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Magnets Make a Difference

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  • davidj95650
    In my message to Yahoo MEG-Builders #1207 dated 03 NOV 2003, I reported the results of an experiment to measure the magnetic field inside the output coil at
    Message 1 of 3 , Dec 19 8:17 AM
      In my message to Yahoo MEG-Builders #1207 dated 03 NOV 2003,
      I reported the results of an experiment to measure the magnetic
      field inside the output coil at locations on the inside and
      outside of the core opening. That experiment indicated that
      there is a large difference in the magnetic field based on
      location which I attribute to the difference in magnetic-vector-
      potential (A) at those locations. Those findings prompted me to
      want to know if there are differences in even smaller regions
      around the output coil since there will be differences in the
      gradient of A for small differences in location. Hence I built
      another setup using ferrite inductors as sense coils spaced
      evenly around the circumference of the core leg. I used short
      inductors to reduce coupling to adjacent core surfaces at the
      top and bottom of the core opening. A drawing of the test setup
      is in the file "ACorTst2.bmp".

      Go to "Files" then go to the folder "MESSAGE ATTACHMENTS", go
      to the folder "Magnets Make a Difference", and open
      "ACorTst2.bmp".

      The core is again a Honeywell AMCC-320, cut core. Honeywell
      cores can be purchased from Eastern Components,
      www.eastern-components.com. A layer of electrical insulating
      tape 0.020-inch thick is placed on the core. On this tape are
      placed 31 inductors side-by-side, centered on the core leg.
      These inductors are Jeffers Electronics part number 19A472-J,
      and are 4.7 mH, 0.194-inch diameter, and 0.450-inch in length.
      Similar inductors can be purchased from Digi-Key
      (www.digikey.com) and other electronics distributors. There is
      nothing special about these inductors, I had a package of them
      in my "junk" box and used them. A layer of insulating tape
      covers the inductors and holds them firmly in place. Over the
      insulating tape is placed a single layer of 0.002-inch brass
      shim stock to provide an electrical shield between the
      inductors and the voltage in the output coil. The ends of this
      brass shield are insulated to prevent it from becoming a
      shorting turn which would be a low-resistance block to any
      changing magnetic field in the core or output coil.

      Two layers of insulating tape 0.0022-inch thick are placed
      over the brass layer, followed by 6 layers of 0.0015-inch teflon
      tape commonly used by plumbers. The teflon layer reduces the
      capacitance between the first layer of the output coil and the
      brass shield. Such capacitance would reduce the resonant
      frequency of the output coil.

      The output coil is wound using #24 enamel-coated magnet wire
      wound bifilar (two wires side-by-side) creating a large
      capacitance between the two wires. The layers are wound 12
      turns of this bifilar configuration per layer, for a total of
      five layers and 112 bifilar turns. Four layers of teflon tape
      0.0015-inch thick are used to insulate between layers. When
      the end of one wire is connected to the beginning of the other,
      the output coil is a total of 224 turns and measures 314 mH.
      The capacitance between the two wires is 4.83 nF, and the
      series resonance of the output coil with the nearest drive-coil
      is 33.5 kHz.

      On the other core half is an output coil of 224 turns wound
      similarly, but not identically, whose series-resonance with its
      drive-coil is 29.5 kHz. Having two resonant output coils
      provides some balance to the changing flux in the core.

      A resistor of 12k ohms was placed on each output coil. A
      filter is placed between the output of the 10x probe (used to
      measure the voltage on the inductors) and the oscilloscope
      input. The 3dB cut-off frequency of this filter is 90 kHz.
      Its purpose is to reduce the "ringing" voltage of the
      inductors (about 300 kHz) to make it easier to measure the
      voltages on the oscilloscope. This ringing is the response
      to the sharp changes in flux induced by the square-wave drive
      pulse.

      There are two drive circuits, each composed of two MOSFETs
      in a half-bridge configuration so that the full supply
      voltage can be applied across the drive coil. Each drive
      circuit produces a half-square wave to its respective drive
      coil to provide large values of dB/dt in the core, and
      consequent large values of dA/dt outside the core.

      The drive logic turns on each drive circuit for about 49%
      of the total period. Thus there is no overlap of drive
      signals, and during most of the time there is drive to the
      core. Providing a small amount of "off" time allows the
      core to discharge if there is any asymmetry in the applied
      drive.

      The voltage at the output coil was kept constant at 1,000
      volts peak-to-peak for all measurements. This level of
      output provides a reasonable amount of dB/dt in the output
      coil, encourages the formation of surface charge and
      requires about five watts of drive power.

      Measurements were made during a single test session to
      reduce any possible changes due to changing environmental
      conditions (temperature, humidity, etc). These conditions
      will slightly change the response because the metal of the
      core will expand or contract which changes the coupling
      between drive coil and output coil, resonant frequency,
      and related effects.

      For the results posted here, the measurements were made at
      a fixed frequency of 31.75 kHz without and then with the
      magnets in place. Because the permeability of the core changes
      slightly with the large magnetic field of the Neodymium magnets
      used, other measurements were made at the different resonant
      frequencies without and with magnets. The results were similar
      to those reported here.

