alternate between 2 compressors wiring q?

[The Cobbler]

I thought I recall a thread on here where the discussion was about wiring 2 compressors (a&b ) so they would alternate a-b-a-b etc, and if the pressure fell below a certain level they would both kick on.
can someone help?
tks

 

[Bert_]

Duplex compressors are common. You would need 2 pressure switches a few relays and an alternating relay.

 

[LXCam]

What do you have for starting the motors. Just a pressure switch or magstarters.

 

[DSMR]

I would use a PLC, 2 pressure switches, and 2 motor starters to do this.

 

[matt_i]

This logic seems a little intimidating at first but its 3 ice cube relays that do all the work. No need to integrate a PLC. There are two pressure switches, the PS-1 is the "top end" switch that deals with low volume needs and it toggles from motor 1 to motor 2 (MS = motor starter, PS = pressure switch, CR = control relay) The PS-2 is lower than that and turns on both motors. Once PS-2 is satisfied the toggle action will continue.



We can walk thru this in english if you are prepared to read a little bit. Assume that all relays are in the non-energized state to start with, as when its first powered up.

PS-1 calls for air.
[Not 60] is true
[Not 50] is true
CR 40 is energized, also energizing the [40] seal circuit.

Skip down to 3rd rung
[40] is true
[Not 50] is true
CR60 is energized, also [60] seal branch is true.

Back to first rung
[Not 60] is false but CR40 continues to be energized because of the seal around it.
Until PS-1 is satisfied. And CR40 drops out and MS-1 stops the motor.

Skip to 2nd rung
[60] is true, being held by the [60] seal circuit in the 3rd rung.
[Not 40] is true
Waiting for PS-1 to call again, which this time energizes CR50 and then MS-2 motor answers the call.

If at any time the pressure drops low enough to close PS-2 then both motors will start until its satisfied, then one motor will stop and the other will top off the compressor tank.

 

[The Cobbler]

ouch... seems over my head
I was hoping there was a simple solution

 

[86turbodsl]

Matt-i, still ladder logic. PLC would be more fun, no?

 

[sberry]

Do they really need to alternate?

 

[matt_i]

It looks like a lot of spaghetti and would be a little wiring. But the wiring diagram is right there. Each line that connects a feature is a wire and its just a matter of running them one at a time. Lets say each relay + base is $15, so you're into $45 plus an enclosure. All of the above could be wired using #16 or probably #18 for that matter. Just landing wires under screws after that. Cheapest PLC I know of now is the Automation Direct's CLICK series, its on the order of $125, need a cable to hook up to the CPU, software is free. A PLC would be useful for other features, could have maintenance interval warning based on retained timers, could setup auto-drain strategy and could also use interlock to lighting or other occupancy sensing to disable the motors when nobody's home.

I bet there's some sort of "toggle box" that one can buy. I'm going to guess its not $50 and its not $100. Probably $3-500. The same basic control is used when you have 2 pumps draining a sump and want to equalize wear. Instead of PS you'd have float switch FS and the FS2 would be the high level that kicks in both pumps.

 

[LXCam]

If you have magstarters on these, it can be just as easy using this with a little reconfiguring. That's why I asked.

https://m.grainger.com/mobile/produ...kwcid=AL!2966!3!281733020606!!!g!539397194111!

But if you're using the pressure switches as across the line starters, matt laid it very well with the exception of pointing out utilizing the correctly rated contractors and isolating the feeder circuits. It totally depends on what you have for compressors.

 

[sberry]

As I recall the poster is a hobby/home type,,, best I can recall so,,,, while this might be a noble idea is it something that is needed or makes sense? This is often for companies with hi air demands, new comp setups etc. I simply have my backup/demand unit set a little lower cut in than the main, if it needs it can catch up and really use the tanks and in fact since I am on 24/7 have the breaker off to the 2nd unit.
I wire wheeled 20 minutes the other day, cleaned 4 wheels just as the air was sagging. Did it from 3 hp.

 

[udderlyoffroad]

You’re describing a fairly standard industrial duty/standby setup.

Let’s step back a second; question is what, exactly, are you trying to achieve?

