Air compressor control question

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ammklq143

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Location
Iowa
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Electrician
I have a customer that has a large commercial air compressor. It has 4 motors and 4 compressor heads and a large control cabinet. Originally, the pressure switch would kick the compressor out but leave the motor running so it wasn't starting and stopping so much. They had someone come work on it. He replaced the pressure switch and then added a contactor so rather than the compressors not compressing air anymore, it just starts and stops the motor like on most compressors. Had anyone had any experience with this type of thing? They want me to make it work like it originally did but I'm not sure how it is supposed to work. I'm trying to get some pictures that I can upload but they're too big so I have to compress them.

From what I can see, there are solenoids that have copper tubing going in one side (not currently sure where that comes from) and another copper tube that goes out the other side and connects to each of the compressor heads. I haven't worked on a compressor that disconnects or shuts down the compressor portion and leaves the motor running continuously. I thought there would maybe be an external clutch somewhere that would engage and disengage the compressor but don't see anything like it. Has anyone ever worked on one of these and knows how it's supposed to stop the compressor from compressing air rather than shutting the whole unit down?

Thank you.
 
You can save large images at postimages.org, then refer to them here by url.

Compressors with an external clutch are very common in automotive air conditioners. When the clutch is not engaged, the compressor doesn't turn. But a clutch is a wear item, and its lifetime might be unacceptable for industrial use.

Trucks with compressed-air systems often have a belt-driven compressor that turns whenever the engine turns. When more air is not needed, the intake valve is held open. The compressor huffs & puffs, moving air in & out of the cylinder but not consuming much energy.

It sounds like you might be looking at air-operated unloading valves, where a small solenoid (valve?) actuates a much bigger bypass valve.

mods: Please delete prior post. I didn't edit fast enough and my five minutes expired.
 
Here is a link to the pictures. The first two are of the small solenoid that has the copper tubes entering it. I don't think you can see it from the picture but there is one going out the back of it. The third picture shows where the copper tubing connects the each of the cylinder heads on the individual compressors. The last picture is the manufacturer nameplate.

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I expect that is some type of unloading valve that prevents the compressor from producing air pressure when the solenoid is activated. The solenoids would be controlled by the pressure switch and let the motor run under an almost no load condition when the pressure switch is not calling for pressure.
 
Four megaPascals is quite a bit higher pressure than the usual shop air.
It's almost certain that these are cascaded multi-stage compressors, possible that the original design activated individual stages one at a time, and likely that it didn't activate any of them until the machine was up to speed and developing adequate lubricating-oil pressure. It might be a good idea to review the original documentation and/or request a site visit from someone with mechanical expertise.
 
Given that this is not a normal air compressor (about 580 PSI) and that it's from China I would say you are out of your league. I agree with @drcampbell , you need someone with experience on that kind of machine.

-Hal
 
As said, we are electricians...not sure why its your problem if another changed things...but as far as just running down through a "popoff" (thats what i call them) that would be horribly annoying.
They can start up and down all day long.
I guarantee you have a compressor dealer in your town- sometimes a 100$ for a pro to come down on your dime to satisfy the customer is very worth it.
 
rambojoe said:
As said, we are electricians...not sure why its your problem if another changed things
Click to expand...
I always tell my guys that being an electrician means being able to figure something out that no one else can. On top of that, fixing a problem that someone else caused or can't figure out is my bread and butter.
...but as far as just running down through a "popoff" (thats what i call them) that would be horribly annoying.
They can start up and down all day long.
I guarantee you have a compressor dealer in your town- sometimes a 100$ for a pro to come down on your dime to satisfy the customer is very worth it.
Click to expand...
It's pretty common for large compressors to run with a bypass valve instead of stopping and starting. It certainly is much easier on the motor.
 
ammklq143 said:
..... I haven't worked on a compressor that disconnects or shuts down the compressor portion and leaves the motor running continuously. I thought there would maybe be an external clutch somewhere that would engage and disengage the compressor but don't see anything like it. Has anyone ever worked on one of these and knows how it's supposed to stop the compressor from compressing air rather than shutting the whole unit down?

Thank you.
Click to expand...
I wouldn't expect to see a clutch anywhere. I agree with Don that there is likely some kind of arrangement that dumps the air coming off the compressor head into the atmosphere instead of the tank when the pressure is high enough.
 
