How to fix air compressor in the shop?

Capacitor.

 

Is it plugged in directly to the outlet, or is it on an extension cord?
Is it 110V or 220V?

 

If it's even semi-portable, drag it to the outlet nearest to your breaker box, plug it in (without an extension cord) and try it again. If it was really truly burned out, I don't think it would run at all, not even for a second or two.

 

Motors have a capacitor that helps provide a little extra "oomph" when starting. A compressor uses a lot of juice to get going - if the cap is bad, then when it tries to start, it will ramp up quick in how much current it draws, and pop a breaker.

Coincidentally a cap is probably a $15 part. I'd try that first, 'specially if the motor spins freely and wiring looks OK.

 

Different motors have different configurations of capacitors. Some use only a run capacitor, some (especially high starting touque motors) have a start capacitor as well. If either is bad, the symptoms you have are possible. If there is a hump (or humps) on the motor housing, that's where the capacitor(s) is (are.) If not, check under the end plates (more disassembly required.)

A capacitor meter would be handy for checking it (I have one built into one of my multimeters) but it is relatively inexpensive to "just do it." Any appliance parts store should be able to hook you up with a replacement. Actually, since it is Sears, you should be able to get the exact replacement if you are patient enough.

 

An oldie, from back when men were men, women were women, and 4HP was a real 4HP.
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Is there pre-existing pressurised air in the tank? If so, the motor will have to work harder (draw more amps) to turn the motor upon start up. Thus it wil draw too many amps & tirp the breaker.

Is the compressor in an unheated area? Cold (thicker oil) will also lead to the motor drawing more amps to get it going...thus tripping the breaker. And even if it's "oil-less" things don't work as well in the cold.

If the environment in which the compressor exists is both cold and you have a tank full of previously compressed air you can add points 1 & 2 together....

I use compressors for many things from spraying paint to driving air driven nailers. Even in the heat of summer they will trip breakers.

1. If at all possible don't use an extension cord...get a longer air hose
2. Drain the compressor after you are done so that it doesn't have to work so hard from cold start-up
3. Make sure it's pluged into a circut that is ample for the load...don't plug a 20 amp motor into a 15 amp circut.
3a. Make sure that if it's on it's own circut it's got capable wire...no 14 gauge crap.
4. If needed, upon the first start of the day, hold open the relief valve on the compressor so that the motor can run free without pressurising the tank...oil or oil-less this will warm up the motor.

In general, compressors require a lot of power at start-up no matter the size or HP. If the circut is not up to the task you will blow the breaker. If it's a stationary unit in your garage it should be on it's own dedicated circut.


FTIW< I was just doing a roof the other week (fall... temps around 35-40)... small 5 gallon compressor to run the air nailer; would blow the fuse 2-3 times before it got warm enough to run freely.

 

Most house plugs are 15amp, just barely enough for high powered equipment. I've grabbed the Clothes Washer outlet which happens to be close by, and is rated around 20amps. The important thing is to have an extension cord that is capable of handling a 20amp load, otherwise anything less than the rated amount will melt the cord!

 

I hear that.

One of the latest homes we've purchased had a 14 amp microwave... plus about 6 ceiling lights.... and a bath fan on a 15 amp circut. Current code (where we are) requires a dedicated circut for built-in microwaves & any ciruct with 14 gauge wire can't be on a breaker over 15 amp.

I've been working with a former state inspector on our additions and remodels and have learned a lot about electricity I didn't know before....best left to the pros.

 

Motor starting capacitor.

Remove the one you have and take it to your local Grainger for a new one.

 

I'm having a problem with my compressor where it runs fine and fills the tank but the motor will not turn off after the tank is full, it just keeps running until the thermal breaker makes it stop. Any ideas, it is the generic version from Harbor Freight.

 

if you will take the model number to a Sears Parts Center, they will be able to give you the exact capacitor that you need.

 

Bad pressure shut off switch.

Careful - don't want to run it that way for long!

 

Exactly. You should be able to get the part from Harbor, their stuff usually comes with a manual with part numbers and a phone number.