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# RE: [femm] Re: some proposals for future developments

- To: "'femm@yahoogroups.com'" <femm@yahoogroups.com>
- Subject: RE: [femm] Re: some proposals for future developments
- From: Mark Smith <mark.smith@linx.co.uk>
- Date: Thu, 25 Sep 2003 09:32:57 +0100

    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

    -----Original Message-----
    From: atreding [mailto:atreding@gwlisk.com]
    Sent: 24 September 2003 15:28
    To: femm@yahoogroups.com
    Subject: [femm] Re: some proposals for future developments

    --- In femm@yahoogroups.com, David Meeker <dmeeker@i...> wrote:
    > Hi Dave - I'm new to the group but I've been watching the posts for
    a while now and I downloaded the "beta" version of Femm 4.0 - my
    question is since the lua scripts from 3.3 are no longer going to
    work with the 4.0 version anyway, have you considered moving to Lua
    5.0 for the new program? It would be a good time to do it since
    scripts need to be rewritten anyway.

    P.S. - This program is great - fully the equal of commercial magnetic
    software I have used. Thanks!

    Also, for everyone else - just a question that has been eating at me
    for a while... We have been using Femm as well as Ansoft Maxwell for
    modeling solenoid actuators. The results agree well with each other
    in general, and when we have actually built the devices for test the
    models do a pretty fair job of predicting forces. However, in
    solenoids where the iron is not very strongly magnetized (B field far
    from saturation, ie. 1T or less) the results overestimate by a large
    amount, as much as 30%. While for saturated designs, the accuracy can
    be 5% or even better. This is true of both of the modeling programs
    we have used - they both overestimate force at low saturation and
    they both do so to roughly the same degree - that is they both give
    similar wrong answers.

    We have gotten around this to some extent by simply overengineering
    the designs when the saturation is low, as well as intentionally
    designing for saturation or near to it. But it is an irritating
    phenomenon and I was wondering if anyone else has ever seen this
    problem. Our thought is that at saturation large errors in H cause
    only very small changes in B, therefore the force calculation is more
    numerically robust when problems are solved at saturation, versus at
    peak permiability where a tiny shift in H could cause a huge shift in
    B. But this would seem to suggest that we should see large errors of
    underprediction as often as overprediction - and we don't. It always
    overestimates.

    Anyone have any thoughts on this?
    Thanks, Andy Reding

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