How to make a fan run with compressor?

Discussion in 'General Electronics Chat' started by sdowney717, Dec 15, 2014.

  1. sdowney717

    Thread Starter Member

    Jul 18, 2012
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    [​IMG]

    The schematic shows the thermostat completing the circuit back to neutral white wire.

    It looks like the compressor is hot all the time and the thermostat closes the switch on the neutral - ground side of the circuit.

    I would like a fan to come on when the compressor runs and go off when the compressor goes off.
    The fan is running off 120 vac that comes from a wal wart. I am adding an internal fan from a computer to move air inside the freezer. The fan is 12vdc.
     
  2. cmartinez

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    If the circuit can handle the load (and I'm pretty sure it does), just connect the fan in parallel with the compressor. If it can't, then I'd suggest you use a solid state relay for the fan that is triggered by the compressor.
    EDIT: I hadn't noticed that the fan you want to add is 12V, that means that you have to add a relay (preferably solid state) that would be triggered when the compressor starts
     
    Last edited: Dec 15, 2014
  3. MaxHeadRoom

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    What is actually switching the compressor off?
    If the compressor is 120vac as suggested and the fan is 12dc from a Wall-Wart, wire an outlet for the the wall wart from across the compressor motor supply itself.
    Max.
     
  4. cmartinez

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  5. MaxHeadRoom

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    I think you have connected across the start capacitor/winding switch and that may not be as easily available as just across the L1 and the neutral.
    If that is the start winding switch it appears to be shown as N.O. instead of N.C.
    There must be something controlling the compressor off the top of the page?
    Also the relay idea still needs the 12v for the fan?
    Max.
     
  6. sdowney717

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    Jul 18, 2012
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    The 120vac compressor is turned on by the thermostat connected to neutral (ground), so is at the end of the circuit.
    The DC fan will run off a 120 vac wal wart.
    If I connect walwart fan in parallel with compressor it will run all the time?
    I dont need a relay?
    So where to connect the walwart so that it gets 120vac when the compressor is turned on by the thermostat?

    Compressors have common, start, run terminals. Your showing a relay connected from the start winding back to neutral. I do think there is back EMF on that start winding, but I think its 50vac

    Anyhow, I think all of them would have power if connected to neutral, even if compressor is off, as then your bypassing the thermostat switch.

    The start winding gets its boost from the potential relay, when the back EMF gets large it opens the relay and the boost is gone.

    I will measure the volts across start and run terminals and see what it reads.
     
    Last edited: Dec 15, 2014
  7. MaxHeadRoom

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    Wire the wall wart to the blue wire on the thermostat and the left hand black live conductor, it will switch on/off with the compressor.
    Max.
     
    #12 likes this.
  8. cmartinez

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    If by walwart you mean a 120VAC to 12VDC adapter, then if you connect it instead of the relay coil as I showed in my previous post then it should work in unison with the compressor
     
  9. cmartinez

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    If you mean conn
    If you mean connect it like this:

    proxy.png

    Then it should also work, but the fan's load would be taken by the temperature control switch, along with that of the compressor... which shouldn't be a problem...
     
  10. sdowney717

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    It works if I measure volts across the run and start terminals, off = 0 vac, on = 121 vac
    So that worked.
     
  11. MaxHeadRoom

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    That will not be across 120v but in series with the motor, the relay is a current sensing relay to switch the start winding in, it is not connected across 120vac.
    As I mentioned, the relay contact would normally be normally closed for start operation, open in run.
    Max.
     
  12. sdowney717

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    It will work good just connected from run to start terminals, why I dont know, but it does
     
  13. MaxHeadRoom

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    Yes.
    I would expect the W.W./Fan to be insignificant compared to the compressor.
    Max.
     
  14. sdowney717

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    I see, it can be done two ways, using run and start off compressor, or hot line to thermostat.
    I think it is easier to do from hot line to thermostat.
     
  15. cmartinez

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    Then problem solved... I think...
     
  16. sdowney717

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    yes, I think so. Sometimes I am kind of dum, I see now how this was easy.

    So can any explain why I get 120vac when measuring across run and start terminals on the compressor with it on, and don't with it off.
     
  17. MaxHeadRoom

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    As I mentioned, the contacts shown would have to be N.C. not as shown open, when in the off state, for normal start cap operation, the start cap/winding normally switches out when in run, so measuring in the run condition you are getting a reading from Live, (run winding) and through the (now) unconnected start winding back to neutral.
    Max.
     
  18. cmartinez

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    I'm a little confused here... what kind of motor is the compressor using? Is it a single phase induction motor like this one?

    tmp9C12_thumb4_thumb.jpg


    Also, I don't see a capacitor anywhere in the original diagram.
     
  19. sdowney717

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    Jul 18, 2012
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    ok, so the power is flowing through the start winding back to common and out through thermostat back to neutral, I get it thanks.
    The resistance in the start winding must not be enough to do bad things to the voltage. BUT I suppose you dont want current flowing through the start winding, it is supposed to be off when the compressor is running, so using start and run for power is a bad idea. That will create extra heat in the windings.
     
  20. sdowney717

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    Jul 18, 2012
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