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Re: [femm] Approximation of non-axisymmetric geometry
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@xxxxxxxx
http://femm.berlios.de/dmeeker