ActionDave said:
I wouldn't expect to see a clutch anywhere. I agree with Don that there is likely some kind of arrangement that dumps the air coming off the compressor head into the atmosphere instead of the tank when the pressure is high enough.
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Likely the pressure switch controls the solenoid valves. But there are several and a considerable amount of pneumatic control tubing. Since there is tubing running around to all the heads they probably open the intake valves to unload. What are those white things that are valved off? What's in the "large control cabinet"?

ActionDave said:
I always tell my guys that being an electrician means being able to figure something out that no one else can. On top of that, fixing a problem that someone else caused or can't figure out is my bread and butter.
Click to expand...

Nahh, my head is too full of useless things as it is, I don't need to see any more.

-Hal
 
I used to design controls for large compressors. Some previous comments are pretty close.
The solenoid valve is “normally open”. That is, the compressor is loaded when power is applied. No power to the solenoid valve sends air to the inlet valve unloaders and the compressor doesn’t pump.
The simplest form of control uses a time delay relay (off delay) that is de-energized with the solenoid valve. Once the relay times out, the motor starter is de-energized. The setting of the time delay relay depends on the motor size. Larger motors have higher settings to limit the stat intervals to a safe value.
 
That's what I was thinking- that there is some kind of delay or logic involved because of multiple solenoid valves. That's why I asked what was in the control cabinet. Maybe you will get lucky and there will a diagram in the cabinet. It's probably pretty easy for some hack to bypass stuff and make it turn off and on with a contactor and pressure switch, but it's another story to put it back to how it came from the factory. This thing is four motors and four compressors. Do you know how it's supposed to work? Is it staged? Alternating? What are they running the pressure at? What kind of place is it in?

-Hal
 
From what I understand, they were unable to find any info on this model from an air compressor dealer that sells large compressors. I believe he said they told him it's been discontinued or something. The dealer was the one that sent them the new pressure switch and contactor and told them to have someone wire that in instead of using the control cabinet. The pressure switch they were sent is a Square D pressure switch like on smaller compressors and looks nothing like the old one. I'll have to find out why they rewired this thing with a contactor if it was just a bad pressure switch. @retirede's comment about the solenoid valve being "normally open" makes me wonder if the pressure switch they put on is different than the original. If the control cabinet expects to see normally open vs. normally closed at low pressure and they assumed something in the controls was bad, the guy may have added the contactor just to start it. He may have been in over his head and decided to bypass everything and just make it start for them so he could get out of there. If that's the case, it would be simple enough to reverse that with a relay DPDT relay. I'll have to have the maintenance guy at the plant find out some information for me. It used to be a bottled water plant and they needed a high pressure air comp for something with the bottles. They don't need that high of pressure now and are only using one of the four compressors.
 
Here is a typical control schematic.
If they’re blowing PET bottles, they likely need 1000 psi or more.

It’s likely that the compressors are double-acting (compress at both ends of the cylinder). If so, it’s possible to run 50% load by unloading one end of the cylinder. They would require an additional pressure switch and solenoid valve. This could explain the multiple solenoid valves.

Allen Bradley style ‘C’ pressure switches were what we normally used for control. Those Sq-D ones did not allow for a small enough differential between open and closed, and I don’t recall them being available in pressures as high as you are talking about.

1cb2bfa7f2531ed15f56975d61f92d35.jpg
 
Another note - the air pressure to operate the unloaders cannot be full system pressure on these high pressure systems. If discharge air is used, a regulator is necessary. Often a separate control air supply (100psi or less).

Using higher pressure on unloader valves can cause damage to the compressor and be dangerous.
 
Defeating the original controls and substituting a simple contactor (or implementing other shortcuts without adequately understanding the system) runs the risk of "making it work" while overlooking the possibility that it starts compressing without adequate lubrication, or that starting all the stages simultaneously puts excessive jerk into the system and strain onto the crankshaft. (et al.) It might "work", but its lifetime might be very short.
 
drcampbell said:
Defeating the original controls and substituting a simple contactor (or implementing other shortcuts without adequately understanding the system) runs the risk of "making it work" while overlooking the possibility that it starts compressing without adequate lubrication, or that starting all the stages simultaneously puts excessive jerk into the system and strain onto the crankshaft. (et al.) It might "work", but its lifetime might be very short.
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100% agree!
 
and holy goodness yes, those are huge units!!!
retirede said:
Another note - the air pressure to operate the unloaders cannot be full system pressure on these high pressure systems. If discharge air is used, a regulator is necessary. Often a separate control air supply (100psi or less).

Using higher pressure on unloader valves can cause damage to the compressor and be dangerous.
Click to expand...
Especially to eardrums....
 
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