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Tommy Tesla
Tommy Tesla, Electrical Technician
Category: Electrical
Satisfied Customers: 139
Experience:  E-1 Master Electrician. Specializing in Commercial ,Industrial, and Residential Wiring Applications.
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Hi, I have a 7HP Craftsman oiless air compressor thats not

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Hi, I have a 7HP Craftsman oiless air compressor thats not working properly. When I turn it on it will run very slow and trip a 20amp breaker. I removed all of the shrouds and the motor/ pump turn easily like the tip of a finger pressure will do it. Nothing seams to be burnt or smelling of burnt wire. I plugged it in again and it turned slowly for maybe 20 revs and tripped the breaker again. The tag says that its a AO Smith eletric motor co. 110 volt. It has not been used very much since new. Hoping to be able to repair it. Thanks

Hello. Thank you for choosing Just Answer! My name isXXXXX am here to help you with all your electrical needs. Have you checked the outlet for any loose connections?

Is this a dedicated homerun or is there other loads connected to the circuit? You can eliminate these by simply plugging it in somewhere else. Also, the start or run capacitor could possibly be compromised, a very common problem with compressors. Motor windings could also be bad....there are a few things, but lets start first by eliminating any outlet issues. Also check the cord on the compressor for any frayed wiring or a broken prong.

Customer: replied 4 years ago.

Hi, This is a dedicated one outlet socket just for the compressor. When I flip the switch on the unit I can tell it is drawing a lot from the slight flicker of the other lights in garage. The cord/plug is in great shape.


Welcome. My name is XXXXX XXXXX would be glad to assist.

What is the Model number of the air compressor?
Should be located on the nameplate.
Customer: replied 4 years ago.

Hi. I found this number on the tag below the motor.NNN-NN-NNNN Thanks

Looking at the manual, the compressor motor does not use a capacitor for starting.


But. please look on the motor or in any box you may see for a round or oval black or aluminum can looking item to be sure.

It is always possible it is left off the parts diagram because the motor is supplied by a 3rd party.

If you find one, look at it, it is stamped on the side with numbers and letters

Example: ?? UF and ?? voltage


There are a few possibilities here to explore:
Start with item #2, then #3 to see if it helps the situation.

1. The start windings themselves could be bad and not allowing the compressor to come up to speed and switch over to the run windings.

2. Manual requires that a 15 amp dedicated receptacle be used and in many cases a 20 amp is needed to overcome any voltage losses due to the wiring.
A washer normally has a 20 am dedicated circuit and sometimes the garage or basement may have one to try.
No drop cords to be used on the compressor, unless they are short and #12 gauge.

Use a lamp and be sure that the receptacle being used, is a 20 amp circuit by turning the breaker off to check and nothing else is on that circuit.

3. The check valve is stuck and will not allow any air to pass into the tank from the compressor.
This will basically stall the motor.
Need to perform maintenance on the check valve
See Page 18 of the manual posted below for that procedure.


While you have the hose disconnected and check valve out for maintenance, you can try to start the compressor for a quick test also to see if the situation changes.

CLICK HERE for manual

Check Valve if needed

See how it goes and post results after.

Let me double check something here also.

You are stating 7HP?

Did you change the motor on this air compressor?

Manual sure does not reflect that for the Model number.


Look end of your motor and see if it has has a cap on the end that can be removed to find a capacitor

Our system is not allowing pictures at the moment, so I will try to add one later.



Customer: replied 4 years ago.

Hi, I opened the drain and released all of the pressure and tried to start it. The same turns only a couple of very slow revs and trips the dedicated one outlet breaker. I took off a two screw cover on the side of the motor and it has a round ( automobile flasher ) looking unit in there. Under the end cover there are a set of points, spring loaded gismo on the end of the motor shaft, and a shallow round black pod. I know this is not the techinal terms. Thanks The sticker sayes thats its 7hp but I don't know how they rate these because the whole motor /compressor is smaller than a 2 hp motor on my other compressor that I have had

Ok, thanks.

The automobile flasher looking item is probably the capacitor for starting the motor.

See if you can find all the numbers and letters written on it, need all.

The spring loaded gismo is a centrifigal switch.

What happens is this:
You turn on the compressor, it uses the capacitor to assist in getting it rotating with a little extra current, then it ramps up to speed, at which point the centrifugal switch opens and then the motor is running on the main windings.

All of that takes place in a matter of seconds when it starts.

Now, the capacitor could easily be the problem, since you have found one.

So get the numbers a post them, will see if we can find one for it.
Note/measure the capacitor too, if round- the length and diameter, if oval then length, width and depth.

It is probably the original motor. Lets assume for now.

What size breaker is it that it tripped?
Customer: replied 4 years ago.

It is a 20 amp breaker that tripped. The motor is original low hours of use. The pump and motor are one unit. I will get the measurments and # XXXXX the capacitor and get back. Thanks

Ok, thanks.

Also, need to remove and clean the check valve as posted earlier.
Use the manual for the guide.
It may be bad and if so, it will keep the motor from running because of the back pressure
Customer: replied 4 years ago.

Hi, I removed the check valve and tried it again, same thing. I have the number off of the capacitor. 30UF+- 6% 370VACB50/60Hz Then on another line 40/070/21 p2 SH Last line 325P306H37M30N4X It measures 1 3/4 Dia X 3" long. I hope this helps. Thank You

Ok, thanks.

Well, with the check valve removed, that rules another item out of the picture.

That one sounds like a Run Capacitor.
Normally the starts will have ratings in MFD.

If you have a multi tester with a CAP settling, yours can be tested before getting one to be sure.

If you do not have a tester capable, maybe there is an HVAC contractor somewhere in the area.
I am sure if you stopped by, they would be accommodating to quickly test your capacitor.

They could also help supply one much faster if it is indeed bad. Since they have quick access.

There may be some building supply stores close by as well.

Would be nice to verify before getting one and still have an issue.

The motor could have a bad set of windings.

To test the motor, A local motor shop could assist or a pool supply store many times has these parts for pool motors. Fairly common.

You have the numbers needed and also can carry it with you.
the key is the 30 UF 370 volt

You can use any the same physical or small size at higher voltage also.
30 UF must be used but you can use a 440 volt as well as a 370 volt.

Many sites do not list the actual dimensions, because they acquire from various resources and the size changes from time to time.

In hand is the easiest way for sure.

1 1/2" x 2 5/8"- CLICK HERE
AssuredElectrical, Master Electrician
Category: Electrical
Satisfied Customers: 4241
Experience: Contractor-42+ Years in the ElectricalTrade
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Tommy Tesla, Electrical Technician
Category: Electrical
Satisfied Customers: 139
Experience: E-1 Master Electrician. Specializing in Commercial ,Industrial, and Residential Wiring Applications.
Tommy Tesla and 3 other Electrical Specialists are ready to help you