7.5hp air compressor Blows breaker


(Pendleton, OR)

I have a 7.5hp air compressor that is rated at 33.2 amps at full load. It starts and fills the 120 gal tank but when the air pressure drops and it starts the second time it comes up to speed then runs for 20 seconds or so and blows the 50 amp breaker. None of the wireing feels warm and neither the motor or the compressor are hot. Any ideas.

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Bill says...

With your air compressor off, drain the tank entirely. Start the compressor. Runs Ok?

Now, with the compressor off, drain the tank completely again, start the compressor again. Runs OK?

Now, with the compressor on, open the tank a bit, or bleed off air to the point where the compressor tries to start automatically. Does the breaker pop?

If yes, look to your UNLOADER VALVE.

Post a comment here to let us know how it turned out, OK?

Cheers,

Bill




Comments for
7.5hp air compressor Blows breaker

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Nov 16, 2011
 10hp compressor
by: Robert Swinburne

Hi, I have a Atlas copco compressor, I bought it second hand, installed it and fired it up, started fine and built pressure like a champ, atleast for the first fill up I found out later.But now that I got all the plumbing the problem was clear.When I started the compressor it like the first time built up pressure fine. When set pressure is reached it shuts off like it should. but seconds later it kicks on and off 2 times and breaks the 240v 33 amp fuse.
What should I adjust to make this go away, I thought it should build pressure,shut off and wait until the tank had been "drained" of pressure until a set level, kick on again and build until set max pressure, NO stop start stop start after pressure is built. Any Advice ???????????

Kind Regards
Robert D.Swinburne

Norway

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My first thought, that since it is the pressure switch that controls the on / off of the compressor, is that your switch has a problem. Pull the cover, turn the compressor on, and watch the pressure switch innards operate. Be very careful as there are live wires in there.

Watch as it reaches the cut out pressure and trips off. Then, continue watching. Does it trip off and on again, or is there an obvious short?
Sep 21, 2011
 rotary air compressor
by: Christian Kuboushek

You bring up some interesting points regarding your air compressor. I have my contracted employees using a similar 7.5hp air compressor, I'd be interested to know how many other contractors do that as well.
Jun 15, 2011
 7.5hp air compressor blows breaker
by: Anonymous

I finally got to the bottom of it. The transformer that supplies my shop is too small. I?m going to try a 5hp with a smaller pulley and see how that works. I should still get about 20CFM.
Jun 15, 2011
 Compessor Blows breaker
by: Anonymous

I have a very similar problem to this, except mine doesn't run at all on the refill. As soon as it calls for more air, the compressor tries to start and shorts the breaker immediately. Does anyone know what this issue is?

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Review the page on UNLOADER VALVE to see if this is the the issue. An easy check and a problem easy to spot.
May 11, 2011
 7.5hp air compressor Blows breaker
by: Anonymous

The unloader is OK. I am using 8 gage wire running about 10 ft. with a 60 amp breaker. The motor is new made in China. When I got it both the run and start capacitor were bad. I replaced them with new ones from the manufacture. The compressor turns easy. The mag starter hasen't needed to be reset. The moter runs fine up to pressure then shuts off and the compressor unloads, (you can completly drain the tank and it doesn't make any difference) when it starts the second time it runs for a few seconds (20 to 30) then blows the breaker. If I let it sit over night it will again run for the first air fill but not the second. It has a 120 gal tank and a large 2 stage Kellogg American compressor head. It puts out between 35 and 40 CFM at 150 pounds I have the pressure switch shut off at 125 pounds and come back on at 90 pounds

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OK, now I'm wondering if maybe you have a pressure switch problem? While I can't nail down the specifics, if it starts at zero PSI and runs up to cut out, then you use air, and when it goes to restart, it runs for 20 seconds and then the breaker pops.... is it reasonable that the compressor pressure switch shorts out some how?

Anyone else have any ideas?

Bill

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