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O/T shop fan question

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  • O/T shop fan question

    I have a 3 speed fan from an old furnace that I want to use as portable shop ventilation.
    I do not have the controls from the old furnace just the fan.I have determined which wires are which speeds but now I need a switch.I would like to use something simple.......ideas?

  • #2
    IM NOT A CERTIFIED ELECTRICIAN. That being said, is it 220V ,or 110V? You can run it a buncha ways. Plug it into a wall then to a switch ,then to motor ,or u can just put a cord on motor and plug it in when u want it on. I ran my furnace blower exhaust fan to a fused box w a cutoff switch but thats only cause i had one layin around.Mine only runs 1 speed so i cant help w using different speeds.

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    • #3
      A three position switch should not be to hard to locate but you will have to choke down on the air outlet from the fan as it will overload the motor pumping free air.

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      • #4
        Originally posted by J. R. Williams View Post
        A three position switch should not be to hard to locate but you will have to choke down on the air outlet from the fan as it will overload the motor pumping free air.
        Or choke down on the air inlet, same result.
        Give us some motor nomenclature and wire colors.

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        • #5
          ??? I am not a fan expert, but wouldn't choking the air flow down increase the load on the motor and increase the current draw? A motor draws the least current at minimum load which is usually also the maximum speed. I could understand the need for some kind of transition flaring to provide a smooth air flow, but choke it down?

          What am I missing here?
          Paul A.

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

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          • #6
            A few years ago, I had the air conditioning and furnace replaced in our house. When the blower was removed, I asked for and received it. I plan on using it as an aux cooler for testing my motorcycle when i get it running.


            If your's is the 115 volt motor, the wiring should be this way:
            Furnace Fan Blower Motor Wiring

            White Common
            Black High Speed
            Blue Medium High Speed
            Red Low Speed


            You'll need to make an enclosure, and possible air filter housing. I'd think a four position step switch would work.

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            • #7
              My understanding is that the power required to run the fan depends on how much work it's doing. Choking the output reduces the amount of air moved and therefore reduces the work done and load on the motor.

              I've watched the guys in the AC class adjusting the plate over the fan output opening while monitoring the current with an amprobe. The current goes down as the opening is reduced.
              Any products mentioned in my posts have been endorsed by their manufacturer.

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              • #8
                I am missing something too. I can see choking down the intake side of a fan. But choking the output would increase the load on the motor rather than decrease it.
                Not any different than a compressor and they have the kick down valve on the intake side.

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                • #9
                  Originally posted by Paul Alciatore View Post
                  ??? I am not a fan expert, but wouldn't choking the air flow down increase the load on the motor and increase the current draw? A motor draws the least current at minimum load which is usually also the maximum speed. I could understand the need for some kind of transition flaring to provide a smooth air flow, but choke it down?

                  What am I missing here?
                  Not sure why that is either, but it works. Buddy in the a/c business would change out single speed motors for two speed when converting mobile home units from heat only to central air. He gave me one of the new single speed motors. If I let it free run, the thermal overload would shut it off after while. Put a piece of duct tape across the intake and it'd run forever. Go figure.

                  Just pick the middle speed and wire it for that and forget the multispeed switch. Probably use it that way most of the time anyway.
                  Definition: Racecar - a device that turns money into noise.

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                  • #10
                    Originally posted by 1-800miner View Post
                    I am missing something too. I can see choking down the intake side of a fan. But choking the output would increase the load on the motor rather than decrease it.
                    Not any different than a compressor and they have the kick down valve on the intake side.
                    A centrifugal pump of any kind fluid will draw less current if you reduce the flow by closing off either intake or exhaust. If you close it off completely it will only see the friction load of the fluid against stationary objects. Completely closing it off will raise the temperature of the pump due to friction.

                    A centrifugal pump is nothing like a compressor which is a displacement pump.

                    John
                    My Web Site

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                    • #11
                      If you block the air discharge on an axial fan, you will increase the amp draw of the motor. If you block the air discharge on a radial fan, you will decrease the amp draw of the motor. The rpm's on the axial fan will be reduced, and will be increased on the radial fan.
                      Last edited by HSS; 12-08-2012, 09:19 AM.

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