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# [FEMM] Solid block vs laminations - does not compare

- To: femm@list.foster-miller.net
- Subject: [FEMM] Solid block vs laminations - does not compare
- From: "robert Macy" <macy@california.com>
- Date: Wed, 26 Nov 2003 11:02:04 -0800
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    I really need help on this problem.  Trying to understand
    how femm characterizes eddy current losses in an inductor.

    The results of the the following analyses suggest that it's
    better to use a solid block than large laminations.
     Somehow, that does not seem intuitively correct.

    First, use a huge slab of magnetic material
      femm predicts fairly good inductance with fairly high
      losses from eddy currents.

      BUT! the block is not an infinite length so set the
      core's current to a circuit value of zero.

      femm then predicts better inductance and lower eddy
      current losses.

    Second, use laminations to simulate limited block length
      Make the core's current go zero due to laminations, but
      make the laminations as thick as the block, or half
      the block.

      BUT! the effective permeability has gone down to 11 !!

      femm predicts a 50 times drop in inductance!

    For laminations femm applies Stoll's equation (as shown in
    the femm manual) to predict a "new" effective permeability
    versus frequency versus lamination thickness.

    Problem is that the difference between analyses using
    laminations and using block material are *WILDLY*
    different!

    As femm uses Stoll's equation, once the laminations are
    more than 3 skin depths thick; the effective permeability
    becomes simply divided by the ratio of lamination thickness
    to skin depth at an angle of 45 degrees.  It is possible to
    drive the effective permeability arbitrarily to zilch.  It
    seems it should have converged to solid block permeability.

    Question:  How do you correlate femm's solutions using
    Stoll's equation for effective permeability to solutions
    one gets using giant blocks?

                              - Robert -

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