      With the magnets in place, a gauss-meter was used to measure
      leakage flux near the output coil. In all areas, especially
      those closest to the magnet stack, the static field was
      measured at less than 100 gauss. When the magnet stack was in
      place, prior to placing transformer laminations to eliminate
      the air gap between magnets and core, placing the probe in that
      gap showed a magnetic field greater than 8,000 gauss. The magnet
      stack is 1.5-inch in length, 0.75-inch width, and height of 1.25-
      inch.

      The drawing, "ATstRslt.bmp" plots the results of my
      measurements versus location on the core leg. As can be seen,
      there is a significant difference in measured B field based on
      the location. Of particular note is that with the magnets in
      place, there is a significant difference in the measured flux in
      the inside core space whereas there is almost no change for the
      measured flux outside the core space. In the inside core space
      there is much more A, and a large gradient in A, as detailed in
      my message #1207.

      This drawing is done so that if the voltage were zero, the
      test point would occur on the dotted line surrounding the core.
      The location of each test point is directly proportional to
      its measurement. Thus the drawing becomes a contour of flux
      measurements versus location on the core leg. It is
      interesting to note the large difference between the sense
      inductors near a corner of the leg, and those in the center of
      the leg. The results are not perfectly symmetrical probably
      due to slight differences in placement of the inductors (they
      may not be the same distance from the core surface due to
      irregularities in the insulating tape, and perhaps forced up
      and away from the core due to force from their connecting
      leads). Also, there are slight imperfections in the bifilar
      winding, sometimes the adjacent wires do not lay against one
      another, slightly changing the circumstances for surface
      charge (whose motion through the gradient of the permanent-
      magnet-induced A is what I believe is responsible for these
      results).

      In message #1207, I noted that I had observed, but not
      quantified, the difference when the magnets were placed in
      the core. Also, I had not observed much difference in that
      test between an inductor at the center of the leg, and one
      placed near the edge. Here, obviously, with better resolution
      of location, and more attention to the measurements,
      differences are very clear.

      As noted earlier, these tests were run under different
      operating conditions and similar or greater differences were
      noted without and with the magnets in place.

      Several questions arise:
      1. What is the back-reaction to the drive circuit from
      this increased magnetic field in the output coil ?
      2. What should be done with this increased magnetic
      field ?
      3. How can this effect be increased ?

      More exploration, hopefully more discovery :-) !!

      David J.

      Files:
      ACorTst2.bmp
      ATstRslt.bmp
    • davedameron
      Hi David, These look like very interesting tests. Do you have a plot of the output of the 32 inductors around the core and inside the output coil, showing the
      Message 2 of 3 , Dec 20 8:11 AM
        Hi David,
        These look like very interesting tests.
        Do you have a plot of the output of the 32 inductors around the
        core and inside the output coil, showing the measurement of the mmf
        or
        the A-field distribution, with load?
        -Dave D.
        --- In MEG_builders@yahoogroups.com, "davidj95650" <djenkins@r...>
        wrote:
        > In my message to Yahoo MEG-Builders #1207 dated 03 NOV 2003,
        > I reported the results of an experiment to measure the magnetic
        > field inside the output coil at locations on the inside and
        ...
      • davidj95650
        Hi Dave, I do not presently have such a plot. I ve been working on a program that would generate such a plot so that I could compare mathematics, as I
        Message 3 of 3 , Dec 22 1:12 PM
          Hi Dave,

          I do not presently have such a plot. I've been working on a program that
          would generate such a plot so that I could compare mathematics, as I
          understand it, and results. The increases in measured voltage at each corner
          are due to greater coupling between the embedded inductors and the output
          coil.

          BTW, I changed the output connections so that the two bifilar wires were in
          parallel, then added a 1.8nF capacitor to make it series-resonant at 44 kHz.
          This changed the observed difference between no magnets and magnets to
          about twice that of the bifilar connection. I also removed the laminations that
          make a tight fit between the magnet stack and the core and the results did
          not vary. Making this observation is limited by the ability to accurately
          measure voltage on the screen of the oscilloscope. I expect that there should
          be a small decrease because there is now a much larger reluctance in the
          path of the magnetic field from the magnets which will cause a decrease in
          the static field in the core. The air gap at the bottom of the magnet stack was
          just over 0.10-inch.

          I am building another setup to concentrate on the changes of magnetic
          field at the core-interior side of the output coil and to determine what physical
          changes increase the no-magnet/magnet difference.

          David J.

          --- In MEG_builders@yahoogroups.com, "davedameron" <ddameron@e...> wrote:
          > Hi David,
          > These look like very interesting tests.
          > Do you have a plot of the output of the 32 inductors around the
          > core and inside the output coil, showing the measurement of the mmf
          > or
          > the A-field distribution, with load?
          > -Dave D.
          > --- In MEG_builders@yahoogroups.com, "davidj95650" <djenkins@r...>
          > wrote:
          > > In my message to Yahoo MEG-Builders #1207 dated 03 NOV 2003,
          > > I reported the results of an experiment to measure the magnetic
          > > field inside the output coil at locations on the inside and
          > ...
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