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Electrical poser; sharpen your pencils.

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  • Electrical poser; sharpen your pencils.

    OK, let's say you want to make a multiplier to extend the range of your tong ammeter to read smaller currents. You make a nice brass bobbin (with a 1" dia eye) and wind it with 10 turns of #16 enameled magnet wire. You connect the coil's start and finish in series with the hot (black) conductor between a standard NEMA 5-15 grounding pluge and a NEMA 5-15 standard grounding receptacle. The load current passes through the 10 turns of the magnet wire increasing the total flux at each turn.

    You connect the male plug to a 115 V 50 Hz wall outlet. You plug a 5 Amp load in the coil's receptacle. You pass the tong of an Amprobe analog tong ammeter through the eye of the bobbin and close its magnetic circuit.

    What will the meter read if 5 Amps is passed through the turns of the multiplier? The answer is easy if the bobbin was plastic - 50 Amps because of the 10 turn multiplier coil. O does it?

    The conductive brass bobbin alters the picture. It's understood the brass bobbin amounts to a shorted turn so far as the flux generated by the 10 turns of magnet wire are concerned but how does that affect the tong meter reading?
    Last edited by Forrest Addy; 03-21-2012, 03:49 AM.

  • #2
    Awnser: Please use a plastic bobbin as we don't want to do complex magnetics math and you did not give us enough specs on wire insulation, winding layers, bobbin diamiter, etc etc etc to figure it out anyway.
    Play Brutal Nature, Black Moons free to play highly realistic voxel sandbox game.

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    • #3
      Weenie. Chicken. Wimp. Wuss.

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      • #4
        It depends on if and when the brass bobbin magnetically saturates. It is probably going to get warm, or hot, acting like a shorted one turn secondary of a transformer. Once the bobbin saturates, it ceases to be a factor in the magnetic field that is generated by the ten turn coil, and the magnetic field that doesn't get used inducing current in the bobbin will be available to induce current in the Amprobe.

        Without saturation specifics, there is no good way to predict the measurable current.

        I get involved with solenoids, mostly DC. In some cases brass tubes are used to guide the plunger. Makes smoother and quieter operation. With a DC solenoid with the presence of the brass tube, about the only noticeable effect that the brass tube has is to slow the response of the solenoid travel, and is usually only measureable in microseconds for the strokes I deal with. It is not common to use a brass tube with AC solenoids. DC solenoids can suffer excessive heating of the brass tube if cycled repeatedly at a high rate (simulates AC)

        There, more information than you ever wanted to know.
        Weston Bye - Author, The Mechatronist column, Digital Machinist magazine
        ~Practitioner of the Electromechanical Arts~

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        • #5
          Brass bobbin won't saturate, since it isn't even partially magnetic.....

          The answer depends on the resistance of the brass..... and the degree of coupling.

          In the perfect case, the current in the bobbin would be such as to exactly cancel the magnetic field of the coil, leading to no net flux and a zero reading.*

          In the real case, there will be a good deal of current flow, but not complete cancellation, so "some" reading will be obtained.

          The answer is calculable, with more physical details, but frankly, I have no wish TO calculate it.... hence the qualitative answer.

          * this is why high current draw from a transformer does not "saturate" it...... the core at full load has LESS flux in it than at no load.
          1601

          Keep eye on ball.
          Hashim Khan

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          • #6
            Forrest,

            I have some 1.5 brass stock, plenty of #16 magnet wire, an amprobe clamp meter, a variable AC source, a load bank to create the 5 amp load and a lathe to make the bobbin.....

            If you want I could also give the results for different frequencys...

            Give me some more detailed specs on the bobbin and I will let you know the answer
            Robin

            Happily working on my second million Gave up on the first

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            • #7
              The effect of the brass bobbin will be almost impossible to predict due to the lack of specs. Measurements, conductivity of the brass, etc. In any case, it will read less than the 10X factor, it may even read less than a 1X factor.

              A brass bobbin is a very poor choice for this idea. Fire that engineer. If you must keep the brass, you could cut a slit in it to eliminate the short. That should bring you back to the same situation as if you had an insulator for the bobbin. Kind of like using laminates for transformers with electric insulation between them (varnish).
              Paul A.

              Make it fit.
              You can't win and there is a penalty for trying!

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              • #8
                ^--- Yea, Thats what I said. *shrugs*
                Play Brutal Nature, Black Moons free to play highly realistic voxel sandbox game.

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                • #9
                  Forrest, I'm willing to help but I'm a little pressed for time for the next few days. I've gotta wash the car, brush my teeth AND all of my hairs. I'll get back to you when I can find the time.
                  - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
                  Thank you to our families of soldiers, many of whom have given so much more then the rest of us for the Freedom we enjoy.

                  It is true, there is nothing free about freedom, don't be so quick to give it away.

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                  • #10
                    Brass bobbin won't saturate, since it isn't even partially magnetic.....
                    Just so. However, the induced current will cause the bobbin to heat and increase in resistance. This will attempt to limit current, to the ability of the bobbin to dissipate the heat, and would make predicting the behavior like hitting a moving target.

                    Paul A. Has suggested the best solution in slitting the bobbin lengthways to open the circuit. Put a strip of bakelite or glass epoxy circuit board or other non-conductive material in the gap as a filler. That done, there might still be some localized eddy currents within the brass that will draw off some of the current in the form of heat, but probably minor if at all.
                    Weston Bye - Author, The Mechatronist column, Digital Machinist magazine
                    ~Practitioner of the Electromechanical Arts~

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                    • #11
                      lol you guys.
                      A good engineer can figure out how to do anything.
                      A great engineer knows when to just say "NO"... Use a plastic bobbin.
                      Play Brutal Nature, Black Moons free to play highly realistic voxel sandbox game.

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                      • #12
                        BM, Ever deal with the auto industry?
                        Weston Bye - Author, The Mechatronist column, Digital Machinist magazine
                        ~Practitioner of the Electromechanical Arts~

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                        • #13
                          Originally posted by Weston Bye
                          BM, Ever deal with the auto industry?
                          With the auto industry, You need a legendary engineer to be able to make something that can actualy be assembled.
                          Play Brutal Nature, Black Moons free to play highly realistic voxel sandbox game.

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                          • #14
                            Originally posted by Black_Moons
                            lol you guys.
                            A good engineer can figure out how to do anything.
                            A great engineer knows when to just say "NO"... Use a plastic bobbin.
                            The original post was NOT "how to solve this problem?".......

                            The original post was "given this setup, which you are not allowed to change, what will happen?"

                            So you get a "0" for your answer.
                            1601

                            Keep eye on ball.
                            Hashim Khan

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                            • #15
                              Originally posted by J Tiers
                              The original post was NOT "how to solve this problem?".......

                              The original post was "given this setup, which you are not allowed to change, what will happen?"

                              So you get a "0" for your answer.
                              If this is just a recreational exercise and not a real, practical problem, then I still say the math involved is almost impossible without EXACT details of the construction of the bobbin. The easiest way to answer this is to make it and measure the results. But even then, it is not a good design because any changes in the brass alloy may have significant effects on the outcome.

                              So Forrest, what is this? Is it a real world situation or is it just a mental exercise? If real world, I am willing to try to help. If just an exercise, then I have better things to use my time for.
                              Paul A.

                              Make it fit.
                              You can't win and there is a penalty for trying!

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