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AC fan speed control by periodic on/off switching

Discussion in 'Electronic Design' started by ectoplasm, May 3, 2007.

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  1. ectoplasm

    ectoplasm Guest

    I have a fan (110 V AC) that I want to control the speed of.

    I want to do this by periodically switching it on/off. Say, period of
    about 5 seconds; and the 'on' period can be controlled as 10-100% of
    this time. I already have a fan that has this built in, and it works
    great, silent too. Basically it's variable pulse width control of an
    AC appliance.

    I probably need a thyristor / triac circuit for this, switching on the
    null-crossings. Would be nice if the period time could also be

    Searches on Google failed because I don't know what keywords to look
    for. Can someone help me out?

  2. Roger_Nickel

    Roger_Nickel Guest

    Should be able to cook something up with a solid state zero crossing
    switching relay and a simple op amp based pulse width modulator. you could
    also use the ever-popular dual 555 to generate the PWM waveform and power
    can come from the mains via a dropper resistor/rectifier/zener regulator
    arrangement. Safety first; plastic enclosure, plastic shaft on control
    potentiometer, plastic mains switch housing and operating lever. Proper
    cord anchorages (get the proper clamping grommets for this). Build it in
    the fan housing if you can to save some trouble. Five seconds for the
    switching cycle seems a bit long; a couple of seconds would reduce
    hunting. Watch for overheating of the fan motor.
  3. GPG

    GPG Guest

    Integral cycle control. Triac drivers wwith this feature are available.
  4. LVMarc

    LVMarc Guest


    the idea for speed control is good. However, instead of crating a few
    cycles of 120 60 hz and then a few off and aFEW ON ETC,, CONSIDER THE
    WAY THAT LIGHT DIMING IS PERFORMED... in this case each 1/2 cyle of the
    60 hv power is allowed to fow of 180 degrees (full speed, or 90 degress
    1/2 speed etc.)

  5. Rich Grise

    Rich Grise Guest

    Integral-cycle control dramatically reduces the hash.

    And it could be better for the fan motor, as well, assuming that this
    is the kind of motor that can be controlled that way in the first place.

  6. ectoplasm

    ectoplasm Guest

    Thanks for these great tips, Roger Nickel.

    I would like to get power from the mains, yes. What do you mean by
    'rectifier', by the way?

    To get a low voltage DC supply from the mains, more must be needed
    than just only a resistor + zener diode, right? Could you elaborate?

  7. ectoplasm

    ectoplasm Guest

    I found a triac driver, the MOC3041, a 6 pin DIP. It has a zero
    crossing circuit, LED optocoupling. I can use this one; I'll need to
    have an extra triac that does the main switching.

    But you say there are triac drivers that have integral cycle control
    BUILT IN. Could you tell a model number? I couldn't find it in Google.
    Are there any online searchable electronics parts catalogs?

    Thanks !
  8. You'd do better with a cycle control system using full cycles.
  9. John Fields

    John Fields Guest

    If you could settle for an on and off (both variable) period of 255
    seconds or less it can be done with a single 8 bit down counter, a
    "D" type flip-flop, the TRIAC driver you have now (I'm not psychic,
    I read your later post ;)) a TRIAC, some switches, and some pullups.

    And a power supply of some kind. Since it's not going to be
    dissipating much power, something using a capacitor straight off the
    mains, a rectifier and a Zener ought to work fine if you can stand a
    non-isolated supply.

    Want a schematic?
  10. MooseFET

    MooseFET Guest

    There is something that I have wanted to try out for quite a while.
    It is a variation on dropping cycles that I think would lead to lower
    losses in the motor. I will explain it and others can shoot it down
    if I am wrong.

    To decrease the operating speed of an induction motor, it is best to
    lower both its voltage and frequency. A device like a triac can be
    fired to allow an alternation through or not.

    The triac it fired on the positive alternation.
    The negative alternation is skipped
    The positive alternation is skipped
    The triac is fired on the negative alternation
    The positive alternation is skipped
    The negative alternation is skipped
    .... repeat

    This lowers the frequency to 1/3rd that of the mains frequency.

    The RMS voltage on the motor would be 1/sqrt(3) of the normal line
    voltage. However, the frequency content will be a bit funny. If I
    did my math right, only 0.4 of that will be at F/3.
  11. I have a fan (110 V AC) that I want to control the speed of.

    Use a light dimmer.
  12. ectoplasm

    ectoplasm Guest

    Nah. LVMarc also suggested it. Not so good to cut up the phase (HF,
    net pollution, might have bad effect on motor). See also Rich Grise's
  13. ectoplasm

    ectoplasm Guest

    If you could provide one, sure, thanks.
  14. ectoplasm

    ectoplasm Guest

  15. jasen

    jasen Guest

    Don't they aren't good for fans. they make triac based speed controllers,
    use one of them.

  16. John Fields

    John Fields Guest

  17. John Fields

    John Fields Guest

  18. They are asymmetrical - output some DC.
  19. ectoplasm

    ectoplasm Guest

    OK. And if later I have a working solution I'll post it. Thanks for
    your offer for the schematic!

  20. We used to have an electric cooker where the heating
    elements was controlled like that..... Bursts of
    full sinewaves separated by gaps.

    Try the keywords "burst control heater" as a starting
    point. You might even be lucky and find a complete
    knob-controlled module that does it.
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