Sberry beat me to it, but If I had to guess, I’d say you recently replaced your first small compressor with a larger one? The new one is ok, but you’ve got…say, a blast cabinet where it quickly uses up the available flow, so you’d like to be able to run both compressors together.. Similarly, there’s nothing wrong with the old one, and you’d like to be able to start that one up on its own to, say, quickly check the tires on SWMBOs car without waiting for the big one to air up. Obviously, you want to do this so they don’t both kick on at the same time, so as to not trip breakers.

I don’t think you need to get into PLCs to do this. I wonder if attempting to set the pressure switches on both compressors so the second one kicks in at a higher falling pressure, and turns off at a lower rising pressure would do it? This would give you 80% of the functionality (I think) you’re shooting for, for $0.

You’d only turn on the second compressor when you needed the greater flow. Ok you wouldn’t be able to select duty/standby, and you’d need to invest in a decent filter regulator setup, and piping upstream to common the 2 compressors together. But you’d need to do that regardless.

Or am I wide of the mark here?

Matt

 

[86turbodsl]

I buy my plcs used on ebay. I think i have 4 now, DL06 and 3 DL205. They are cheap with a little hunting and good buys. Software is free up to 100 lines i believe.

I agree, it's overkill, but it makes adding features cheap later too.

 

[The Cobbler]

OK, thanks for all the advice
I guess I should say this idea I have is probably a pipe dream .
I have several air compressors, in fact more than I need . (lots of portables for work)
I have a 5 hp v4 cylinder Jenny Frankenstein that I have most of the parts for but am thinking of selling that .
most of the time my air needs are quite low. on occasion I need more volume so that's what spurred on this thread.
I have a twin cylinder 30 gallon ( Craftsman) ingersol rand piston compressor that suite most of my needs . I just acquired a similar brand new Sanborn twin compressor that had a broken flywheel . my thought was to hook them both up so each one gets similar use, and if my demands got higher than normal both would work.

 

[sberry]

Right, got it. If its a sturdy little pump would put some Amsoil or similar to it and pump an auxiliary tank when I wanted a lot of air. I would learn to time and stage the work so once it came on the breaks were to recover pressure prior to shut off and resume work which allows it to run as continuous as possible at the highest pressure. Valve it on so you needed it when you needed it, use a single tank for tasks and go to bigger one as needed.

Set air tools at correct pressure saves a lot of wasted power. The extra tank isn't an end all but gives it a lot more range if the pump can run continuous than if it cycles,

 

[sberry]

A modest 30 gal 240V unit makes 2x what a 120V one does. The tanks are small which is ok for short run tools, direct hose type systems but the on off cycles are frequent on the small tank and instead of it being off can be on for way longer cycles. If the pump is running and tanks near full the pump is making 1/2 enough for the tool with full tanks, allows for longer run and pacing by a skilled operator. With big tanks I can do body work easy with 3 hp, its on the edge of a constant challenge with less but really body paint stuff is 5 hp.

 

[sberry]

I guess I should say this idea I have is probably a pipe dream .

It was or is done. You don't need it and not worth it. I don't have it. I want rather plug and play, hook them together with a t. Add a valve if you want to isolate it. Look at my pic above, No common controls. Breaker for each unit. If I am walking out to sandblast can energize the second unit, tank already, no breaker starting even. Can charge either compressor with either one and be seamless.

 

[sberry]

If I really needed more would hook up a 5 if I had it and could. 5 is a real standard and about twice the one you use now.

 

[wyliesdiesels]

seems like a solution in search of a problem.

If you dont have a need for it, move onto something that you do have a need for.

 

[udderlyoffroad]

Sounds to me like the 5hp is all the compressor you’ll ever need. If I were in your situation, I’d fix that up so it runs sweet and have it as my main compressor and be done with it? Saves screwing around with pressure switches and relays. The time and effort I saved could be put into building an external compressor shelter, thus freeing up space in my shop.

I know most of the time 5hp will be overkill. But even if you value your time very low, it’s still going to be a better investment in your time to get one big compressor running right than the electricity it’s going to ‘waste’ by being capable of too much flow for most of your use.

