Compressor motor hums eventually starts

[homeputter]

My 1 hp 120vac compressor motor sometimes just hums and does not start up until the breaker trips. Reset the breaker and it will start right up. Sometimes it hums for a few seconds and then starts. I replaced the capacitor, even though the old one tested good. No change.
Is this a winding problem? Any ideas?
Thanks

 

[Steevo]

What type of compressor is it?
Is the pressure switch releasing head pressure like it should?
Each time it stops running, you should hear a "ssssssssss" that is the pressure switch releasing head pressure so it can start without the resistance.

 

[nehog]

Sounds like the unloading valve is not unloading and the compressor is trying to start while fully loaded. There is a chance the start capacitor is failing, but that would be less likely given that sometimes it works, and some times it doesn't.

 

[phiftyseven]

I agree with the above respnses. Usually the first suspect is the unloader valve which releases head pressure after each cycle. Without that release, the motor is trying to turn the pump which has pressure in the cylinders, and will overload your motor. The capacitor will also cause a similar non-start situation, but you indicate you already replaced that, so it's probably air realted. One last possibility is the tank check valve. If that goes bad, or leaks, it would also feed air back into the cylinders, again creating a hard start situation.

 

[homeputter]

The compressor is a single stage, cast iron compressor.

How do I check the unloading valve?

I don't believe that is the problem, because the symptoms will happen when 1st starting after the compressor has been unplugged for months. The compressor is at my cabin so I can't go out an check it until later this week when I make the trip to the cabin.

If I can figure out how to check the unloading valve, I will check it anyway, when I get back up there.

Thanks for all the suggestions. Any other possibilities?

 

[larry_g]

To check the unloader one can usually listen for the 'hiss' of escaping air when the compressor reaches cutoff and stops. The release of air is the air escaping from the pump and fill tube.

One other thing that has not been mentioned is the centrifugal switch (CS) in the motor. The CS engages the start capacitor and helps with the initial start torque. When the motor approaches operating speed the CS opens removing the start cap from the circuit. If the contacts in the CS are dirty then you can get the symptoms of intermittent bad starts.

lg
no neat sig line

 

[Milton Shaw]

Check the switch, may find spiders or wasp nest in the motor, especially if it's one that is not used very often. If there is no pressure in the tank and still having the problem then its in the motor and switch instead of the air system.

 

[homeputter]

Thanks! I will check the centrifugal switch.

 

[rlitman]

Thanks! I will check the centrifugal switch.

Yeah, if the switch isn't engaging, the start capacitor and windings never gets power.

 

[Steevo]

The fact that it will exhibit these symptoms even after month of disuse may indicate a different issue than head pressure.
Is this compressor always plugged into the same receptacle, or does it get moved around and plugged into different ones?
Are there some that it starts easily on and some it doesn't?
I have a small 120v oil-less compressor that will not start (just hums/groans) when it is plugged into a receptacle that is too far from the electrical panel, or if it is plugged into an extension cord. It starts right up when plugged into a receptacle near the panel or one with adequate gauge wiring.