Compressor just hums and barely turns on?

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?

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Feb 04, 2017
Compressor just hums
by: Doug in s.d.ca.

Dave -

Have you found a solution to your problem?

Dec 28, 2016
Dave's shorts
by: Doug in s.d.ca

The plumbing analogy is a little tricky. But try this:

Say there's a lake, with a dam and generator. That's the power company.

The water normally flows through some device and does work. Making electricity, that is.

Then some one "shorts" the generator by blowing a big hole in the dam right next to the generator inlet pipe. The water takes a path of least resistance and goes through the hole instead of the generator.

Likewise, your motor is a relatively small hole, and a short is a big hole, so the electricity goes there in large amounts.

Hope that helps.

Dec 28, 2016
Thanks...
by: Dave L.

...for your explanation. :^)

Why is it that a short circuit can draw big current, but a motor can't (as easily)? Is there a plumbing analog to an electric motor that would help me understand?

Dec 27, 2016
Re: blowing circuit breaker
by: Doug in s.d.ca

Nearly any wire can deliver thousands of amps - for maybe a thirtieth of second - before it overheats and melts or vaporizes.

In your situation, there is the current, but with a voltage drop due to the inadequate size of the wire, which reduces the power available to the motor, which "tries" to draw more amps, but cannot because of the loss (resistance) in the wiring to it.

It's a little like a clogged pipe - the pressure is there, but not much flow because of the clog, or a too small pipe, getting to the delivery point.

Dec 27, 2016
wire gauge and circuit capacity
by: Dave L.

Thanks, Doug. I guess that make sense. So how can one trip a 15 amp breaker if the wiring can't even deliver 15 amps?

BTW, it's a Central Pneumatic (Harbor Freight) model 90221 -- 4 gallon, 2 HP peak pancake compressor.

Dec 26, 2016
Hmmmmm
by: Doug in s.d.ca

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.

Dec 26, 2016
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.

Dec 26, 2016
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?

See how to check your capacitors on this site.

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Compressor motor hums

by Frank
(Farmville, NC)

compressor is plugged directly into outlet.(no extension cord)
took belt off and the motor will start (no load).
unloader valve seems to be working ok.
compressor turns by hand with some resistance.
sometimes the compressor will start and sometimes it hums.

(See http://www.about-air-compressors.com/humming-air-compressor.html - Moderator)

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Motor Humming not starting

When you turn it on it just humms and cuts off and humms and cuts off.

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May 05, 2015
Compressor motor humming
by: Bill

See the troubleshooting section, page about why air compressor motors hum. Do the checks, then post a comment here with the results if you still cannot get the compressor motor going.

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