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My take on inductor spacing

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  • My take on inductor spacing

    From time to time when the subject is inductor spacing for crossover assemblies, like in the "Copper" thread, I will post a comment saying that a 2" spacing is perfectly adequate with air core inductors laid down flat, not any of them needing to be stood up and/or rotated at 90 degrees. And, I have criticized some of Troels recommendations. Thus, I think it's only proper for me to elaborate.

    I've replicated Troels' experimental measurements for illustration. He took a pair of same-value inductors, connected them in series, and measured their total inductance while separating them at different distances. I've done the same thing using 3 pairs of same-value inductors, two at 6 mH (15-gage), two at 4 mH (18-gage) and two at 0.3 mH (20-gage) (all labeled values). The reference total series inductance for each pair was measured with the two inductors spaced 24" apart. Following that, the separation distance was reduced to 20 cm, then 10 cm as measured CTC. Last I spaced them so the perimeters of each pair of inductors was set at 2". For the reference measurements (at 24" spacing) the two 6 mH coils measured 12.35 mH, the two 4 mH coils measured 8.26 mH, and the two 0.3 mH coils measured 0.595 mH.

    With the spacing reduced to 20 cm CTC, the pairs of coils respectively measured 12.33 mH (-0.2% change from reference), 8.25 mH (-0.1% change) and 0.595 mH (-0.2% change). With spacing reduced to 10 cm CTC, the respective measurements were 12.16 mH (-1.5%), 8.22 mH (-0.5%) and 0.595 mH (no change from reference value). Last, with the spacing set at 2" between perimeters, the respective measurements were 12.23 mH (-1%), 8.22 mH (-0.5%) and 0.596 (+0.2%).

    If there is enough room there's no reason not to separate air core inductors as far apart as possible, but a spacing of 2" between adjacent perimeters is entirely adequate from a mutual inductance standpoint. The percentage change in this inductance is obviously influenced by the size and value of the inductors, at least according to my experiment. If you are forced to crowd your components, then standing up inductors and rotating them will certainly be necessary to minimize mutual inductance.

    Another type of mutual effects to consider is voltage crosstalk, and this is another reason I've stated that a 2" spacing is entirely adequate. A number of years ago there was an article in either Audioexpress or Speaker Builder where a person measured the voltage crosstalk between adjacent air core inductors laid down flat with different spacings. His tests showed that a spacing of 2" between the coils' perimeters created a voltage crosstalk of -50 dB, or ~0.3%.

    In summarizing, with a 2" spacing between the perimeters of air core inductors both laid flat, we're dealing with mutual inductance changes varying from 0.0 to 1.0%, much smaller variations than potentially will occur from typical tolerances of the capacitors (5-10%) and inductors (2-5% we all use.

    Hope this is useful,
    Paul

  • #2
    Re: My take on inductor spacing

    Not to nitpick, but did you observe the winding polarities? if you use 2 adjacent coils with differing winding directions, they can actually cancel each other, just like magnets. Since you were getting negatives, this is why I'm asking. In other words, were both adjacent coils wound clockwise or counter-clockwise and not differently?
    This does make a difference.

    Later,
    Wolf
    "Wolf, you shall now be known as "King of the Zip ties." -Pete00t
    "Wolf and speakers equivalent to Picasso and 'Blue'" -dantheman
    "He is a true ambassador for this forum and speaker DIY in general." -Ed Froste
    "We're all in this together, so keep your stick on the ice!" - Red Green aka Steve Smith

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    • #3
      Re: My take on inductor spacing

      If mutual inductance with the coils out of phase is reducing the total inductance by 0.X % then mutual inductance with the coils in phase will only increase the total inductance by the same 0.X %.
      Craig

      I drive way too fast to worry about cholesterol.

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      • #4
        Re: My take on inductor spacing

        I think the Crosstalk between inductors would be a bigger issue then the posibility of mutual induction. A 2nd order electrical crossover would have the inductor in series with the woofer transmitting energy to the tweeter inductor; most likely not in phase with the tweeter filter output. I think -50dB would be the minimum allowed crosstalk (more would be better). My current speaker I'm designing will have a seperate crossover board per driver and be located far apart from each other in the enclosure.
        Shawn

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        • #5
          Re: My take on inductor spacing

          If you look at Troels' measurements, what I was replicating, he got the same direction of changes as I did, a decrease in the total inductance as the inductors were brought closer and closer together. Each pair of the inductors I used were the same brand and part number. You'll also note that Troels inverted one of the inductors for the series of tests with both of them laid down flat and got a positive change in total inductance. You'll also notice that Troels labeled as "problematic" a 10 cm spacing that exhibited a -0.6% change which I think is a ludicrous label.
          Paul

          Originally posted by Wolf View Post
          Not to nitpick, but did you observe the winding polarities? if you use 2 adjacent coils with differing winding directions, they can actually cancel each other, just like magnets. Since you were getting negatives, this is why I'm asking. In other words, were both adjacent coils wound clockwise or counter-clockwise and not differently?
          This does make a difference.

