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# Re: [FEMM] Need help in analytical fitting of steel B-H curve

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- Subject: Re: [FEMM] Need help in analytical fitting of steel B-H curve
- From: "Chris Holt" <chris.c.holt@aeat.co.uk>
- Date: Wed, 21 Apr 2004 11:35:45 +0100
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    Tsarafidy,

    Birke's expression is:

    Z(J) = X(1) * (1 - EXP(-((XLOG(J) - X(2)) / (X(3) - X(2))) ^ X(4)))

    where Z is used for B, J for H and X(1)...X(4) are constants and
    I believe XLOG is the common log function.  I found that I had
    greater control of the permeability by introducing a fifth constant
    as a factor within the exponential term.  I also used two of the
    constants to offset the calculated B values, i.e.

    Z(J) = X(1) * (1 - EXP(-((X(5)*XLOG(J) - X(2)) / (X(3) - X(2))) ^
    X(4)))
              + X(1)/X(2)

    The constants were chosen heuristically, rather than by regression.

    Regards

    Chris

    > Thanks for your reply. I plan to try this way.
    > But Could you tell me more about this factor on the
    > exponential term ?
    > Regards !
    >
    > Tsarafidy.

    > > One approach is an exponential function like that
    > > described in Paul Victor Birke's 1998 post to
    > > sci.physics.electromag, which can be seen at
    > >
    http://groups.google.com/groups?selm=364EE79A.F279CFE%40home.com&oe=UTF-8&output=gplain

    > >
    > > I've also tried this approach with a further factor
    > > on the
    > > exponential term and have obtained some reasonable-
    > > looking B-H curves.
    > >
    > >Chris
    > >
    > > > I'm searching for an analytical form of a function
    > > > with known derivative, other than piecewise
    > > > polynomial, fitting B-H curve of a given steel.
    > > > Any paper or advice about this will be
    >> > appreciated.
    > > >
    > > >Thanks in advance.
    > > >
    > > > Tsarafidy.

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