If you still felt ambitious, you could keep one of the smaller compressors (the one that needs the most parts), for its air tank alone, to add some extra receiver volume, and gradually acquire parts to rebuild this unit into your ‘backup’ compressor. Obviously you’d fix up and sell the easier-to-repair compressors to fund this…

That’s my £0.02 anyway…

Matt

 

[TractorJeff]

Mostly retired now so the electric fills my bill but when more air is needed I tee in my engine drive.
I agree in the theory of just going with the larger unit but also adding receiver volume and getting the correct pressure out on the lines.
One unit, one maintenance schedule, more floor space!

 

[sberry]

If its a hobby to fiddle with equipment than it doesn't make much difference, if you want it to get something done and cost is an issue then its another. A worthwhile long term move is to fix something up that is long term and meets demand. Low duty cycle makes oversized equipment last forever.
If I am a small operation want a simple system as was suggested, single machine, single electric connection, simple plumbing with a hose goes where I need it. I have a full system but back in the day slumming around painted a lot from a compressor and 100 ft hose, fix up a whole semi.

If you are trying to sand, running the little comp an hour a day nonstop then you need a bigger unit. If you had to wait 5 minutes on air last month then its probably not a big deal. Air is for time and money. Its not efficient but is to let the man do a lot of work fast. It cost 3 grand to paint a car an extra 5$ in electric to knock off a days labor isn't squat.
We do use electric 4 1/2 grinders though and since we got to buy wheels its a lot cheaper to run than air for the same work. More convenient and a fraction of the operating cost, both power and maintenance.

 

[sberry]

I got a bud with a substantial repair shop. He has 3 or 4 guys tops and has 1 good comp, a 10 hp sized just right for him. He is on power that can support it. He has the skill and the ability to make it any way he wants but has decided buying the right plug and play unit is worth it.
I bought some new back in the day,,, today would shop for used to get the same or better for a fraction of the cost.

 

[sberry]

You can sand steady or more than a guy can stand to sand with 5 hp. 5 with 80 gallon is a real standard anymore. The tank will deliver to big air gun and hi pressure for airing up truck tires, can run all the common air tools.
The 5 have a 3/4 port on the tank I think and the old 3 hp 2 stage mechanics comps have 1/2 for hooking to small shop hard lines where they didn't worry about losses or often a hose right off the tank. They would run a 3/4 air gun a little and air up truck tires just fine.
The Judges, the trucker here was setting up shop and was fussing over air comps. He owns 48 tires but a 3 hp would have worked about as good as a 10 and a 5 would have worked with the electric service he was on. He can remove or inflate during any given cycle for the most part. Unless he had to have more for additional crews the ability to do more wasn't going to help.

 

[The Cobbler]

Okay, you guys are talking me in to keeping the 5hp. LOL
all I need for it is the motor pulley, belts & a mag starter . then assemble all the pcs.
when I say Frankenstein, it's all new parts, except the motor that was a return from Princess Auto that was gently used. It looks unused to me but I wasn't going to argue with them . I think I paid $100 for it and no visible install marks . and yes, it does run fine .
I acquired the v-4 jenny pump on clearance for I think $50 as it was missing the bolt that holds the flywheel to the shaft . it was 1/4 20 LH , I since have acquired that
brand new 80 gallon tank .

 

[dogdog]

Not sure what are the sizes and pressure of each tank...

If you wanted to use the larger one as stand by and wired in... it's simple logic..

adjust the larger compressor with cut in pressure lower (cut in pressure is when the compressor trigger on) than the smaller one... and pipe the regulated output of the larger compressor to a shutoff valve --> check valve --> Smaller tank at a regulated pressure no greater than the smaller tank's rating.

keep the larger compressor off when you don't needed it...

when both compressor are on...

Both tanks will supply air, the larger tank will supply a regulated pressure via a check valve, when the system pressure drops below 100PSI the smaller compressor kicks in...
when the larger compressor (set at lower cut in pressure) drops down the cut in pressure the larger compress runs... if the larger tank pressure is less than the smaller tank.... the check valve will prevent the air from the smaller tank to enter the larger one...

