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  • Calling all electronics experts

    I bought three "2HP" variable speed motors from some surplus place in the U.S. some years ago. One I mounted on a small, table top drill press. (I also lengthened the round stand.) It really works well.

    The other in on my son's grinder I made for him. The third is still waiting a use, maybe a medium size Chinese lathe I bought.

    Anyway, the potentiometer on the grinder burned out. I have no idea of the specs. on it. The (probably useless) numbers on the back are:
    00281921 and R1378602. I can find all sorts of potentiometers here in Brazil, but... how many K should I buy?

    The pot hooks up to the three terminals near it in the pic, in the upper left hand corner, labeled P1, P2, and P3.

    I'm hoping by looking at the circuit board, general knowledge, shooting in the dark etc. someone may be able to help me.

    I have used the pot from the third motor for the time being, but I can see I will need another one soon.






    Another problem is that the green resistors seem to be betting too hot and will probably go south on me soon. Are some some similar ones with some kind of heat sink?

    These boards were never very good, with lots of components needing re-soldering, but they do work. Also, the motors, while rated at 2HP don't seem to really give that kind of power. I think these were surplus treadmill motors.
    Vitَria, Brazil

  • #2
    Remove one of the remaining good potentiometers from its circuit board. Measure the resistance across the two outermost terminals on the good potentiometer. Round that number to the nearest standard potentiometer value. For example, if you measure a resistance of 9987, that would be rounded up to 10,000 = 10k.

    As far as the other two resistors go: You could always upgrade them to ones with a higher watt rating. Typically, heatsinked resistors exceed a 25-watt rating, which is quite large.

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    • #3
      Please excuse my ignorance here.

      When you measure the resistance, do you do it with the "knob" turned all the way to the left (counter clockwise) or to the right (clockwise)?
      Vitَria, Brazil

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      • #4
        Originally posted by davidwdyer
        Please excuse my ignorance here.

        When you measure the resistance, do you do it with the "knob" turned all the way to the left (counter clockwise) or to the right (clockwise)?
        Makes no difference.
        (That's the center terminal.)

        DJ

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        • #5
          Yeah, what DJ said.

          The center terminal is what varies in resistance.

          Within the pot, there's basically a resistive wire running in a circle from one of the side terminals to the other side terminal. The center terminal connects to a 'wiper' which slides along this wire. As the wiper is moved closer to one terminal, the resistance between the wiper and that terminal decreases, while it increases between the wiper and the other terminal. Potentiometers are spec'd by the overall resistance of that wire (i.e. from one end to the other) which is why you measure from the two outer terminals and ignore the middle one.

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          • #6
            Speed control pots are usually from 5k to 10k, make sure you get a linear version, not logarithmic.
            If you ever have to replace these drives, the KB and Baldor versions have been around for a long time and are relatively cheap.
            Max.

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            • #7
              I got a "nothin" on that part number from google which is pretty rare

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              • #8
                Aha!!!!!

                That explains it. I think the one on the drill press will be easier to get to.

                How about the one which I took out? Could I just measure that? The defect was that it would start slowly as normal, but when you moved it a little bit, the motor took off uncontrollably. Could I still use it to measure?
                Vitَria, Brazil

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                • #9
                  Still another dumb question. How do you know it the pot is "linear" or logarithmic?

                  Here, the people who "help" you at the counter don't know much and are not very helpful.
                  Vitَria, Brazil

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                  • #10
                    The remaining pot on the circuit board has absolutely 0 relation to the one thats fryed and hence has absolutely NO REASON to be the same value.

                    Id measure the two most outer terminals of the fryed pot, they should retain the pots value if its only the wiper that went (Most common failure mode)

                    If the resistive material itself is fryed in a spot, Use the outside terminal and wiper and put it about midrange, measure the resistance, and double it. (One outside terminal will likey read open to the wiper, thats the damaged side)

                    As for the smoken hot resistors, they look to be.. 1/2W? Maybe 1W.

                    You can replace em with higher wattage unheatsinked resistors, but you'll likey need to stand them up verticaly, or maybe at an angle, to make room.
                    Play Brutal Nature, Black Moons free to play highly realistic voxel sandbox game.

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                    • #11
                      Originally posted by davidwdyer
                      Still another dumb question. How do you know it the pot is "linear" or logarithmic?
                      The application

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                      • #12
                        Originally posted by davidwdyer
                        Still another dumb question. How do you know it the pot is "linear" or logarithmic?

                        Here, the people who "help" you at the counter don't know much and are not very helpful.
                        If you have someone who serves you over the counter and are not that electronic savvy they may not know what you are talking about, it should be listed in the spec if they have it.
                        I don't think 5k/10k it is going to be very critical in your application, I have used 5k or 10k with no problem, the input impedance of the circuit is probably fairly high.
                        Max,

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                        • #13
                          Uhhh, maybe I didn't ask the right question. I meant, when I am going to buy one, is there some number, symbol or distinguishing characteristic for me to know which is which?

                          I'm not doubting what you recommend I should use, but how am I, a rank beginner, to recognize what I am looking for. As mentioned, the store clerks probably don't even know the difference.
                          Vitَria, Brazil

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                          • #14
                            PS you can check if its linear or log by moving the wiper while measuring resistance. Or if the wipers dead, open it up and measure one of the outside contacts and put your probe right onto the resistive material at verious spots.

                            You also might be able to just SEE if its linear/log after opening it up. Linear typicaly has a consistant resistive material, Log will have one that gets wider all of a sudden, or thicker. (Usally log isent really log, but more like a log aproximated by 2 or 3 lines on a graph)

                            When you go to buy one, Its what you ask for. Linear will be defacto normal.
                            Log might be called 'audio taper' or 'audio pot' as that is there most common use.
                            Play Brutal Nature, Black Moons free to play highly realistic voxel sandbox game.

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                            • #15
                              Well I guess if the store does not have any details, you could take a meter with you and just measure the resistance in the centre and half way either side, the reading should be fairly linear, which is what you want.
                              In a log or antilog type, the resistance will increase or decrease exponentially.
                              You need the linear type for your application.
                              Max.

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