Shop air compressor Electrical issue

[da prez]

  I have a 3HP piston compressor. Bought it in December of 17. The other day , it would not go to speed. It would blow the circuit breaker.  I removed the belt and the motor  would start and run. I tested the start capacitor . It checked good along with the run capacitor. Reinstalled and put the belts on. Same thing. I purchased a new start capacitor. Same thing. I did not replace the run capacitor as it also checked good. I hooked to a different power supply. same issue.
 WHAT AM I MISSING.

                                               da prez

[Factor]

With a replaced start capacitor that still doesn't get it rolling, I'd look at either the electric motor having too much resistance (bad bearing? rotate by hand to check) or the actual piston/crank having too much resistance (rotate by hand, check oil, pull head off and look at cylinder walls). I guess it could also be some poor electrical connections that are limiting the amps from the start capacitor getting to the windings of the motor. Also check your motor bushing and clean up the contact surface.

[WintrSol]

You say you disconnected the compressor from the motor by removing the belts, right? That means it is all in the motor. An electric motor will draw as much current as it can to overcome resistance to its designed run speed, blowing the breaker, which means it is unlikely the brushes (if it has them). That leaves something binding between the rotor and stator. Something could have been sucked in with the cooling air and jammed between them, or a bearing may be failing; if it uses solid bearings, one could be worn out of round, letting the rotor touch the stator. It is possible the brushes (if) are shorting, and cause the same effect - easy to check on inspection.

OK, I re-read your post, and it said the motor seemed to run fine when the belt was removed. So, back to the compressor ...

[Challenger]

Have you checked to make sure the compressor unloader valve is functioning? I had the exact symptoms and had to replace mine.  Too much torq needed to start with pressure on heads.

[JimC]

Prez,
I am guessing that you checked this already.
I had one do the same thing, ended up being clogged air intake. I had just finished painting a car and it filled the filters solid with paint.
Jim

[da prez]

I checked the unloaders,  air filter and the compressor pump
 I misstated initially
 The motor would not go to speed on its own. After two attempts,  it started working
 This is not my first time with electric motors. I am still wondering what I missed. If this compressor has 50 hours on it , I would be amazed.

                 da prez
 

[WintrSol]

I checked the unloaders,  air filter and the compressor pump
 I misstated initially
 The motor would not go to speed on its own. After two attempts,  it started working
 This is not my first time with electric motors. I am still wondering what I missed. If this compressor has 50 hours on it , I would be amazed.

                 da prez
 

OK, back to something that got jammed between the rotor and stator, or a rotor making intermittent contact with the stator. Can you open it up? I know many motors that size can't be opened, and then still be able to be put back together.

[Steel cowboy]

Try this, you stated you used another power source. I’m assuming you meant another outlet, but is the new outlet on the same circuit as the first? You could have a weak circuit breaker. If it’s a GFCI circuit that could be a problem too.

[WintrSol]

Try this, you stated you used another power source. I’m assuming you meant another outlet, but is the new outlet on the same circuit as the first? You could have a weak circuit breaker. If it’s a GFCI circuit that could be a problem too.

Also, since this changed recently, it could be the wire terminal at the breaker has loosened, which creates heat, which not only reduces power to the motor, but will pop the breaker.

[da prez]

Different outlet , different circuit. I'll have time tomorrow.  Thanks for the help.

                 da prez