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# Re: some proposals for future developments

- To: femm@yahoogroups.com
- Subject: Re: some proposals for future developments
- From: "atreding" <atreding@gwlisk.com>
- Date: Thu, 25 Sep 2003 15:27:31 -0000
- In-reply-to: <86916FE08FDA1C4BA606D0331B6F4D4680BE90@stivesmail.linx.co.uk>
- User-agent: eGroups-EW/0.82

    --- In femm@yahoogroups.com, Mark Smith <mark.smith@l...> wrote:
    > Hi
    > I might be off the ball here but many years ago I was trying to see
    if it
    > was practical to measure the air gap in a solenoid valve by
    measuring the
    > inductance, which my quickfield model suggested should be possible.
    I found
    > that my measured inductance's were much lower than expected which
    turned out
    > to be that the permeability at low field strengths was much lower
    than my
    > material model due to the fact that the B-H curves are for ideal
    shaped
    > fully annealed samples & my cores were "as machined". I went to the
    expense
    > of having some parts hydrogen annealed and the results were much
    more in
    > agreement with the FEA analysis. Saturation field strengths were
    not so
    > different.
    > Regards
    > Mark

    Thanks Mark. Yes, we figured it was partly due to changes in
    permiability at low levels so we actually had some ring samples of
    the iron tested and made BH curves from them, in both the annealed an
    unannealed condition. There were differences (mostly at low levels)
    but not enough to explain the kind of errors we were getting.
    (Although maybe for inductance it makes more difference - we were
    only concerned with force.) One thing that did help (although not
    completely) was to treat the iron as a weak permanent magnet oriented
    against the coil. That is, we took the Hc of the iron from the
    samples we tested and put that into the materials library as a new
    material. This was enough to partly fix things and I've done it on
    low saturation models since. But it still is somewhat unsatisfying as
    this "solution" is really just a fudge factor. From what you are
    saying, it seems that maybe the finished part itself needs to be
    annealed to bring it in line with the model, implying that the
    machining stresses are causing a larger Hc than our sample measured.
    That would be unfortunate since there would be no way to predict it,
    and annealing is too expensive an operation to do for some of these
    units.
    But thanks again for your insight - it is nice to know that others
    have had similar problems and that something as simple as
    annealing/machining stresses might be enough to explain it - even if
    I can't calculate around it.
    Andy

- References:
  - RE: [femm] Re: some proposals for future developments
    - From: Mark Smith <mark.smith@linx.co.uk>

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