Controlling DC motor when load is ON

Discussion in 'General Electronics Chat' started by DJ Fahed 1, Sep 4, 2016.

  1. DJ Fahed 1

    Thread Starter New Member

    Sep 1, 2016
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    Hello everybody,

    In the circuit attached, I want to turn on the DC motor (valve), which needs 24V as power supply, only and only when the switch is closed, i.e. the AC motor is ON.

    The problem is that I don't have access to the AC load, I can only place a current sensor (ASC712) which delivers an image of the current drawn by the load. If there is a current in the AC circuit, the DC motor goes ON, otherwise it stays OFF.

    Can anyone please help me at least with idea to how to do that? Thanks.
     
  2. cmartinez

    AAC Fanatic!

    Jan 17, 2007
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    I'm not sure what you're asking. Are you asking about the what type of control you need to accomplish that? Or do you want to design said control?
    Also, may I suggest you take a look at this sensor? There's no need to cut the wire leading to the motor in order to sense the current passing through it.
     
  3. DJ Fahed 1

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    Sep 1, 2016
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    I want to design the said control. I want at least the general design of the control box, not necessarily in details.

    The ASC712 is the only one I have for the moment, I don't want to purchase others.

    Thanks.
     
  4. cmartinez

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    Ok, no problem.
    1. Is your 24V motor AC or DC?
    2. What's its rated current?
    EDIT: never mind question #1, I've just noticed in your diagram that it's DC.
     
  5. DJ Fahed 1

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    Sep 1, 2016
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    It's DC, the valve's motor (electric valve)

    1A max.
     
  6. MaxHeadRoom

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    A couple of things I can see, if you are controlling a 1ph 240v motor why not use the switch to control a contactor (DP BTW) an auxiliary contact on the contactor can pick up the DC motor?
    Unless I am missing something, this would be the customary way.
    Max.
     
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  7. cmartinez

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    Then it's a piece of cake... I'd use a simple SSR (solid state relay) to control the thing. Just like this one.

    54135-8221778.jpg
     
  8. cmartinez

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    Another question. You implied that your valve is motor-controlled. Do you need to reverse the voltage to close it after the other motor (M1) stops running?
     
  9. MaxHeadRoom

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    Or a simple power mosfet to drive the motor direct.
    You might want to wire a BEMF diode across the valve, if not reversed.
    Max.
     
  10. cmartinez

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    Yup... your suggestion might be the simplest answer ... my personal and humble opinion is that I don't like mechanical contactors, I very much prefer SSRs.
     
  11. cmartinez

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    Something tells me he's gonna need an H-bridge. That's why I think your contactor suggestion might be best.
     
  12. MaxHeadRoom

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    Although if the contactor aux was used to turn on a Mosfet, the current would be trivial if not wishing to switch the 24v direct.
    Not sure from the Diag whether the AC source is phase pairs or a live and N?
    If the latter a single pole power relay with aux would suffice.
    Max.
     
  13. DJ Fahed 1

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    Sep 1, 2016
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    Actually, the switch is inside the motor. I can't access it, I can only pick up the current drawn by the motor.

    The AC source is L & N.
     
  14. cmartinez

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    Does the motorized valve include limit switches or sensors when it reaches its open and closed positions?
     
  15. DJ Fahed 1

    Thread Starter New Member

    Sep 1, 2016
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    No, when the circuit closes, the valve closes.

    But how to control the SSR using the signal delivered by the sensor?
     
  16. DJ Fahed 1

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    Sep 1, 2016
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    No.
     
  17. cmartinez

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    See the picture I posted in post #7? Ground would be connected to terminal #4 of the SSR, and your sensor's output would be connected to terminal #3. Whereas your power supply's +24V output would be connected to terminal #1, and terminal #2 to the motor.
     
  18. cmartinez

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    Then how is the valve's motor supposed to stop when it reaches it's fully open position? And how does the valve close after you remove current to its actuating motor? Through a spring?
     
  19. cmartinez

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    Maybe it would be best if you posted your valve's datasheet, so we can take a look at it.
     
  20. DJ Fahed 1

    Thread Starter New Member

    Sep 1, 2016
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    The valve has only two positions : ON & OFF.
    When there is DC voltage across it, it turns fully ON, when there is no DC voltage it closes. It is inside a compressor, and there is no reference number on it.

    The ASC712 has a typical sensitivity of 185mV/A, which means that if a minimal current of the motor (load) of 100mA is drawn from the AC source, the sensor will give a signal of 18.5mV. I don't think that's enough the close the SSR?

    Other thing, the current drawn by the load is alternative, which mean that it's image is also alternative. How to use that to control the SSR?
     
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