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  • Ac speed control

    First off, I didn't use O.T. because I think all these questions about THINGS in
    our shops is right on topic for ... Home SHOP machinist.

    Anyways I have this fan in my shop that is rather industrial. So I want it
    to run at a slower speed to quiet it down.

    I have tried those HF router speed controllers and even though it slows it down
    it is still really noisey .. for some reason.

    Right now I have a variac controlling it and it is perfect. But I think the
    variac is over kill. I mean it is just supplying a smaller ac voltage.

    Would there be a difference in the results if I just used some ohm's law
    and put a resister in line with the fan ? Any pot ok ?
    John Titor, when are you.

  • #2
    Any resistor or pot will have to be rated at the power drawn and will dump excess energy as heat rather than switching the power on and off
    If you benefit from the Dunning-Kruger Effect you may not even know it ;-)

    Comment


    • #3
      Single phase AC motors are meant to run at a certain speed based on the line frequency. You can force them to do otherwise, but you are compromising them. Which usually means overheating. So keep an eye on it.

      And there will a limit as to how low you can force it.

      Picking a pot is kind of tricky, but:
      - measure the voltage that the variac is supplying at the minimum speed you want
      - subtract it from 120 to get the voltage that will have to be dropped at min speed
      - measure the current that the fan is using on the variac at minimum speed
      - divide the drop voltage by its current to get the pot's resistance value. With the pot at zero the fan will run full speed. At its full setting the fan will run the slowest.
      - the other, critical, pot parameter is its wattage. This determines the current that it can handle. You will need a pot whose wattage is equal to the current that you measured, squared, times the pot's resistance. If you don't do this you will surely burn open the pot.

      HTH,
      Bob

      Edit - also measure the current when the fan is running full speed. If this is greater than the current at minimum speed, use it to determine the pot wattage. No use trying to figure which will be greater - just measure it.
      Last edited by Bob Engelhardt; 12-03-2017, 11:47 AM.

      Comment


      • #4
        As macgicniner says, yes it will work, but wastes heat and power.

        I have used that same technique of a resistor with small fans in products, to create a 2-speed fan. If your fan is one of those floor-standing fans that is 20 or 24" diameter, that is going to waste quite a bit of power, compared to the small fans I have done that way. The only "pot" that will work is going to be one of those large ceramic wire wound controls that will be in the 6" diameter, or larger, size range. They are not cheap.

        I do not know what your fan draws as far as power. Fans generally are a "cube law" device, meaning that doubling the speed increases the power required by 8 times. If you decreased the speed to 3/4 of normal, the power drawn would be about 40% of normal, the current and voltage would likely be about 64% of normal, and the resistor would have to dissipate about 25% of the normal power drawn by the fan.

        If you can determine what voltage you are setting the fan to, you may be able to wire a multi-voltage transformer as an "autotransformer" to supply that voltage, so the variac can return to its usual uses.
        1601

        Keep eye on ball.
        Hashim Khan

        Comment


        • #5
          Most router motors are universal motors. A light dimmer will work for controlling the speed of universal motors. The Chinese router speed controller is a glorified light dimmer.

          If your fan is not a universal motor, you'll need a fairly sophisticated and expensive doofarb to control the speed effectively. Simply reducing the voltage is not enough. Some types of motors rely on line frequency, as noted above by Bob. Others must have the frequency increased as the voltage is decreased - this is what the expensive controllers do.

          I respectfully suggest swapping out the motor on your fan. Use a reclaimed smaller motor for the fan and save the "industrial" motor for a fun project.

          Comment


          • #6
            The typical "regular fan" uses either a PSC or a shaded pole motor.

            Both those types, while they are induction motors, are fairly easily operated in "slip controlled" mode by varying voltage. They are also (for the same reason) not usually very good for machine tool purposes, although the PSC can be OK.
            1601

            Keep eye on ball.
            Hashim Khan

            Comment


            • #7
              Residential HVAC blower motors are typically 3 or 4 speeds and might be a easy swap for your motor. That could give you multiple speeds with just a switch to select the speed. Most units scrapped have a perfectly working blower motor. (nema 48 frame and 1/2 shaft being very common) Just something to consider.

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              • #8
                You might just consider wiring a light bulb in series with the fan if you cannot measure the desired voltage.

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                • #9
                  All this because using a variac is overkill.. or is it? I use a variac in the summer to make my 3-speed fan run at e.g. 1.5 which is about right. Variacs come along just like other shop items. I currently own 3 and don't have a dime into them. I consider a variac to be an elegant solution, not overkill at all.

                  metalmagpie

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                  • #10
                    Ok .. requirements don't seem too bad.

                    115v down to 90 gives me a drop of 25v
                    @ 1 amp

                    sounds like 25 ohm @ 25 watt .. Thats pretty do-able I think.

                    metalmagpie .. I know what you mean ... that variac works so perfect, I'm in
                    no hurry to change it. But it's a reeeaaallly nice 1000 watt variac.
                    John Titor, when are you.

                    Comment


                    • #11
                      Originally posted by Mike Amick View Post
                      Ok .. requirements don't seem too bad.

                      115v down to 90 gives me a drop of 25v
                      @ 1 amp

                      sounds like 25 ohm @ 25 watt .. Thats pretty do-able I think.

                      metalmagpie .. I know what you mean ... that variac works so perfect, I'm in
                      no hurry to change it. But it's a reeeaaallly nice 1000 watt variac.
                      Or you could find a transformer with a 25VAC secondary and wire it as a buck transformer. A secondary rated for 1.25 Amps or more would
                      be fine. If you go the resistor route, you'll want something larger than 25 Watts, probably at least 30 Watts. The transformer technique won't
                      generate nearly as much heat.

                      Comment


                      • #12
                        The variac is essentially a variable transformer. As such it wastes a lot less power than a series resistor would. And that power that is lost in a resistor will become heat in your shop: I would assume you are using the fan to cool yourself so more heat would be counterproductive.

                        I consider that resistor to be an inelegant and wasteful solution while the variac is an elegant one. Besides, it works. Why fix it if it isn't broken?
                        Paul A.

                        Make it fit.
                        You can't win and there is a penalty for trying!

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