Compressor motor hums and blows breaker

Compressor motor
(Last Updated On: July 31, 2019)

This issue:

The compressor worked just fine until I shut it off for about 1 minute and tried to turn it back on. The pump motor makes a humming or buzzing sound, not the usual pump sound. If I leave it buzzing for too long (maybe 5 seconds) it trips the breaker, so I don’t do that for fear that I’m burning something up internally. I unplugged it, drained air pressure, and let it cool down overnight for 12 hours. The problem still exists. There were no sounds or physical indications that the pump or compressor had a problem before shutting it off when it reached 100psi.

compressor motor hum

It was plugged directly into an outlet without an extension cord. I’ve been using it in this manner for 3 years without problems. I’ve tried several outlets, (GFCI, non-GFCI, and unused circuits), but there is no change. It is a 25 gallon, Sears/Craftsman, 175psi air compressor. It has a red lever switch for OFF and AUTO positions. I haven’t located any reset switch.

Are there internal fuses? Is there a reset switch? Any other ideas or something I can check?

Thank you.

_________________________

Our Answer:

First, thanks for writing in.

Second, thanks too for such a good description of the issue.

From the sounds of it, you either have a failed capcitor or a failed motor.

Since compressor motors (along with all AC electric motors) require an “inrush” of energy to start, a capacitor is used to “jump start” the motor, since there’s not enough amperage in the typical home circuit to start the motor.

If the capacitor fails, then the motors can’t start, and it sits and hums until it pulls too many amps and pops the breaker.

Could be the motor, but I’d bet on the capacitor. Since the capacitor is cheaper to replace than a motor, if you want to do it yourself, start there. If you are taking it to a shop, they can tell you pretty quickly which is causing your problem.

Cheers,

UAC

Hello, I am Bill, the Compressed-Air-Man. I have years of experience in industrial and residential compressed air applications, air compressors and general pneumatics. I created this site to help professionals, students, and DIYers understand and properly implement and maintain compress3ed air systems.

12 COMMENTS

  1. I have one doing the same thing. Turn it on, it tries to run, the piston is moving, the fan turning and pop, there goes the breaker. Is this the motor or the switch? Please help. I need it. Shawn

      • Where can i get a start capacitor? The local sears parts store said i could no longer get the motor for my compressor. I am wondering if it is worth trying to fix. I hope so as it has been a good one.

        • Shawn, use the term start capacitor and then the model number of the one from your compressor as a search string on your browser. You will find sources for them, and more knowledgeable ones I expect, that will be able to tell you whether or not their model matches yours. The capacitors have a rating, and as long as the rating is similar, and the new cap fits where the old one when, you should be good to go.
          Bill

      • Thanks. It is a 30 gal 5.5 hp compressor. It has been used almost everyday for about ten years. It has always been on the powerstrip. Where can i get the on off switch?

    • Bill. I put on the new start capacitor. It turned over about ten times and pop, flash from the on and off switch. Now nothing. The switch does not appear to be burned. I am wondering if it is not the motor. Before i started having this trouble it would chirp a little when i first turned it on. Does it sound like the motor or the on off switch? Shawn

      • Shawn, if you have a pop and a flash with an electrical component, to me that sounds like a short. You can check the ON/OFF switch for continuity with a multi-meter to see if the switch has been burnt. If so, it may have been a short in the switch itself.

        You say the compressor turned over about 10 times… do you mean that the compressed air was being used, the pressure dropped in the tank, the pressure switch tripped and sent power to the motor, the motor started and the tank refilled and the compressor stopped, about 10 time before this last incident?

        If that is the case, avoid the tendency to think that the various scenarios are the same. I tend to think that your air compressor has developed another problem, and that may be in the motor.

      • It is not building air . With the cover off you can see the rod and piston go up and down then pop. Now it will not do anything and burnt the power strip that i had it plugged into. I plugged it directly into the wall and instantly popped the breaker in my house.

        • Shawn, I did note in an earlier post that it is not a good idea to use a power strip as a power supply to an air compressor. And, in my last response, I asked about what you meant by the compressor “turning over”.

          In reviewing your posts I can’t find where you say what make or size your air compressor is. Nor do I see how old it is. Based on the difficulties you are experiencing with your air compressor, and making an assumption that it has a lot of use in it already, maybe it’s time for a new one? Good luck.

  2. Bill – You are right! I had the same issue with one of my compressors. I opted for a replacement compressor because I wanted a more portable and quieter unit. I use the tank from my failed compressor if I am doing a project that requires more air.

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