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Inductance of a Bar Core Solenoid

Discussion in 'Electronic Design' started by Stephen, Jul 22, 2003.

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  1. Stephen

    Stephen Guest


    I hope this isn't the stupidest question all week, but here it goes:

    I'm trying to determine the relative permeability of a soft magnetic
    material by measuring the inductance of a solenoid for which it is the
    core material.

    My simple plan was to saturate the core using a large permanent
    magnet, measure the "air core" inductance, remove the magnet, and
    measure the non-air core inductance. I then assumed (and this is
    probably the stupid part) that the relative permeability of the core
    material would simply be the ratio of these two measurements.

    The material of the core in this case is Ni(45%)Fe(55%) whose relative
    permeability should be well above 1000 if it was prepared correctly.
    My measurements and calculations are giving a permeability value of
    ~10. Not good.

    The core is simply a bar with its middle third wrapped with Cu wire,
    and one third sticking out on each end. There is no closure path
    other than air for the flux.

    Can anyone out there steer me towards a web site where inductance for
    such a structure can be calculated. Or if someone out there can
    provide the calculations here, that would be even better.

    Thanks in advance,

  2. Sorry, but this is a hopelessly bad way to measure such a high
    permeability. All the flux passing through the coil passes through a
    lot of air as well as the core material, so the air will dominate the
    inductance, and slight variations in core permeability (say, between
    100 and 1000) will have little effect on the total magnetic reluctance
    of the flux path. This test will be a lot more sensitive to core
    geometry than it is to core permeability. This sort of test is
    usually done of a toroid made of the material in question, to exclude
    all other material from the flux path.
  3. BTW, I'm going to be of no help, but where did you get that Ni-Fe core
    material? Is it annealed and otherwise intended for magnetic use?

    Best regards,
    Spehro Pefhany
  4. Stephen

    Stephen Guest

    Thank you very much for this confirmation. After I posted the
    question, I continued to look for answers on the web, and found a
    treatise on inductance calculations by Marc Thompson that provided the
    same answer you gave. I see, now, the error of my ways. In the end,
    this is good news for us, since we were thinking that our material had
    very bad magnetic properties, and now I see that another test is

    Thansk again!

  5. Stephen

    Stephen Guest

    This material was electroplated. We didn't use any magnets to align
    it during the plating process, but the B-H loops look reasonably good,
    with Hk values in the 6-8 Oe range.

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