2 x Air Compressor Automated Changeover Switching?

Discussion in 'The Projects Forum' started by Petrol Head, May 2, 2016.

  1. Petrol Head

    Thread Starter New Member

    May 2, 2016
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    Hi everybody

    I'm hoping you can help me with a project i need to do in my workshop.

    Not having the luxury of 3 phase, i can't invest in the ideal large Air Compressor plant i'd like to, so instead i have 2 x 240v Single phase compressors, each with their own air tank and pressure isolator switch.

    For those not familar with that device, it is merely a double pole switch that turns on when the tank fed pressure valves senses air use down to a certain lower limit, triggers on to make air, and cut out again at the high limit. They also have an outright manual on/off plunger switch normally on top

    they can be seen very simply here



    as you can see they are simply supply and load double pole contacts, controlled by the pressure spring trigger.

    Now, because of my heavy usage of air, it can really push on the compressor duty cycle, so having 2 of them, i would like to have an automated changeover so that they cycle the make air duty between them, giving them both adequate cooling periods whilst the other is active.

    Have i explained that well enough?


    So from the beginning, at the point of using air, the pressure in Compressor Tank A drops, and triggers the motor pump to make air. At reaching its air made level, the pressure switch turns off Motor A, but in doing so triggers a changeover so that the next compressor to make air will be Compressor B, and then reversed again, so the compressor cycle to make air will always be A B A B A B A B A etc etc etc


    I'm sure you are all far smarter than I am at the electronics, relays, circuits etc, but i am an able and competent person of certain abilities, but forgive me if i had instructed granny how to suck eggs!

    I hope i've made the project clear, and i thank you in advance for any help and advice you can offer me

    Cheers

    Shrew
     
  2. tcmtech

    Well-Known Member

    Nov 4, 2013
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    So why not just have both of them cycle on and off together with a 2 - 3 second delay for the second unit to start and stop over the first one?

    They will thusly see equal run time and equal cool down times as the A - B switching system would give them for the same volumes of air being pumped.

    And why can't they run continuous duty? o_O
     
  3. Petrol Head

    Thread Starter New Member

    May 2, 2016
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    , Three reasons really,

    firstly, because between them they would draw around 30amps at peak running together, and i don't have that size supply, so its has to be one at a time

    secondly, they are not identical compressors, just very similar, but one makes air a little quicker than the other one and has a slightly different size tank.

    thirdly, when i'm really using air, they could both be running constantly rather than dropping one tank at a time.
     
  4. tcmtech

    Well-Known Member

    Nov 4, 2013
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    Um.. What?:confused:

    You don't have the electrical capacity to run both the same time but when you are using a lot of air,
    both are running at the same time?

    So which is it? You can or can not run both at the same time and if it is only one at a time isn't that one running continuous duty until the demand for air drops below its supply capacity allowing to catch up and turn off?o_O
     
    #12 likes this.
  5. Petrol Head

    Thread Starter New Member

    May 2, 2016
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    right now i know i'm overdriving them, they are for general workshop use mostly, which is fine, but i shotblast aswell, and thats when whichever one i'm using at the time, can end up running constantly until i give it a break through mechanical sympathy.

    they are not what i'd call professional quality compressors, so i am trying to make the best of a bad situation, and they won't last too long constantly running which is a little pointless as they only just provide enough cfm to as it is for useful pressure for shotblasting

    if they cycled alternately, i'd still have to break off occasionally but far less down time and easier life for the both of them.


    do you have a circuit/switching in mind?
     
  6. gerty

    AAC Fanatic!

    Aug 30, 2007
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  7. tcmtech

    Well-Known Member

    Nov 4, 2013
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    Not really. I'm more of a 'solve the real problem not put a crutch on it' sort of guy. :(

    What is your actual in use current draw for each compressor? If they are both combined just short of your 30 amp line limit you may be able to cheat and free up a few to several amps with nothing more than a simple power factor correction capacitor on each unit thusly allowing both to run simultaneously without issue. ;)
     
  8. Petrol Head

    Thread Starter New Member

    May 2, 2016
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    I didn't say i have a 30amp line limit, i said

    "between them they would draw around 30amps at peak running together, and i don't have that size supply"

    i only have a 20amp supply, which is a just a few amps spare of the peak draw of the slightly bigger compressor


    neither did i say both are running at the same time, i said

    "they could both be running constantly" if i did as you suggested and simply had them running alongside each other.

    both running constantly, and neither compressor having some down time is the anti of what i'm trying achieve here!


    The alternating relay is definitely in the right direction.

    I'm trying to find a UK 240v version, and whilst searching i see there is also timed versions?

    i assume these will simply switch power from one to the other at a predetermined amount of time, regardless of whether the compressor is required for air or not, it will simply cycle A B A B A B supply possibilities, yes?


    picture this as my future simplified set up, one storage of air, two motors and pumps to fill the tank (joined tanks) but they cannot run together at the same time, they must alternate,

    is this relay going to be the best solution?

    twin motor compressor.jpg
     
  9. tcmtech

    Well-Known Member

    Nov 4, 2013
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    Well now that you have given all the relevant information relating to the what and why of your problem then yes a simple timed relay that switches between the two motors every X amount of time would work just fine.
     
  10. Petrol Head

    Thread Starter New Member

    May 2, 2016
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    I cant seem to find many at 240V and none so far above 10amp!

    i think it will be easier to move house and organise 3 phase supply! :D
     
  11. tcmtech

    Well-Known Member

    Nov 4, 2013
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  12. Dr.killjoy

    Well-Known Member

    Apr 28, 2013
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    Imo I would sell the unit and buy the right unit fo you..
     
  13. ifixit

    Distinguished Member

    Nov 20, 2008
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