Pressure switch has reached set pressure

by Norman

I have built compressors for several years but i am having a problem in the electric motor restarting after the pressure switch has reached set pressure and shuts the motor off.


I have seen some compressors which have a pressure valve that keeps the intake valve open until the motor restarts, and some which allows the motor to run free until the pressure drops.

I would like to find a supplier for one of these valves. i would like your help in this matter, thanks.






Hello Norman...

Thanks for your question.

What I think you need is an unloader valve, which automatically discharges the air pressure over the piston after the compressor has reached cut-out pressure.

If you don't unload that pressure over the cylinder, it makes it very hard for the electric motor to start. Since the electric motor already has an inrush requirement greater than many electrical supply circuits, without the 'capacitor' (I think that's the name) to provide a boost when the motor starts, the circuit breaker or fuse would pop each time the motor went to start.

Here's more information on unloader valves.

The unloader valve mechanism is commonly part of a compressor pressure switch assembly. You can get a combo pressure-switch / unloader valve from a compressor parts supplier.

Cheers,

Bill

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An unloader issue.

by NORMAN LEIGH
(POMPANO FLORIDA USA)

I HAVE BUILT SEVERAL ELECTRIC MOTOR DRIVEN COMPRESSORS OVER THE YEARS. AT THIS TIME I BOUGHT 3 MOTORS RATED AT 3 HP. ONE OR TWO HAVE 220 VOLT SUPPLY AND OTHERS HAVE 115/220 VOLTS SUPPLY. THESE MOTORS ARE OF CHINESE ORIGIN AND WHEN THEY TRY TO RESTART THEY BLOW FUSES. I HAVE ELECTRIC CUTOUTS HOOKED UP SO THEY CUT OFF AT 100 PSI. MY COMPRESSORS DO NOT HAVE DECOMPRRESSORS TO RELIEVE LINE PRESSURE FOR RESTART,DO YOU THINK I HAVE TO INSTALL THEM? I JUST READ SOME OF YOUR READERS QUESTIONS AND I SEE THAT THEY ALSO HAVE SIMILAR PROBLEMS. THE SITUATION IS THAT I DONT KNOW WHERE TO GET DECOMPRESSORS, I ONLY SEE FILTERS AND OTHER PARTS. NEED YOUR ADVICE ,THANKS.
NORM0126@yahoo.com if possible.






Hi Norm....

Hey did you know that in the world of emails, all caps means you're shouting! :-)

All electric motors have an inrush current when they start, and then once they are going, draw less amps. Some motors come with starters that are basically (if I understand my electrician friend correctly) that allow a quick boost of energy to the motor when it goes to start. This reduces the inrush current, and helps prevent blowing fuses or popping breakers when the motor starts.

By not "unloading" the piston type compressor, you are adding load to the start up current requirements, and that may be why you are overloading the motor on start up and blowing the fuse.

The compressors have an unloader valve to bleed off the compressed air over the piston when the compressor stops, so that there's less load when it goes to start.

Your electric cutouts are shutting off the motor, but not unloading the compressed air over the piston. You should be using a pressure switch that's built for compressors, and when you do, it comes complete with the unloader valve.

Or, you can use the cutout to operate a small 2/2 valve that will perform the same function, but putting that into the circuit will probably cost more in materials and frustration than buying the pressure switch and plumbing the compressor that way.

Cheers,

Bill


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Adjusting the unloader valve on a recip comp. 75hp

by Rick
(So.San Francisco, CA)

How do you adjust the unloader valve on an old 75HP IR ES-1 compressor? I cannot seem to make it work.






Bill answers...

Hi Rick;

Short answer...why would you adjust it?

The unloader valve opens when the pressure reaches cut-out, and stays open until the pressure in the tank reaches the cut-in pressure level.

The pressure switch trips in response to the low pressure, turns on the electric motor, and at the same time, closes the unloader valve.

There is no adjustment needed.

If the unloader valve isn't working, replace it. If it is part of the pressure switch, you may have to replace the pressure switch to fix the unloader valve problem.

Cheers,

Bill

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Unloader valve leak issue.

by Robert DeLaPlaza
(Orlando Florida USA)

I have a two stage compressor with an unloader valve, after several years of work the unloader valve got full of gummy residue, I think from the little amount of oil leaking from the compressor in to the tank.

