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# [femm] Re: BH data

- To: <femm@yahoogroups.com>
- Subject: [femm] Re: BH data
- From: "Chris Holt" <chris.c.holt@aeat.co.uk>
- Date: Wed, 23 Oct 2002 09:58:21 +0100

    Hi Michael and David

    The Brms/Hrms suggestion should work OK for high current
    devices; however, for low current devices (such as eddy
    current probes, LVDTs etc.) the changing field will only
    take the material around a subsidiary B-H loop about a
    point on the magnetisation curve. The incremental
    permeabilities associated with such subsidiary loops are
    much lower than the magnetisation curve would suggest.

    The magnetisation curve for a carbon steel will indicate
    relative permeabilities in the 500 - 5000 range, whereas
    the incremental permeabilities that provide reasonable
    results when used in modelling low current devices are
    more like 80 - 300.

    I doubt you'll find incremental permeabilities listed for
    the material in question. If you have (or can borrow) an
    impedance analyser, you could carry out some measurements
    with a test coil and then iteratively use femm to estimate
    the permeability you should be using in your application.

    Chris

    Chris Holt BSc CPhys MInstP Mem. IEEE
    Senior Consultant, Engineering Solutions

    AEA Technology plc, E1 Culham, Abingdon, Oxfordshire OX14 3ED, UK
    Tel: +44 (0)1235 464089 Fax: +44 (0)1235 463799
    Email: chris.c.holt@aeat.co.uk
    Web: http://www.solutionsinengineering.com and http://www.aeat.com

    >>> dcm3c@earthlink.net 22/10/02 21:38:24 >>>
    This is a good question. Traditionally, people have used the initial
    magnetization curve as the BH curve in magnetostatic simulations. If
    you have this info, it is OK to use it as your BH curve for
    magnetostatic simulations.

    However, it sounds like you are actually interested in AC simulations.
    Even though the program accepts a nonlinear BH curve, the nonlinear
    BH curve is ignored for AC simulations and the linear values of
    permeability are used instead. (strictly speaking, harmonic analyses
    are well-defined only in the case of linear materials) In this case, a
    good approach would be to pick a characteristic RMS value of H that
    your material would see and then pick the corresponding point on the
    BH curve corresponding to this value, calling it Brms. The
    permeability that you'd use would for your simulation would be Brms/Hrms.

    Dave.

    --- In femm@y..., "HUTCHINGS Michael" <mhutchings@s...> wrote:
    > Hi David (Meeker) and others,
    >
    > I am trying to include Radiometal in the materials library. I have the
    > manufacturers data for d.c. induction but it is in the form of an
    > initial magnetisation curve. This is unlike the mumetal and supermalloy
    > materials in the femm library, which show a high initial slope /
    > permeability, declining steadily as saturation is approached. These
    > plots ignore hysteresis.
    >
    > I am tempted to input data from the radiometal d.c. hysteresis loops
    > which I also have, shifted to pass through the origin. At least then the
    > plot shape is correct. Is this likely to give acceptable results for
    > modelling of low frequency fields and inductances?
    >
    > Perhaps the included BH curves are similarly derived. I feel it will be
    > better to do this than use a description with an initially increasing
    > permeability with H.
    >
    > BTW thank you for an excellent free tool, and all the inputs to an
    > interesting discussion group.
    >
    > Michael J Hutchings
    > Engineer
    > Solartron Metrology
    >

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