How to Control Well Pump with Microcontroller?

Discussion in 'The Projects Forum' started by bt101, Mar 7, 2013.

  1. bt101

    Thread Starter New Member

    Apr 21, 2011
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    Hi

    I need a few tips on controlling my well pump with a microcontroller.

    I have a typical 240V, 1/2 hp submerged well pump that is controlled by a pressure switch.

    My MAIN goal is to be able to have the microcontroller DISABLE the pump (mostly to prevent a flood when I'm away).

    I'm having difficulty with the high voltage end of things.

    I see a couple viable options (but let me know if you have other ideas).

    Option 1:
    Leave the things wired as-is and insert a 240V relay in series with the power going to the pump. My problem with this is that I do not want to have a relay that is energized for days at a time (do they make a flip-flop relay, cuz I can't seem to find one).

    Option 2:
    Remove the pressure switch from the 240V cct and use it in a low voltage cct as an input to the microcontroller. The microcontroller would then control a 240V relay to run the pump for its normal operation. At least this way the relay is only energized when the pump needs to run.


    Lastly (for either of the above options or any other proposed options) what is the proper way to have the uC switch the 240V cct. Yes I would use opto-isolation, but I am confused about relays and contactors. Do I need one or the other or both? When I look at specs of contactors, they all seem to be switched by high voltages, so I assume I need an intervening relay to drive the contactor? And they never seem to specify the coil current for contactors, so I don't know how much current the relay needs to supply. Or do I even need a contactor, is there such a thing as a 12V driven 240V relay?

    Thanks in advance.
     
  2. pilko

    Active Member

    Dec 8, 2008
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    20
    "My MAIN goal is to be able to have the microcontroller DISABLE the pump (mostly to prevent a flood when I'm away)."
    The safest and most reliable method would be to just turn off the breaker before you leave.
     
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  3. crutschow

    Expert

    Mar 14, 2008
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    A solid-state AC relay of the appropriate voltage and current rating can be directly and safely controlled from a microprocessor output. There is no problem with leaving such a solid-state relay on continuously. It takes very little power to do this and has no effect on its lifetime.
     
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  4. MrChips

    Moderator

    Oct 2, 2009
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    Most microcontrollers operate at 3V or 5V.

    Yes, you can buy an optically isolated solid-state relay (SSR) that requires 3V or 5V input and controls 240VAC.
     
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  5. bahubali67

    New Member

    Mar 15, 2012
    28
    1
    I have made auto on and off of water pump(2hp)(2min on ,30min off). I have used microcontroller and relay. the ckt will be on 24*7. its just working fine. I have placed ckt at the place of pressure switch.
     
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  6. bt101

    Thread Starter New Member

    Apr 21, 2011
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    I would have to walk down to the basement at last twice a day (so I would likely forget or be in a hurry and say the heck with it) and would have no way to disable the pump when I'm away from the house. Unfortunately it would end up being the least reliable method. I know some people loop the pump wiring cct through their garage and put a switch at that location. However I already have a computer system that knows when I'm home and away so I'd like to take advantage of it. Plus I can access the system from anywhere in the world.
     
  7. bt101

    Thread Starter New Member

    Apr 21, 2011
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    I notice that the consensus seems to be to use an SSR.
    I should double check... are you guys absolutely sure that an SSR would be suitable for a reactive load? I was under the impression that the SSR causes sharp edges on the AC wave as it crosses zero (ie it doesn't completely pass the original wave). Firstly is this correct and secondly when these sharp edges hit the coils on the motor will they shorten the life of the pump?

    I've attached a picture of an SSR with which I am familiar. I've used then for resistive loads. Is this the type that you guys recommend? It only has one pole, so do you just put it on one leg of the 220V cct?
     
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  8. tracecom

    AAC Fanatic!

    Apr 16, 2010
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    Most SSRs consist of an opto-triac and a larger triac to actually switch the load. Look for an SSR that has a zero-crossing opto-triac.
     
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  9. MrChips

    Moderator

    Oct 2, 2009
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    Not correct. If your load is highly inductive, zero-crossing relay will not work.
     
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  10. tracecom

    AAC Fanatic!

    Apr 16, 2010
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  11. crutschow

    Expert

    Mar 14, 2008
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    Zero-crossing relays turn on at the zero crossing of the voltage so I see no particular problem from that into an inductive load. That is the most benign way to apply power to such a load. All triac-based SSRs also turn-off at the zero crossing of the current, so that shouldn't be a problem either.

    One problem mentioned in tracecom's reference is that motors can generate back EMF when turned off so that voltage surge may need a suppression circuit.
     
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  12. MrChips

    Moderator

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    The problem with highly inductive loads is that the current lags the voltage and hence it is not at zero when the voltage crosses zero. The triac will not turn off.
     
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  13. crutschow

    Expert

    Mar 14, 2008
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    But when the inductor (and Triac) current is at zero the voltage across the Triac is also zero (no current, no voltage drop). The voltage across the inductor at that point is not zero but that should not affect the Triac turnoff.
     
    Last edited: Mar 11, 2013
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  14. bt101

    Thread Starter New Member

    Apr 21, 2011
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    Thanks guys, I'll give the SSR a try.
     
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