One day I noticed that the compressor was running for too long to reach the stop pressure.

I checked the compressor while it was running, and it seemed like it was running at idle.

Then noticed a small leak at the unloader valve, and when I put my finger over the unloader valve, just were it was leaking, the compressor sound changed, and started pumping really well again and reached the stop pressure in less than 3 minutes.

Took apart the unloader valve and clean it really well with solvent, then I put it back together, after checking that all the o-rings were sealing.

After installing the unloader valve all hell broke loose... it is totally out of adjustment, it has 3 points were it can be adjusted.

1.- A knob on top, a real adjustment intended by the manufacturer. What it adjusts?

The valve has 3 sections

2.- So the top and middle sections can be adjusted and set with a locknut.
to adjust what? and how to set it ?

3.- The middle and bottom sections, can also be adjusted and set with a locknut.
to adjust what? and how to set it ?

when I change the adjustment on the bottom parts it chatters. Fiddling with it I can make it to unload the compressor but then it doesn't pump until I move the adjustment of the valve again... I feel really lost with this.


HOW TO ADJUST A UNLOADER VALVE 3 Sections and top knob?






Bill answers...

Hi Robert:

Gosh, this is impossible to do without having the switch here and playing with it on the compressor.

You don't indicate the brand of compressor, so I can't even refer you to the factory, if you could find it.

Go to your local "big box store" that sells compressors. They probably won't have a compressor repair depot, but what they will have is a local company that they use for warranty repair of the compressors they sell. Find out who that company is.

Take your pressure switch to them and either ask them to show you how to set it, or given that you've taken it completely apart, it might be a better idea to ask them for a replacement for it.

Do you remember the pressure settings of the compressor before you started having problems? You want to set the OEM replacement switch to those same levels when you reinstall it.

Good luck.

Bill

Comments for Unloader valve leak issue.

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Feb 28, 2009
About "Factory Authorized" Service
by: Robert DeLaPlaza

I already searched the internet, specifically looking for an Ingersoll Rand "Authorized" repair shop, I found one in my area and called them, only to find out that this guys are; well..., not good.
The guy wanted to charge for the 2 way trip time; from his shop to my place, back and forth at $85/hour rate, then for the labor time, 1 hr minimum. JUST TO CHECK THE THING, so I could see that this guys are CROOKS. Hopefully not all of them are, but BEWARE. ask befor you get the sh..ft.

Doing my homework on unloaders I found that there are several types.

In compressors operated by gasoline motors.
They switch the "load' off and on the gasoline motor.
The unloader valve makes that the compressor head valves stop pumping when the "stop" pressure is reached without actually stopping the gasoline motor and then back to pumping when the pressure drops under the preset "on" value.

At least 2 systems for electric operated compressors.

1.- Centrifugal: a set of spining wheights inside the compressor crankcase operate the unloader valve, that is, when the compressor reaches the full operating speed the spining weights move and open a valve connected to the heads, which turn on the compressor head valves and it starts pumping.

2.- Check valve and unloader on the electric pressure switch.
The compressor pumps in to the tank through a check valve that doesn't let the pressure back in to the unloader, then when the electric pressure switch stops the compressor, at the same time it pushes a little pin valve that releases the pressure in the line between the compressor and tank check valve; so when it starts again there is no pressure in that line.

The electric pressure switch with included 1/4 in unloader valve is worth about $ 30.00 at Grainger which is not one of the cheapest places but they have it.

Pressure switches come in 2 basic flavors.

Fixed and adjustable "differential".

Differential is the pressure difference between the turn off and turn back on of the compressor.

The "adjuster" in the pressure switch look is most of the time a nut pushing on a spring.

The adjustable differential type has two of them, a big spring and a small spring.

The big spring nut adjusts the stop pressure, and it should be adjusted first, then the "on pressure" or differential, is the smaller spring/adjuster, and should be adjusted later.

If your compressor doesn't have an unloader be careful and don't set the on pressure too high or the compressor motor will stall on start and probably burn if unattended.





An excellent post. Thank you very much, Robert.

Bill

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Compressor Issue - Leak near pressure switch

by Larry

My compressor pumps to cut off at 120 PSI then a little switch on the side of the regulator kicks in and drains air off till it reaches cut in pressure 100psi and this goes on over and over.

