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# Re: [femm] Approximation of non-axisymmetric geometry

- To: femm@yahoogroups.com
- Subject: Re: [femm] Approximation of non-axisymmetric geometry
- From: David Meeker <dmeeker@ieee.org>
- Date: Sat, 09 Aug 2003 09:16:05 -0400
- In-reply-to: <bgtgmu+6rn3@eGroups.com>
- References: <bgtgmu+6rn3@eGroups.com>
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niallcaldwell wrote:

> `Hi all,
> Firstly thanks to David Meeker for writing such a great program- I was
> using ANSYS for years, writing scripts for parametric optimisation of
> solenoid valves. It is very powerful but a complete pig to use. I
> have moved to femm because it is so much quicker to set up and change
> geometry. The inclusion of Lua means it is just as good as ANSYS for
> most design optimisation.
> My question is not really femm-specific, but maybe some of you have
> tackled it before...
> I have a mainly axisymmetric solenoid valve. The moving pole in real
> life has radial slits in it (for non-magnetic reasons) leaving it only
> 50% solid in places. I am wondering what the best way is to
> approximate this non axisymmetry.
> One thought I have had is to define a new steel, based on the B-H
> curve of 1006 but with the B values divided by half (making sure that
> the final gradient equals mu0). I would use this new 50% steel over
> the slitted area. By building a library of 30%, 50%, 70% etc. steel I
> could model the slitting percentage across the radius of the moving
> pole by dividing it into zones of different material.
> I suspect there is a fatal flaw in this somewhere.... anyone got a
> better idea (apart from 3D modelling that is!)
> thanks
> Niall Caldwell
> `

There are some cases where you could make this sort of approximation, but this could be very problem-specific as to whether or not it's a good idea. FEMM implements this idea for the modeling of laminations with a fill factor of less than one, by computing an adjusted BH curve. To get the program to do this "automatically" for DC problems, select "laminated in plane" and then pick the fill factor that corresponds to your case.
However, there are some things to watch out for--specifically, how does the flux get onto and off of the part with the radial slits? For example, if you assumed that the slotted plunger is a solid cylinder and flux gets onto the cylinder by traveling radially across the gap, you'd have the wrong reluctance for the air gap, which would affect results in a big way. You might be able to fix this by modifying the air gap thickness or plunger diameter to give the "right" air gap reluctance, but again, whether or not this sort of thing is a good idea is very problem-specific.
Dave.

    --
    David Meeker
    dmeeker@ieee.org
    http://femm.berlios.de/dmeeker

- References:
  - Approximation of non-axisymmetric geometry
    - From: "niallcaldwell" <niall.caldwell@ed.ac.uk>

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