If your tool consumes at larger rate that drops the system pressure below both compressors cut-in then both compressor turns on... the faster filling one (probably the larger one in certain scenario would also recharge the smaller tank)..

If the smaller tank recharge faster... the larger tank will still run to recharge... the check valve would have prevent the air from smaller tank to enter the larger tank... effectively stopping the smaller compressor from charging the larger compressor.

That is in theory... not sure in practice.... never encounter a situation I would need this... of cause if you find an electronic valve... and two pressure transducer... you can have some fun with the valves as well...and some electronics to make your compressor smart

at least with this setup your wiring is independent of any other wiring... you will needed to set each compressor up with a proper fuse/breaker and wires independently... it's purely mechanical... all you needed is a ball valve for shutoff the pressure between the two tanks... and a check valve... and of cause know how to adjust the cut-in....

 

[alfredeneuman]

To get the compressors to both run at the same you'll need to put a couple of pressure switches (wired in series) in an airline anywhere after the combined compressors come together. 1 normally open, for the low pressure, and the second, normally closed, for the cutout pressure
After the switches, connect the coils of 2 properly HP rated contactors.
The input side of the contactors will be hooked up to the input lines of the compressors' respective motor contactors, or if no contactors, tap off the incoming wires.
The load sides should be tapped to the wires going to the motors.

This will temporarily bypass the alternating relay, and take it out of the picture.

 

[DSMR]

To get the compressors to both run at the same you'll need to put a couple of pressure switches (wired in series) in an airline anywhere after the combined compressors come together. 1 normally open, for the low pressure, and the second, normally closed, for the cutout pressure
After the switches, connect the coils of 2 properly HP rated contactors.
The input side of the contactors will be hooked up to the input lines of the compressors' respective motor contactors, or if no contactors, tap off the incoming wires.
The load sides should be tapped to the wires going to the motors.

This will temporarily bypass the alternating relay, and take it out of the picture.

Have you ever done this before?

He wants to alternate between compressors (you mention running at the same time). I could probably draw out what you are saying but it is wrong. Maybe you can draw it out and it would make sense.

 

[alfredeneuman]

From the original post:
"and if the pressure fell below a certain level they would both kick on"

 

[alfredeneuman]

Yes, I've done this before. The compressors powered pneumatic A/C dampers.
Night work in the rooftop equipment room of 12 story building a block from UCLA.
You had to get there between 10:30 and 11:00. The elevators shut off at 11, and didn't come back on until 6.
It was a logistical nightmare with parking, (the van was too tall for the parking structure) building access, 12 stories of stairs to go down when you were finished and having to have it up everyday by 5:30.
All the EC supplied was an alternating relay of the wrong voltage. I had to order the rest.

 

[Diver_John]

Duplex compressors are common. You would need 2 pressure switches a few relays and an alternating relay.

Two small air 12v compressors, inside a sealed vented enclosure, floating on the water, proving surface air to two divers, no deeper than 10-15 ft. Flow and Motor/Pump heat, are the obstacles. A single compressor is underrated for the task at hand. (due to cost and power consumption. Combined, 15 AH is fine.

I'm still trying to figure out, using two ("A"+"B") matched small air compressors, sharing one load/demand, and how only "A" could run, while the "B" is cooling, and vise versa. (based on duty cycle 50%). Both should be running together as much as possible.

I dont know how to wire the the two pressure switches, relays, and an "alternating" together so that its fully automatic, providing the proper flow and keeping the motor'(s) heat under control. I thought I could switch between both motors using specific auto reset, "temperature" on/off switches, mounted to the cyl heads, in conjunction with a single pressure on/off switch. but that's not enough.

All the other single gadgets in the enclosure, are easy.

ANY IDEAS/HELP WOULD BE APPRECIATED..

​

 

[Diver_John]

ouch... seems over my head
I was hoping there was a simple solution

me too.

 

[Kmp259]

Go to Ingersoll Rand and buy a alternative switching box....

I have been running one for 27 years....with 2 each 5 hp IR 80 gallon compressors....great set up