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  • Question on old fan motor

    I pulled an old Robbins and Myers fan motor out of my junk thinking of repurposing it but the wiring has me confused and I thought someone would know. It is 115v. 1.3 Amp 60 hz 1725 rpm continuous duty. It has 4 wires coming out, one obviously ground and the other 3 with blade connector ends. An ohmmeter showed:
    Red to Green is 135 Ohms
    Red to Brown is 150 Ohms
    Green to Brown is 100 Ohms

    So what am I looking at and how would I wire it, Please?

  • #2
    It might need a run capacitor, or perhaps it is a two speed motor. You may need to experiment with connections. If your ohm readings are correct, it won't draw any more than 1.3 amps on 120 VAC for any configuration. But if the motor does not run, there will be no BEMF and it could dissipate 100 watts or so. Just apply the voltage briefly and remove it if it does not start.

    I searched for "two speed fan motor connections"

    https://www.hunker.com/13414432/how-...o-speed-motors

    https://content.greenheck.com/public...ndamentals.pdf

    https://ourpastimes.com/2-speed-120v...g-7943821.html

    Not very definitive, but you might search further and find a wiring diagram that matches what you have. Is there a nameplate on the motor?

    More info:

    https://groups.google.com/g/sci.elec...VZ3t0lK0?pli=1

    https://www.afcaforum.com/forum1/14930.html

    Last edited by PStechPaul; 02-20-2021, 05:57 PM.
    http://pauleschoen.com/pix/PM08_P76_P54.png
    Paul , P S Technology, Inc. and MrTibbs
    USA Maryland 21030

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    • #3
      Originally posted by PStechPaul View Post
      It might need a run capacitor, or perhaps it is a two speed motor. You may need to experiment with connections.
      With the nameplate marked 1725 RPM, I didn't think it would be 2d speed, but I didn't think of a run capacitor. Hmmmm.
      Thanks for the reply.

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      • #4
        This sounds like (just a guess) it could possibly be a permanent split capacitor motor ("PSC") which is what I turned out having here:

        https://bbs.homeshopmachinist.net/fo...trical-experts

        Apparently these are commonly used in things like fans that often have a constant speed and not a lot of torque requirement. I was lucky in that mine indicated the required capacitor value on the spec plate, I just had to suss out how to connect it. If the resistance between two of the terminals was roughly twice that of the value between the other pairs, that would suggest that this is even more likely. Any other info on the spec plate or even a photo might help.
        "A machinist's (WHAP!) best friend (WHAP! WHAP!) is his hammer. (WHAP!)" - Fred Tanner, foreman, Lunenburg Foundry and Engineering machine shop, circa 1979

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        • #5
          Just to follow up, I experimented with the motor. I tried all three combinations of the three connectors on 110 and nothing. Then I tried hooking brown and green to 110 and put a capacitor between red and brown. It was a 30 microfarad 450 volt I had lying around. The motor ran smoothly, but after a couple minutes the capacitor got too hot and started smoking. Anyway, it looks like the wiring does need a cap of some value between 2 of the wires.

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          • #6
            Originally posted by Gary Paine View Post
            Just to follow up, I experimented with the motor. I tried all three combinations of the three connectors on 110 and nothing. Then I tried hooking brown and green to 110 and put a capacitor between red and brown. It was a 30 microfarad 450 volt I had lying around. The motor ran smoothly, but after a couple minutes the capacitor got too hot and started smoking. Anyway, it looks like the wiring does need a cap of some value between 2 of the wires.
            Was the cap non-polarized?

            -js
            There are no stupid questions. But there are lots of stupid answers. This is the internet.

            Location: SF Bay Area

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            • #7
              Originally posted by Jim Stewart View Post

              Was the cap non-polarized?

              -js
              As-in, NOT a motor start type, but a motor run type..... Most motor run caps are OK with going right across the line, but the wrong size capacitor can resonate, and may then have a much larger current than intended going through it (and a high voltage across it, too.)
              2801 3147 6749 8779 4900 4900 4900

              Keep eye on ball.
              Hashim Khan


              It's just a box of rain, I don't know who put it there.

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              • #8
                Thanks for the responses. Well, I had thought it was non polarized. Metal can with lugs and two identical non marked terminals in the middle of the can. After the question, though, I looked closer and the printing on the can said can negative. I've some large non polarized, but of a pretty large size for the small motor. Any guess what value range I might want?

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                • #9
                  A reasonable value would be that which exhibits a reactance the same as the resistance of the run winding. So if that is 100 ohms, a capacitance of

                  C = 1 / ( Xc * 2 * PI * f) = 1 / (200 * 3.1416 * 60) = 26.5 uF

                  Probably anything between 20 and 30 uF will work. You can try a few values and choose one which draws the least current (or highest power factor).
                  http://pauleschoen.com/pix/PM08_P76_P54.png
                  Paul , P S Technology, Inc. and MrTibbs
                  USA Maryland 21030

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                  • #10
                    Just as a refresher..

                    On a four wire system dont ever call common ground , even though.. Safer not to JR
                    My old yahoo group. Bridgeport Mill Group

                    https://groups.yahoo.com/neo/groups/...port_mill/info

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                    • #11
                      i know little if anything about electric motors, but replaced a start capacitor on my 1940 something Craftsman drill press a couple years ago. Doing some reading (only enough to be dangerous), I saw where some motors use only a start capacitor. There is some kind of weighted centrifugal switch that disconnects the start capacitor once it gets up to a certain rpm. They make a "click" sound when you shut the motor down or start it up. Could your motor have a stuck centrifugal switch inside causing the capacitor to be connected too long and overheating? Just a wild guess.
                      S E Michigan

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                      • #12
                        On a motor that size a 10 mfd run cap would be about the right size or even smaller. We used 35's on AC compressor motors larger than 1 hp. Yes and Brown is usually the cap wire.
                        Retired - Journeyman Refrigeration Pipefitter - Master Electrician - Fine Line Automation CNC 4x4 Router

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