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

- To: femm@yahoogroups.com
- Subject: Re: [femm] Re: some proposals for future developments
- From: Keith Gregory <k.gregory@lboro.ac.uk>
- Date: Fri, 26 Sep 2003 07:55:20 +0100
- In-reply-to: <bks9kl+9f89@eGroups.com>
- References: <3F6DAC59.1090003@ieee.org>

`Another thought - how is the solenoid put together? Have you got any small parasitic air-gaps that are not being modelled?`
`Keith.`
`At 14:27 24/09/2003 +0000, you wrote:`

>     --- 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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- Follow-Ups:
  - Re: some proposals for future developments
    - From: "atreding" <atreding@gwlisk.com>

- References:
  - Re: [femm] some proposals for future developments
    - From: David Meeker <dmeeker@ieee.org>
  - Re: some proposals for future developments
    - From: "atreding" <atreding@gwlisk.com>

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