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

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- Subject: Re: [FEMM] Solid block vs laminations - does not compare
- From: "robert Macy" <macy@california.com>
- Date: Sun, 30 Nov 2003 10:47:30 -0800
- In-reply-to: <3FC9716F.2020807@ieee.org>
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    Thank you for responding.

    I'm having difficulty getting femm to model an inductor
    whose inductance is dropping off at a frequency lower than
    expected.  The inductance also drops at half the slope vs
    fequency as expected.

    Any ideas of where to adjust my model?

                  - Robert -

    On Sat, 29 Nov 2003 23:26:23 -0500
     David Meeker <dmeeker@ieee.org> wrote:
    > The bulk lamination model only applies when the
    > lamination is thin enough so that all of the eddy
    > currents are flowing in the plain of the lamination.  The
    > eddy current induced in the "usual" direction (i.e. into
    > and out of the screen) can be neglected if the
    > laminations are thin. In the lamination model, the flux
    > tends to get confined to a thin layer adjacent to the
    > insulation between laminations, and flux in any skin
    > region near the interface of the laminated region with
    > air is neglected.  Conversely, a solid block model
    > assumes that all of the current is flowing in the into
    > and out of the page direction, neglecting the impedance
    > of currents that must travel in the plane of the
    > simulation to enforce current constraints.  In the solid
    > block case, all of the flux flows in a thin skin regions
    > near the surface of the material (i.e. the mode which is
    > neglected in the bulk lamination model).  If you use the
    > solid block model to model sections that are physically
    > short in the into-the-page direction, or if you use the
    > bulk lamination model to model sections that are thick
    > relative to the other dimensions of the problem, you'll
    > get nonsense.  Solid and laminated results don't converge
    > in the "nonsense" region, because they are making
    > different assumptions as to where the eddy currents and
    > flux are flowing.
    >
    > robert Macy wrote:
    >
    > >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?
    >
    >
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- References:
  - Re: [FEMM] Solid block vs laminations - does not compare
    - From: David Meeker <dmeeker@ieee.org>

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