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  • Questions about box fan motors.

    Can these be safely run with the shaft in the vertical position?

    Can they be directly wired to mains power through a simple on/off switch (not the speed control switch) to run at a fixed speed?

    How does the speed control switch work to control the speed?

    If I don't use the motor with the fan blade on it will it overheat under a very light load?

    I'm thinking of using a box fan motor to make a simple centrifuge. Maybe I should use the speed control or make a continuously variable speed control. Thinking of using pulleys so it will run at different chosen speeds if I don't use the speed control. The box fans are so dirt cheap but maybe there's a better way. Suggestions?

    Probably a DC motor with controller board would be ideal but doubt I could get them as cheaply as the box fan motor.

  • #2
    If you want variable speed and 120vac supply, how about one of the Universal motor hand tools such as one of the dremels of a suitable size?
    Depends what the loading is?
    AC fan motors are usually simple shaded pole motors so do not posses all that much torque at start up.
    Max.

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    • #3
      Originally posted by pgmrdan View Post
      Can these be safely run with the shaft in the vertical position?
      I think that might be the real issue. The rotor is designed to float horizontally and there is no provision for it to support a vertical load.
      Last edited by topct; 06-03-2013, 04:52 PM.
      Gene

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      • #4
        The thrust bearings are simple fiber or plastic washers, so vertical operation is not going to work long term well, especially if there is additional load besides the blades. The speed control is a simple triac phase control to give a variable voltage down to about 50% of normal. This works because the motor's voltage-torque curve matches a fan's speed-torque curve well enough to give a nice variation in speed with voltage change. With a constant-torque load like a centrifuge, it's not going to give any speed variation, provided it can get started. The motor is designed for "air-over" cooling which means that it can't self-cool with out a fan on the shaft.

        A DC motor/speed control is he way to go.

        RWO

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        • #5
          For my centrifuge I used a multi-speed fan motor, the kind with the 'elephant ear' clear plastic blades used for room 'air conditioning'. I see them all the time at thrift stores. They have enough torque to bring a bit of mass up to speed in a reasonable time. Chances are by the time you have a suitable drum or something worked up, there will be a considerable inertia to overcome. I don't think a boxer fan motor would really be up to it.

          Of course it depends on what you want to do. My project has swinging buckets that you can drop a little container into. I use it for de-airing resins, etc. I drop in two throwaway containers, fill them about equally with whatever mixture I'm working with, then hit low. Once a bit of speed is built up, I hit med, then high. High was giving me about 650 rpm or so. Having the buckets swinging means that the mixture stays level in the cups as the speed builds up. The buckets go quite horizontal, even at the low speed.

          The first mistake I made was to slow it down too quickly. It's a good thing I built the whole thing into a decapitated propane tank (my containment vessel) since the epoxy I was de-airing went washing out all over the inside of the tank and ran into the bottom.
          I seldom do anything within the scope of logical reason and calculated cost/benefit, etc- I'm following my passion-

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          • #6
            The fan motors MAY be changing number of poles. But they might (cheap ones) also be just changing effective voltage and depending on being high-slip motors..... Fans are cube-law devices and will find their own speed given the power input. All the motor needs is enough back EMF not to burn up.

            Some are cap start-run (PSC) types and probably are changing poles, once the motor gets well started turning, you can switch it to where the cap isn't even used, and it will run as a straight induction motor. You use the cap winding for added poles. We set up a client with a control doing that for a squirrel-cage fan at 1 HP. Gets you two speeds, anyhow.
            1601

            Keep eye on ball.
            Hashim Khan

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            • #7
              Originally posted by topct View Post
              I think that might be the real issue. The rotor is designed to float horizontally and there is no provision for it to support a vertical load.
              It may not be a very large force, but it has to resist the thrust of the fan blade.

              Dave

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              • #8
                A little more info would be handy-

                My centrifuge has the motor in the bottom and the driven shaft above it on its own bearings. A flexible coupling connects the two. I suppose if I was careful about attaching the coupling, I could have the upper part suspending the motor shaft as well. Many of these motors have some end float, so it's entirely possible to remove all the pressure that would normally work against the motors own thrust bearing.
                I seldom do anything within the scope of logical reason and calculated cost/benefit, etc- I'm following my passion-

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                • #9
                  You probably don't WANT to use the motor bearings anyway.... odds are that if you use your own shaft and thrust bearing, as Darryl did, it will work more smoothly.

                  And you can choose a shaft of a size proportionate to the load you plan to put on it. Sounds like an excellent plan to do it the way Darryl did.
                  1601

                  Keep eye on ball.
                  Hashim Khan

                  Comment


                  • #10
                    As I mentioned earlier, I built mine into an old 20 lb propane tank. I figured that it would be prone to vibration since it would be so easy to load the cups differently, so I cut some legs from plywood and mounted them like fins around the outside. Kind of makes it look like a rocket.
                    I seldom do anything within the scope of logical reason and calculated cost/benefit, etc- I'm following my passion-

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