# 2 compressor alternator



## garfield (Jul 30, 2009)

Anyone made an alternator for a 2 compressor setup? How about a recommended product. I need to run a then b unless air is below a certain threshold then run both. There's an ingersoll rand unit that isn't working right. I haven't called them but doubt i will get much help.


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## telsa (May 22, 2015)

Cycling back and forth with your compressors is a poor design scheme.

1) You want to use strictly one compressor for 90% of your demand. Unlike living organisms, machines are happy to stay busy -- even 24 hours a day.

2) The second compressor should be set to come on when the pressure is too low.

This scheme eliminates the 'alternator.'

It also means that when compressor A finally conks out, you still have compressor B... which can soldier on while compressor A is attended to.

If there is one thing that you ought to consider: it's an auxiliary blower aimed at the compressor head -- that continues to run after the compressor stops... A temperature sensing control, like that on high end automobiles, would do the trick.

This _one_ thing would hugely extend the life of your gear.


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## micromind (Aug 11, 2007)

telsa said:


> Cycling back and forth with your compressors is a poor design scheme.
> 
> 1) You want to use strictly one compressor for 90% of your demand. Unlike living organisms, machines are happy to stay busy -- even 24 hours a day.
> 
> ...


I agree.

Given the choice, I never set 2 things up to alternate. 

Look at it this way, if they alternate, then there will be roughly the same amount of time on each unit. When one fails, the only one left has a ton of time on it and very likely it'll fail soon. Then you'll have none........

I usually set 2 compressors up with each unit having its own pressure switch. One unit is set a bit lower than the other so if the air demand is too much for one unit, the other will kick on and catch up. This way, if the first unit fails, the backup is in good condition.


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## garfield (Jul 30, 2009)

I can see the logic of running one compressor each day because the oil would be warmer etc. On the contrary the electric motors would like starting better if they were colder. I still need something to accomplish this. Could use a PLC and some transducers but surely there's a premade product.


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## inetdog (Apr 13, 2016)

garfield said:


> I can see the logic of running one compressor each day because the oil would be warmer etc. On the contrary the electric motors would like starting better if they were colder. I still need something to accomplish this. Could use a PLC and some transducers but surely there's a premade product.


The motor on a typical compressor is designed to handle both continuous running and frequent starting. 
Just don't short cycle it (several times per minute) by using too small a tank for the air usage pattern.

Sent from my XT1585 using Tapatalk


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## sbrn33 (Mar 15, 2007)

I do this all the time with waste water pumps. pretty standard. Honestly I your situation I would buy a $100 Zelio or Logo and program it. If you don't like that idea look up duplex pump stations. 
If you want help please PM me.


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## sbrn33 (Mar 15, 2007)

telsa said:


> Cycling back and forth with your compressors is a poor design scheme.
> 
> 1) You want to use strictly one compressor for 90% of your demand. Unlike living organisms, machines are happy to stay busy -- even 24 hours a day.
> 
> ...


You have no idea when it come to reliability.


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## Southeast Power (Jan 18, 2009)

micromind said:


> I agree.
> 
> Given the choice, I never set 2 things up to alternate.
> 
> ...


The same logic would dictate the alternating setup would last twice as long.
I've seen dual compressors set to alternate both daily and weekly.

I like the setup of using compressor #2 for make up pressure.


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## splatz (May 23, 2015)

sbrn33 said:


> I do this all the time with waste water pumps. pretty standard. Honestly I your situation I would buy a $100 Zelio or Logo and program it. If you don't like that idea look up duplex pump stations.
> If you want help please PM me.


I have worked on some of those pump controllers too. They call the pump that comes on first the lead and the one that comes on second the lag. There are all different configurations, you can get controllers that will alternate the lead and the lag, etc. 

I'd might hesitate a little to use pump controller for compressors just because it might be seen as hack if someone else works on it, even if they serve the same function. So out of curiosity I just googled "lead lag compressor controller" and it looks like they're out there. If they are not out of the budget, they're probably the way to go. 

