM.

  6. #6
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    When I had that problem it was an unloader valve sticking shut. Unloader valves release air in the cylinders so the motor isn't starting against pressure. Does the compressor, when stopping release a small amount of air? Had it previously? Otherwise a marginal circuit or circuit breaker would be a likely candidate. Could you run a heavy duty extension cord to the compressor from another circuit?

  7. #7
    A 2 HP motor will do poorly on a 20A 120 volt circuit, a 30A or larger circuit is required which is doable but it is much simpler to run it at 240V, 1 1/2 HP is the limit for common 120 volt circuits.

  8. #8
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    Quote Originally Posted by Rollie Meyers View Post
    A 2 HP motor will do poorly on a 20A 120 volt circuit, a 30A or larger circuit is required which is doable but it is much simpler to run it at 240V, 1 1/2 HP is the limit for common 120 volt circuits.
    Thanks Rollie. FWIW, the compressor has a NEMA 5-15 plug on it.

    Edit: This compressor is also rated at 1500 Watts, it's in the model name P1.5IU-A9, so thats 12.5 Amps. Perhaps the 2HP figure is from unscrupulous marketing.
    Last edited by mark mcfarlane; 11-14-2019 at 1:29 PM.
    Mark McFarlane

  9. #9
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    I don't want to interrupt the troubleshooting flow here but there are a couple of minor points I want to bring up.

    Quote Originally Posted by mark mcfarlane View Post
    ...check startup current with clamp on ammeter...
    Not many clamp meters are capable of capturing inrush current. If yours is not (and it should be in the manual if it is) and you motor has a proper nameplate you'll find it there. There will be a code letter for the Locked Rotor Current (LRC). It's usually labelled "CODE". Look up the code letter in the table:
    LR Codes.JPG
    Then make the appropriate calculation. For example, if your motor has a code G and a rated voltage of 115V then you start current is 2HP x 5.9kVA/HP x 1000VA/kVA / 115V = 103A.

    Quote Originally Posted by mark mcfarlane View Post
    ...This compressor is also rated at 1500 Watts, it's in the model name P1.5IU-A9, so thats 12.5 Amps. Perhaps the 2HP figure is from unscrupulous marketing.
    Both HP and Watts are units of power. Watt is metric and HP is Imperial. Motor ratings generally use a rounded number of 750 Watt per Horsepower. So 2HP x 750W/HP = 1500W. That is the rated LOAD not the input.

    Input power is output load divided by efficiency and is also measured in Watts. Apparent input power is input power divided by power factor and is measured in Volt-Amps. Input current is then apparent input power divided by input voltage. For example, a 2HP (1.5kW) motor with 85% efficiency and 90% power factor would have an input current at rated load of 1500/.85/.9/120 = 16.3A.
    Beranek's Law:

    It has been remarked that if one selects his own components, builds his own enclosure, and is convinced he has made a wise choice of design, then his own loudspeaker sounds better to him than does anyone else's loudspeaker. In this case, the frequency response of the loudspeaker seems to play only a minor part in forming a person's opinion.
    L.L. Beranek, Acoustics (McGraw-Hill, New York, 1954), p.208.

  10. #10
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    Quote Originally Posted by David L Morse View Post
    I don't want to interrupt the troubleshooting flow here but there are a couple of minor points I want to bring up.



    Not many clamp meters are capable of capturing inrush current. If yours is not (and it should be in the manual if it is) and you motor has a proper nameplate you'll find it there. There will be a code letter for the Locked Rotor Current (LRC). It's usually labelled "CODE". Look up the code letter in the table:
    LR Codes.JPG
    Then make the appropriate calculation....
    Thanks David, I can't see the language you have referenced on my compressor motor.

    IMG_4493.jpg
    Mark McFarlane

  11. #11
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    Quote Originally Posted by mark mcfarlane View Post
    Thanks David, I can't see the language you have referenced on my compressor motor.

    IMG_4493.jpg
    No, I don't see it either. Those are IEC specs not NEMA on that nameplate, but it's not complete.

    The laboratory method for measuring locked rotor current is to literally lock the rotor and then measure the current. You usually don't have a lot of time before the breaker opens but most ammeters are fast enough to get a measurement. It's not difficult, you just need a good way to keep the rotor from spinning.
    Beranek's Law:

    It has been remarked that if one selects his own components, builds his own enclosure, and is convinced he has made a wise choice of design, then his own loudspeaker sounds better to him than does anyone else's loudspeaker. In this case, the frequency response of the loudspeaker seems to play only a minor part in forming a person's opinion.
    L.L. Beranek, Acoustics (McGraw-Hill, New York, 1954), p.208.

  12. #12
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    I doubt that the apparent leak in the network is related to the breaker thing. The circuit itself is the first place to look. Even though this machine will run on 120v, it's still wanting it's own circuit so if it shares, it could merely be a coincidence of timing. It could also be a bad breaker. It could also be something like Curt describes.
    --

    The most expensive tool is the one you buy "cheaply" and often...

  13. #13
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    The unloader is your 1st place to look. They get "sticky" (and a bit later just stop working altogether). They are not difficult to replace.

  14. #14
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    Quote Originally Posted by Lisa Starr View Post
    The unloader is your 1st place to look. They get "sticky" (and a bit later just stop working altogether). They are not difficult to replace.
    Thanks Lisa Is this the unloader? It does actuate when the compressor shuts off.

    unloader.jpg

    Does one typically just replace the little brass fitting with the valve, or the actuating arm assembly which is part of the pressure switch? Unfortunately my manual and IR's web site don't even show this valve as one of the parts.
    Mark McFarlane

  15. #15
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    The first thing I would try is to lube the bearings/bushings on the motor, whether they are designed to be lubed, or not. I have a motor on a wet tile saw, that was doing the same thing, that has kept working for several years, so far.

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