Off Topic: Problem with 60 gallon Craftsman Compressor

Kenster

Member
I have a 60 gallon, 3.2 HP upright Craftsman Professional Compressor. It seems to be having a problem kicking on when it hits the low pressure setting of 90 PSI. It tries to start but then pops the onboard breaker. I can see the pulley trying to turn over when it does this. I thought the belt might be too tight so I backed off the tightness a wee bit.

It seems that when the tank is completely empty it starts okay, though sometimes I have had to give the pulley a little help with my hand. But it just won't start when it tries to kick on at 90 PSI.

Oil is good. Filter is new. Belt is not too tight. 240V on twin 40 amp breakers.

Could the low pressure side be set too high? Any guidance will be greatly appreciated.
 
I think it"s called an unloader valve.....supposed to make it easier to start under pressure.....must not be working right.
 
If You have to help turn the pulley at all with 0 pressure, yet everything turns normal? Its the motor. It doesn't mean the motor is shot; but the Coil, or capacitor might be burnt. I don't know how much it would cost to replace one. I wouldn't go to Sears if possible. Try an independent Electrical shop. They might carry a better motor, or Load Capacitor.
 
Look at the electrical control box and you will see a small tube going into it. It connects to a unloader valve that releases pressure so it can start easier under pressure
 

Is the unloading valve a DYI project? I found another reference to the unloading valve but it only said to replace it if it is leaking air, which it is not.
 
The problem could also be the check valve. It's a one way valve that where the air goes into the tank. With the compressor aired up loosen the pipe that goes from the pump to that valve and see if there is any air leakage. There should be none. You might also take the belt off the motor and make sure the problem isn't with the motor. The motor may need a capacitor. As far as it going down to 90 psi before it starts that would be the pressure switch. Some of them you can adjust. Just be sure you stay within the specifications for the compressor.
 

If it is a single phase motor and it has a problem starting at zero pressure it is most likely the start winding switch. They are almost always just under the back end plate.
 
use enough air to get the tank down to 90 psi, and then get the compressor going . Stay near the compressor. When the high pressure switch engages and shots off the compressor listen for al oud HISS. That is your unloader working. No Hiss, no unloader.
 
A quick and easy check, look at the capacitor,
check the connections, look for evidence of
burning around the terminals, split case, burned
smell. It may have 2 capacitors.

If the caps look ok, remove the tension on the
belt and remove the back end of the motor. Look at
the contacts on the centrifugal switch. If they're
burned or not making contact they can be replaced.

Parts can be bought at a motor rebuilder/supply
house. They can also check the caps.
 
IMO 3.2 hp is not enough motor to start a compressor at 90 psi.
90 psi is often the factory set cut out pressure on these cheap compressors.
The higher the pressure the more starting torque one needs.
 
(quoted from post at 19:04:05 07/04/14)
If it is a single phase motor and it has a problem starting at zero pressure it is most likely the start winding switch. They are almost always just under the back end plate.

The OP said it would start OK with zero pressure in the tank, so that takes us back to probably the unloader valve.

Someone mentioned the check valve, before going any farther, I would check it as described.

An electric motor shop should be able to get the unloader, (or go to Sears) and usually not too complicated to replace, if you have some mechanical skill.

Myron

[b:7706bde9c5]Edit[/b:7706bde9c5] - I see Gordo made a post while I was fumbling keys on my 'puter. Good point, that is a sure way to check that valve. That also will tell you where it is located,
 
Unloader usually built into pressure switch on that type of compressor? Has a Schrader valve that is mechanically opened to bleed off pressure when pressure switch cuts out.
 

There is a short HISS when the pressure switch cuts off on the high end. The is no continuous leak of air at the unloader valve after the compressor kicks off, which, to me, indicates that the check valve is good.

I will open up the back plate and look for problems when I get a chance.

My buddy down the road (also an 8N guy) is a good at things like this. His first thought is that the 90psi Cut In setting is too high. I know that the pressure switch is factory set but I'd like to lower it anyway. Is 50PSI too low?

I think I can adjust the switch if I could just figure out to get the cover off!
 
The unloader and check valve appear to be working OK, from your check, so that leaves the starting capacitor (if it has one) as a possible culprit. That should be mounted under a "bump" cover on the outside of the motor.
If the start switch inside the motor is bad, I doubt that it would start up at all without spinning the motor by hand to get it going.

You can probably adjust the pressure down, but it won't run air tools very well at 50 PSI. Most air tools want at least 90 PSI to work well.

Myron
 
(quoted from post at 15:39:46 07/04/14)
(quoted from post at 19:04:05 07/04/14)
If it is a single phase motor and it has a problem starting at zero pressure it is most likely the start winding switch. They are almost always just under the back end plate.

The OP said it would start OK with zero pressure in the tank, so that takes us back to probably the unloader valve.

