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  • fan motor wiring

    Greetings! I'm building an engine test stand, and need a big fan to push air through the radiator. Just pulled out an old squirrel cage motor I found in a dumpster 20 years ago. Unit is 18H x 12W x 16D. Motor is GE open frame, looks about 1/4 to 1/3 hp. Label on motor is unreadable. Three leads: white, black, red. White to black = 3 ohm. Black to Red = 100 ohm. White to black and black to red appear to be in series. No sign of a capacitor, but that may have been in a control box that was removed. How can I figure out how to wire this without blowing something up?

  • #2
    The 3ohm is the run winding, the 100ohm the start winding, you need a series cap with the start winding.
    You could try a 10ufd motor run-rated capacitor in series with the 100ohm winding.
    Black looks like the common.
    Max..

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    • #3
      Kinda... there may BE no cap for the start winding, and no need for one.

      With that much resistance, it may be a "resistance start" motor. Uses the resistance of that winding to keep the current of that winding nearly in-phase, and then the inductance of the main winding makes the phase shift.

      A lot of people call "resistance start" motors "split phase", although that is not the best name for them.
      CNC machines only go through the motions.

      Ideas expressed may be mine, or from anyone else in the universe.
      Not responsible for clerical errors. Or those made by lay people either.
      Number formats and units may be chosen at random depending on what day it is.
      I reserve the right to use a number system with any integer base without prior notice.
      Generalizations are understood to be "often" true, but not true in every case.

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      • #4
        J Tiers: I think that may be it. I have another big fan, almost the same dimensions, that runs. The motor is also a GE, and it looks exactly like my mystery motor except that it is multi-speed and the wiring diagram is readable. Like the mystery motor, it has no capacitor and no centrifugal switch. You plug it into 115V and it runs. I wonder if the mystery motor would start and run the same way. Would there be any harm in connecting the start and run windings in parallel, plugging it in and seeing what happens?

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        • #5
          Originally posted by alsinaj View Post
          J Tiers: I think that may be it. I have another big fan, almost the same dimensions, that runs. The motor is also a GE, and it looks exactly like my mystery motor except that it is multi-speed and the wiring diagram is readable. Like the mystery motor, it has no capacitor and no centrifugal switch. You plug it into 115V and it runs. I wonder if the mystery motor would start and run the same way. Would there be any harm in connecting the start and run windings in parallel, plugging it in and seeing what happens?
          Do I understand you to mean there is no internal start switch? Because if that is the case, it MAY be different.

          In that case it MAY indeed need a capacitor. But the capacitor may be inside. Depends how old the motor is. Capacitprs used to be expensive, and inductors cheaper. Now it is the reverse.

          Fan motors, especially multispeed ones, may be either with or without capacitors. Many have them, and use a multi-tapped winding to get the various speeds, while the capacitor is connected to a winding that does not get switched.

          There is a type which has no capacitor, and only a tapped winding. But that usually has an external inductor with it. I have a couple of "hassock fans" that are set up like that..

          Wjat does the motor look like? Does it have any sort of a motor tag on it?
          Last edited by J Tiers; 03-11-2020, 10:30 PM.
          CNC machines only go through the motions.

          Ideas expressed may be mine, or from anyone else in the universe.
          Not responsible for clerical errors. Or those made by lay people either.
          Number formats and units may be chosen at random depending on what day it is.
          I reserve the right to use a number system with any integer base without prior notice.
          Generalizations are understood to be "often" true, but not true in every case.

          Comment


          • #6
            Here's a photo of the motor. No sign of a capacitor outside the motor, and hard to imagine how one could be hidden inside.
            You may only view thumbnails in this gallery. This gallery has 1 photos.

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            • #7
              I have a motor of about the same vintage. Mine does have a run cap, but the wiring is: White=neutral; Black=High; Red=Low. That's it. On mine you don't want to run more than one winding at a time or it runs crappy and gets hot quickly. Just white + either black or red. Not both.
              Southwest Utah

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              • #8
                Chipmaker: how is the run cap wired in?

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                • #9
                  Yes, that is commonly the way they are connected. But usually there is another winding, and a capacitor to provide the phase shift to start and run the motor. This motor does not appear to have that.

                  It would make more sense if it were a shaded pole motor, which are often used for much smaller fans, and which are also able to run at different speeds with a fan load by inserting a resistance in series.
                  CNC machines only go through the motions.

                  Ideas expressed may be mine, or from anyone else in the universe.
                  Not responsible for clerical errors. Or those made by lay people either.
                  Number formats and units may be chosen at random depending on what day it is.
                  I reserve the right to use a number system with any integer base without prior notice.
                  Generalizations are understood to be "often" true, but not true in every case.

                  Comment


                  • #10
                    Result: Motor starts and runs with the start and run windings wired in parallel. Doesn't seem to overheat. Looks as if J. Tiers was right -- it's a resistance start motor.

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