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  • Electric Motor Question...

    I have a shop built ambient air filter in the shop that was built around a cast off HVAC air handler. The blower is mounted in a steel case that is about an inch larger all around the blower. The blower is a squirrel cage fan with the motor mounted inside the fan. The unit is set up as a two speed 240 VAC and moves 1,100 cfm and 700 cfm. I don't have 240 in the shop and would like to run the unit on low speed at 120 VAC. The wiring coming out of the motor is: Black - High Speed, Red - Low Speed, & Yellow - Common. I can't get at the wiring inside the motor to see if it can be jumper-ed to run on 120. I'm just ignorant enough to wish I could connect the low speed and common wires up to 120 VAC and run the motor that way. Educated advice please. Click image for larger version

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    Jim Frye
    The Nut in the Cellar.

  • #2
    Do you have any idea of what kind of motor that thing has?
    My impression is that squirrel cage blowers usually have induction motors. Is that what it has?
    If that is the case how is the speed change made? Are half the poles bypassed?

    I have some book knowledge from college and after for motors, but not a whole lot of hands on.
    Last edited by LCHIEN; 03-19-2020, 12:39 PM.
    Loring in Katy, TX USA
    If your only tool is a hammer, you tend to treat all problems as if they were nails.
    BT3 FAQ - https://www.sawdustzone.org/forum/di...sked-questions

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    • #3
      Yes, I believe it's an induction motor as there is no way to service brushes or wipers. I would guess that the two speed function is accomplished by utilizing two different windings, one connected to the red feed and others connected to the black feed. The speed change is done by a double pole double throw toggle switch I added when I built the thing. Flip the switch one way and the high speed windings are fed voltage. Flip it the other way and the low speed windings are energized. The unit was a throw away from an office remodel at a former employer's facility. I'm a scavenger with no self respect, so I grabbed the unit when no one was looking. That was nearly twenty years ago.
      Jim Frye
      The Nut in the Cellar.

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      • #4
        OK some random thoughts here.
        Being a 240 V fan-motor combo, it will have been sized to draw near its rating Amps at 240 V when the fan is running and moving near the maximum amount of air.into a slight resistance, i.e. not blocked off.
        Assume its rated for Y amps.
        Induction motors are near synchronous, they operate with slip a bit less than the number of poles times the line freq.
        Thus to have a two speed fan I assume they use or not a second pole. to put in when working at half speed.
        So working at half speed means that the air movement is very roughly half. so 1100 CFM dropping to 700 is roughly half. Unloaded the slip will be less and the air volume more so 700 is the right direction from not quite half.
        Now. the motor will draw less current because its working at roughly half the work.load. So at 240 it might draw around Y/2.
        Being that you want to run off 120 V then the voltage is halved but the workload stays the same (remember the ROM and hence airflow will be more or less unchanged.
        So at 120 V I would expect the current requirement to double and be nearly Y again.

        So if I am right, then if you plug it in at 120 V (using what you think is half speed) the current will be roughly the same as the rating so you won't burn up any wired from excessive current, the speed will be half of what is was at 240 V full speed and it will move around half the air,

        I would check this out by using a current meter and running it for a while and make sure that the current Y is not exceeded. If not, they you are probably good to go.
        As fans go you know, blocking off the duct and killing the airflow will result in a drastic current reduction. I would guess around Y/4 very roughly.

        Mind you, I've been known to be wrong but that's my best guess. Take precaution, use a fuse or better yet an autotransformer if you have one.
        Last edited by LCHIEN; 03-19-2020, 09:06 PM.
        Loring in Katy, TX USA
        If your only tool is a hammer, you tend to treat all problems as if they were nails.
        BT3 FAQ - https://www.sawdustzone.org/forum/di...sked-questions

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        • #5
          Good guess! I went ahead and connected the cord from the switch to the 120 VAC circuit I had run when I built the shop. Looking at the wiring I had done years ago, the hot wire in the cord is switched between the high and low speed leads to the motor. The common lead from the motor is connected to the neutral wire wire of the cord. So, wire everything up and flip the switch for a smoke test. No abnormal current draw, nor breaker trip. Ran for five minutes on low speed with no issues and switched to high speed for five minutes, again with no issues. As soon as I see my neighbor, I’ll borrow his clamp on amp meter, but I doubt there will be any surprises. I would say the motor is indeed turning at half speed judging by the apparent air flow coming out of the vents. I used to run the unit on low speed before, so high speed now is pretty close to that. Certainly enough for my small shop. Now as soon as I locate the blast nozzle for the air compressor, I can do a good spring cleaning in the shop ( a Coronavirus activity) that I’ve been putting off because I was too lazy to hook up the air filter.
          Jim Frye
          The Nut in the Cellar.

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          • #6
            Just a last post on this. Today, I fired up the air compressor and turned on the ambient air filter on high. Spent a couple of hours blowing off/out everything in the shop. I let the air filter run for two more hours to clear the air completely. No issues with the machine. Mathematically, the filter is completely passing the air in the shop every 3 1/2 minutes. That should keep the air pretty good. The filter is a 4” thick computer room pleated filter. Thanks again for the assistance.
            Last edited by Jim Frye; 03-22-2020, 01:41 PM.
            Jim Frye
            The Nut in the Cellar.

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