Replaced regulator same thing happens.






Bill comments...

Howdy Larry...

You sound a little frustrated...and I sure understand that when things that are supposed to work, don't.

The little switch that's on the regulator, I'm not familiar with it, any chance of a picture, or, does it look a little like
the schematic shown on this PRV page?


I'm guessing here, but it might be that your PRV (pressure relief valve) is pooched, and that it's cracking open at a much lower pressure than it's supposed to. Not sure though. Usually the PRV's found in the pressure switch circuit.

In any case, PRV's come with a little spring inside sometimes, so that when the pressure drops far enough, it closes and stops the air from bleeding. Then, when the pressure reaches the set point,(usually well above normal system pressures) it cracks open, allowing air to bleed out of your system before you have a catastrophic failure.

In your case, it might be opening far too soon.

That's all I can think of.

Hope this helps...

Bill

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Jan 07, 2012
hissing air
by: phil

hi, when my husky(Air Scout 1.5 gal.) compressor reaches max pressure air slowly leaks until it reaches it's kick in pressure. i can use the compressor, but it recycles every few minutes. It's brand new. Is that normal? Help.

______________________

If the air is leaking from the unloader valve, shut the compressor off, dump all the air from the tank, remove, clean and replace the check valve. Did this do it?

Dec 11, 2009
May not be the PRV
by: MEP1

This sounds more like the "dump valve" is doing its job and is bleeding continuously because the check valve in the tank is stuck open or damaged. The dump valve relieves the pressure on the compressor head so it can start with no load, and the check valve in the tank keeps the pressure out of the head. The pressure relief valve should have a ring on it that you can pull to open - if this is not the valve that's leaking, your problem is elsewhere. The dump valve on most small compressors is located on the side of the pressure switch with a tiny lever that pushes on the bottom to open it.

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Unloader valve leaking

by Nels
(Mannford, OK.)

I have a Speedaire Model 12935A 2 stage compressor.
When it reaches pressure and shuts down the unload valve blows back until compressor starts again. Air comes out of a fitting on bottom of valve on the center of the compressor, should there be something in this fitting it it a 3/8" by 1/4" NPT bushing?






Bill answers...

Hi Nels.

When your compressor reaches high pressure cut out, the pressure switch shuts off the compressor. This also opens the unloader valve to atmosphere, allowing the compressed air trapped over the piston head to vent.

When your compressor head is pumping air into the compressor tank, it passes through a check valve (also known as a one way valve), that's usually in the fitting that's ported into the tank.

That check valve is there to ensure that compressed air stays in the tank when the unloader valve opens.

In your case, it sounds as though the check valve hasn't seated properly, allowing air in the tank to flow back up the line and out the unload valve.

Take the air line / check valve assembly apart and clean it. If that doesn't work, replace it.

Also, make sure your tank drain valve is closed.

Hope this helps.

Cheers,

Bill

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Compressor bogging down and blowing the circuit breaker

by Donald Marinelli
(Clarence, New York)

Compressor starts up and runs good on an empty tank.

When I go to use air it replenishes itself and when the motor starts up it bogs down and break the breaker.






Bill answers...

Hi Donald:

What pops the breaker? Something electric. What's electric on your compressor? The motor and the pressure switch.

As your compressor crunches air into the tank, the job gets harder and harder for the compressor to do. At first, pulling in free air and compressing it into the tank is almost effortless. As the pressure in the tank increases, the motor and compressor have to work harder to get air into the reservoir.

You compressor tank pressure doesn't reach high set point before the breaker pops, right?

It sounds like something is affecting the motor's operation, causing either a short of some kind or a thermal overload situation, and this pops the breaker.

Your problem might be mechanical, and the reciprocating or cycling of the piston may be lugging, putting pressure on the motor shaft and causing motor meltdown.

Or, it could be just that the motor itself is wearing out.

One of the problems with the lower cost DIY type compressors is that they are just that...lower cost. Therefore, if you take your compressor to a repair shop, the labor rates and parts costs today might easily add up to more than the compressor was worth in the first place.

Consider this when you decide to fix a major component on a low cost compressor.

Good luck with it.

And... I just reviewed this question and response, and I would now certainly refer you to the unloader valve page (linked from sitemap) for more information on what may be the cause of this issue.

Bill

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