You can certainly program a PLC that will do the job. If there are other controls you want to combine, the PLC would probably be the best choice. For example a cooling fan would be easy to add. Otherwise it might be a waste reinventing the wheel, usually a packaged product is a lot faster and cheaper than rolling your own.


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## sbrn33 (Mar 15, 2007)

splatz said:


> I have worked on some of those pump controllers too. They call the pump that comes on first the lead and the one that comes on second the lag. There are all different configurations, you can get controllers that will alternate the lead and the lag, etc.
> .


I just did that also. That is the ticket. If money was a big deal I still might program a little Zelio or Logo, but if you could install one of those prebuilts you would look like a stud. 

http://www.airbestpractices.com/sys...ystems-small-and-medium-sized-air-compressors


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## telsa (May 22, 2015)

sbrn33 said:


> You have no idea when it come to reliability.


Heat kills equipment.

That's why compressors have some sort of fan mechanism built in, among a slew of devices.

So, I actually have a pretty good handle on the principles.


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## JRaef (Mar 23, 2009)

Standard product available from most of the bigger motor controller mfrs, called a "Duplex controller", consisting of two starters + an alternator relay. Used for pumps or compressors, the only difference being whether the control input is from a float switch or a pressure switch.

You can build your own if you want to and UL listing of a "control panel" is not an issue. You would need an Alternating Relay. Again, most of the bigger players have them. The absolute simplest one to use though is one originally sold by Furnas, but when Siemens bought Furnas they dumped off all of the pressure switches and alternating relays to Hubbell Industrial Controls. 
http://www.hubbell-icd.com/icd/components/files/5247PL.pdf


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## splatz (May 23, 2015)

JRaef said:


> Standard product available from most of the bigger motor controller mfrs, called a "Duplex controller", consisting of two starters + an alternator relay. Used for pumps or compressors, the only difference being whether the control input is from a float switch or a pressure switch.
> 
> You can build your own if you want to and UL listing of a "control panel" is not an issue. You would need an Alternating Relay. Again, most of the bigger players have them. The absolute simplest one to use though is one originally sold by Furnas, but when Siemens bought Furnas they dumped off all of the pressure switches and alternating relays to Hubbell Industrial Controls.
> http://www.hubbell-icd.com/icd/components/files/5247PL.pdf


Would an alternating relay work? 



> I need to run a then b unless air is below a certain threshold then run both.


I would think that would of course alternate the load between compressors, but it would lack the ability to run both at once during especially high demand. The lag kicks in at a second, lower setpoint.


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## telsa (May 22, 2015)

The simplest design is two compressors set at different cut-in and cut-off points, using the pressure in the tank as control.

An extensive radiator to condense entrained water is also a worthy idea.


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## mitch65 (Mar 26, 2015)

http://www.eaton.com/Eaton/Products...trol/RelaysTimers/AlternatingRelays/index.htm


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## Jackofmany (Oct 10, 2015)

All you need is an alternating relay such as a Macromatic ARP120A3R.
This relay will alternate two compressors and provide a lead/lag function with two pressure switches.
Be sure to follow connection diagram as lead and lag pressure switch must be connected to correct input.
Available from Granger for about $55. Cat# 6MPN9 for 120 volt control, other voltages available.


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## JRaef (Mar 23, 2009)

With even the simplest alternator that changes the "Lead" compressor, all you need is a second pressure switch set to call for the "Lag" compressor. Since the lag unit is always going to be whichever one is not already running, all you do is have the 2nd pressure switch close a control relay that turns on BOTH compressors. One is already running so that's irrelevant, but it adds in the one that was on standby. Have that relay seal itself in from the output of the main pressure switch circuit so that once you engage both compressors, they stay on until the pressure is satisfied.









This diagram doesn't reflect that seal-in version, so the Lag device would drop off and leave only the lead unit, but you hopefully can get the concept.


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## garfield (Jul 30, 2009)

Is Macromatic a good quality component? I have never used that brand.


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## JRaef (Mar 23, 2009)

Yes. They are mostly in the industrial instrumentation and signal conditioner world, that's why you may not have heard of them, but what they make is good stuff.


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