          Later,
          Wolf

          Comment


          • #6
            Re: My take on inductor spacing

            I won't argue with your statement about -50 dB being the most one would want allowed. There are so many variables in drivers as well as crossover component accuracies, to worry about 0.3% voltage crosstalk or 1% changes due to mutual inductance really is nitpicking to the extreme. Still if you have the room, separate everything as much as possible, but if you don't, orient components the best possible way. Just be realistic and logical.
            Paul

            Originally posted by Shawn_K View Post
            I think the Crosstalk between inductors would be a bigger issue then the posibility of mutual induction. A 2nd order electrical crossover would have the inductor in series with the woofer transmitting energy to the tweeter inductor; most likely not in phase with the tweeter filter output. I think -50dB would be the minimum allowed crosstalk (more would be better). My current speaker I'm designing will have a seperate crossover board per driver and be located far apart from each other in the enclosure.
            Shawn

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            • #7
              Re: My take on inductor spacing

              Interesting data and thanks for conducting the experiment and publishing your results.
              How would the coupling be affected by higher voltages going through? Would the coupling increase as voltage increased? How much voltage is applied by your test measurements?
              -Mike

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              • #8
                Re: My take on inductor spacing

                Just run a signal through one and measure the coupled signal on an adjacent one.

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                • #9
                  Re: My take on inductor spacing

                  Assuming the voltage would not be so high as to cause some sort of breakdown or arcing, the crosstalk voltage in terms of percentage or dB should remain the same even though the absolute amount of voltage crossing-over would be larger. I simply used my RLC meter and don't know how much voltage was being applied by it to the pairs of series-connected inductors, but I can measure it easily enough (or maybe it's specified in the manual).
                  Paul

                  Originally posted by Alien_Shore View Post
                  Interesting data and thanks for conducting the experiment and publishing your results.
                  How would the coupling be affected by higher voltages going through? Would the coupling increase as voltage increased? How much voltage is applied by your test measurements?

                  Comment


                  • #10
                    Re: My take on inductor spacing

                    Thanks Paul.
                    " To me, the soundstage presentation is more about phase and distortion and less about size. However, when you talk about bass extension, there's no replacement for displacement". Tyger23. 4.2015

                    Quote Originally Posted by hongrn. Oct 2014
                    Do you realize that being an American is like winning the biggest jackpot ever??

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                    • #11
                      Re: My take on inductor spacing

                      John, correct me if I'm wrong here, but isn't the magnetic field generated around an inductor voltage dependant? A inductor measurement is a pretty low voltage test. My concern is that inductance might change with signal level due to other nearby inductors.

                      Originally posted by rone View Post
                      Just run a signal through one and measure the coupled signal on an adjacent one.
                      This, I think is a good idea to measure cross-talk. Run a frequency sweep through your amp connected to inductor and 8 ohm load. Then record the sweep measured at the output of the inductor. Then repeat, but this time measure across an inductor and 8 ohm load nearby, let's say 2". This should give an accurate representation of cross-talk. I think a change in inductance is more of a concern than cross-talk, which I'd expect to be pretty minimal.
                      "I just use off the shelf textbook filters designed for a resistor of 8 ohms with
                      exactly a Fc 3K for both drivers, anybody can do it." -Xmax

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                      • #12
                        Re: My take on inductor spacing

                        All of this would be moot if we all used toroidal coils.... :D
                        Later,
                        Wolf
                        "Wolf, you shall now be known as "King of the Zip ties." -Pete00t
                        "Wolf and speakers equivalent to Picasso and 'Blue'" -dantheman
                        "He is a true ambassador for this forum and speaker DIY in general." -Ed Froste
                        "We're all in this together, so keep your stick on the ice!" - Red Green aka Steve Smith

                        *InDIYana event website*

                        Photobucket pages:
                        https://app.photobucket.com/u/wolf_teeth_speaker

                        My blog/writeups/thoughts here at PE:
                        http://techtalk.parts-express.com/blog.php?u=4102

                        Comment


                        • #13
                          Re: My take on inductor spacing

                          Originally posted by Wolf View Post
                          All of this would be moot if we all used toroidal coils.... :D
                          Later,
                          Wolf
                          But then we'd all be arguing, I mean conversing, about core saturation ;)

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                          • #14
                            Re: My take on inductor spacing

                            in fact, as current increases through a coil, so does the emf generated by the coil which is in direct proportion to the increasing current. Play around with this link - there's some great info here:

                            HyperPhysics Inductance
                            -Mike

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                            • #15
                              Re: My take on inductor spacing

                              Not if it's non-ferrous.... :p
                              Wolf
                              "Wolf, you shall now be known as "King of the Zip ties." -Pete00t
                              "Wolf and speakers equivalent to Picasso and 'Blue'" -dantheman
                              "He is a true ambassador for this forum and speaker DIY in general." -Ed Froste
                              "We're all in this together, so keep your stick on the ice!" - Red Green aka Steve Smith

                              *InDIYana event website*

                              Photobucket pages:
                              https://app.photobucket.com/u/wolf_teeth_speaker

                              My blog/writeups/thoughts here at PE:
                              http://techtalk.parts-express.com/blog.php?u=4102

                              Comment

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