Someone mentioned the check valve, before going any farther, I would check it as described.

An electric motor shop should be able to get the unloader, (or go to Sears) and usually not too complicated to replace, if you have some mechanical skill.

Myron

[b:0a0b371bce]Edit[/b:0a0b371bce] - I see Gordo made a post while I was fumbling keys on my 'puter. Good point, that is a sure way to check that valve. That also will tell you where it is located,

OH 961 go back and read the last part of the OP.
 

I don't run any tools on it. We live out in the country and it's nice to not have to drive into town to fill up the air bottle when we need to air up a tire on one of the lawn tractors or the 8N, or the other vehicles.
 
Sorry to disagree, but 3.2 horsepower should be plenty to start the compressor - even at 90psi. I have several smaller compressors that run similar pressures - only at smaller volumes and smaller tanks. Two are running on 1 to 1.5hp motors.

The cut-in pressure should not be the issue. Starting capacitors or starting windings/contacts are of more concern. The unloader should make the cut-in pressure irrelevant. Motor should not be under any extra load when it starts, as it should not be under any pressure until it starts running, and then not until it builds pressure in the line to the tank pressure.

The real issue in this case should center around the starting circuit in the motor.
 
showcrop,

From the OP:

"Could the low pressure side be set too high? Any guidance will be greatly appreciated."

I'm not sure what you are getting at. A cut-in pressure of 90 PSI is pretty conservative already. That should not be a factor at all. Many compressors operate at higher cut-in pressures than that.

Myron
 
(quoted from post at 00:11:03 07/05/14) showcrop,

From the OP:

"Could the low pressure side be set too high? Any guidance will be greatly appreciated."

I'm not sure what you are getting at. A cut-in pressure of 90 PSI is pretty conservative already. That should not be a factor at all. Many compressors operate at higher cut-in pressures than that.

Myron

Myron, It just seems to not be able to restart with the low side set at 90. If I bleed the tank all the way down to 30 or less, it starts easier, though a hand pull on the pulley seems to help. So, I was wondering if it makes sense to lower the low side for an easier restart.
 

Kenster,

You could do that if it suits your purposes, but that is only a band aid on the problem. There is something else going on that may get worse instead of better.
Is there a start capacitor (or 2) on the motor? There isn't a good way to test them yourself. If the motor is so equipped, you could take them to an electric motor shop to have them tested. It might be better to take the whole motor so they can check for other problems as well.
[b:5ce3e207ec]Be careful with handling them[/b:5ce3e207ec], because they can hold enough charge to shock you pretty good.

Myron
 
Kenster,

Yep, the 2 humps are capacitors. You should be able to get the covers off by removing a couple of screws. They are normally wired with push-on connectors, so not hard to disconnect for testing.

Myron
 
Did a second test today just to be sure.

I bled off all air in the tank down to Zero. Turned on power. Had to give it the fly wheel a little nudge again, then it fired right off.
Filled up to 125 PSI then shut off. I instantly heard a loud HISS at the unloader valve that lasted about three seconds or so.

That should tell me that the unloader is working properly, right?

Since the fly wheel needs a little nudge even at zero, but the unloader seems to be working fine, doesn't that tell me something? Like check out the two capacitors? No sign whatsoever of burning, dirt, etc. Connections are bright and tight. The motor itself looks spotless inside. No sign of burning, mud dobber nests, etc.

FWIW..This is set up on two 40amp fuses. Short run 12g cable to outlet.
 
(quoted from post at 11:09:42 07/05/14) Did a second test today just to be sure.

I bled off all air in the tank down to Zero. Turned on power. Had to give it the fly wheel a little nudge again, then it fired right off.
Filled up to 125 PSI then shut off. I instantly heard a loud HISS at the unloader valve that lasted about three seconds or so.

That should tell me that the unloader is working properly, right?

Since the fly wheel needs a little nudge even at zero, but the unloader seems to be working fine, doesn't that tell me something? Like check out the two capacitors? No sign whatsoever of burning, dirt, etc. Connections are bright and tight. The motor itself looks spotless inside. No sign of burning, mud dobber nests, etc.

FWIW..This is set up on two 40amp fuses. Short run 12g cable to outlet.

Of the hundreds of single phase motors that I have worked on for no start, I would say two were capacitors, the rest were start winding switches. They are usually right behind the back end plate and not difficult to get at.
 
I know you guys have been losing sleep over this for the past three weeks so I thought I better report back to you.

With help from here, I was pretty sure the problem was the starter capacitor. I ordered a replacement. It arrived this morning. I immediately installed it and flipped the breakers. I let just a little air out of the tank to get to the "ON" switch setting of 90. It fired right up! Problem solved. Capacitor was only $16 with free shipping.

Thanks for all your help!

Ken
Bellville TX
 

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