Compressor motor just hums and compressor will not start

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by Dave L.
(Becket MA)

Compressor runs fine plugged into one (GFCI) outlet in the garage. But that wasn’t convenient, because of the outlet location. I installed an outlet off of another circuit, a circuit that’s used for lighting in and outside of the garage.

When the compressor is plugged in there, it hums but generally won’t start up. It’s essentially the same problem as when I use a long extension cord, that is to say it’s not getting enough current, I think.

My question is how can this be on a circuit with no extension cord? Wouldn’t it pull as much current as it needs? Wouldn’t it throw the breaker if it couldn’t get enough? Instead, it just doesn’t seem to be getting enough juice. How can that be?


Compressor hums
by: Bill

If the start capacitor is degrading, and there is slightly less current to one socket versus another, that could explain in.

Does the problem occur in the new location if you try and start the compressor with the tank empty of pressure? Please advise.

Also, see how to check your capacitors here and run a check, will you? What did you find?


Acts the same when trying to start on empty tank
by: Anonymous

Yes — with empty tank it acts the same.

I wonder if the fact that the power is switched via a three-way configuration (two light bulbs and the compressor via two three-way light switches) is the problem. Would such switches limit the available current? I doubt the bulbs matter, since they’re low-wattage CFL’s.

To be clear: I energize the circuit by flipping one of the three-way switches, THEN I turn on the compressor, so it’s not competing for “start-up juice” with the bulbs’ ballasts.

Can light switches passively limit current throughput?

I wouldn’t have wired the outlet that way, except that there’s no full-time power at the three-way switch box that I tapped into for the new outlet.


Hmmmmm (pun intended)
by: Doug in

You don’t ID your compressor, so that complicates the answer, but I can say this:

Light circuits are usually wired for 15Amps and use 14 gauge wire. You should find (or install a breaker for) a 20Amp circuit, with 12 gauge wire.

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My 4hp compressor just hums/buzzes and blows the circuit breaker. I’ve replaced both capacitors, and the pressure switch. It has it’s own dedicated 20amp circuit. It does run occasionally, like once every five tries, but when the pressure goes down, when it should start up again, it just buzzes, and tries to start, but then blows the circuit breaker after a second. Any ideas?

Sounds like the motor may be getting only 110-120V.
To check, open the pressure pressure switch. You will find two sets of contacts (maybe three, but only two wired).
Measure the voltage from one of those screws to the other. Should be 220 or whatever the socket is supposed to be.
Note that measuring from ground to either contact will show 110, but does not mean the voltage is 220 ACROSS the contacts.

Hey, Doug. It’s a 110v compressor, not 220, so that’s not it. Sears Craftsman 4hp 25gal tank. ALWAYS worked but I moved and it sat unused and unplugged in my garage for about 5 years till I built a shop.

Hey Gary –
Can you give an actual model number for this machine?

Still sounds like low voltage.
Can you hang a meter across the power terminals, preferably as close to the motor as possible, and see what the voltage is both before and after it tries to start?
If it’s less than 105V when starting, you probably need bigger wires to the outlet.

BTW, must be 4HP peak, as there is no way to get 4HP from a 110v outlet, which would be at least 27 amps.

Hey Doug.Yea it’s a Craftsman Model # 919.152922. The dedicated circuit has a 20 amp breaker run with 12G wire. It’s only a 2 foot run from my panel box, in my utility room in the basement, so I don’t think power is the problem. Thanks.

OK. That’s a pretty conventional setup for an oil-less.
That pretty much leaves the motor.
I don’t think you ever said, nor have I asked – does it start *consistently* with an empty tank?

Hey Doug. I’ve tried it twice with the tank empty and it started up both times. Then I let it run for a minute, turned it off, and tried to turn it back on and the breaker blew. I think you’re on to something! There’s a guy on YT that talks about cleaning the contacts on the clutch inside of the motor and it fixed the same problem I had. Does that sound logical? Thanks for all the help!

Yes, it does. Often those contacts will get so dirty, or weld, that the motor blows the capacitor or at best will not start at all. Sounds like yours may have some crud that keeps them from making contact for a while, but eventually they do. So sure, look in there and clean those contacts. Can’t hurt, as long as you don’t bend or break them in the process. Also, once you get it starting OK, You may find it blows the panel CB once in a while. It will get worse or time. When it gets intolerable, put a… Read more »

Hey Doug. Just an update. I finally got around to cleaning the contacts on the clutch inside the motor and it’s working! Starts when the pressure drops to 90 stops at 120. So it seems like that was the problem. Thanks for taking the time to respond and helping me out. Have a good Thanksgiving and a safe Holiday!

Cool. Thanks for the update. Really appreciated.
Happy holidays to you, too.