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# Re: Multiple group membership for nodes

- To: femm@yahoogroups.com
- Subject: Re: Multiple group membership for nodes
- From: "David Meeker" <dmeeker@ieee.org>
- Date: Fri, 05 Sep 2003 16:36:04 -0000
- In-reply-to: <bj8o2v+ijhu@eGroups.com>
- User-agent: eGroups-EW/0.82

    --- In femm@yahoogroups.com, "niallcaldwell" <niall.caldwell@e...> wrote:
    > Firstly, many thanks to David for his advice over the slotted
    > armature problem I had. I was not aware that the "lamination" feature
    > essentially did this BH scaling that I was trying to do. The point
    > about radial gap is well taken- this is the case in most of my
    > designs. I presume that, if the material is working in the linear
    > portion of its BH curve, and the gap is small in comparison with the
    > major dimensions of the armature and the slot circumferential width,
    > then a good approximation to obtain the same gap reluctance is to
    > increase the radial gap by the same ratio as the "lamination factor"
    > at the outside of the armature?

    It might be reasonable to fudge the gap width in this circumstance.
    However, I'd suggest using Carter's Coefficient as a guide for picking
    the effective air gap. I'll have to look up the formula--I don't
    remember it off the top of my head. Anyone have a good web reference
    for Carter's Coefficient.

    > One other point which has come up. I often want to parameterise
    > models so that I can sweep through a design space to find an optimum
    > of two or more variables. I want to use Lua with nested "for" loops
    > to do this, for instance generating armature force for a range of
    > diameters and lengths of the armature.
    >
    > To do this from the starting point of existing geometry I need to
    > select nodes by group in Lua- all the nodes on the outside diameter
    > are in group 1, while all the nodes at the top end are in group 2.
    > The trouble is, there is always at least one node which wants to be a
    > member of both groups (ie the outer top corner). Is there any way to
    > assign multiple groups to entities?

    Well, you can't assign multiple groups, but you could assign a
    different group number for the elements that are supposed to be in
    more than one group (e.g. group number 21 for elements that are
    supposed to be in both groups 2 and 1 or something...). Then, you can do
    selectgroup(1)
    selectgroup(21)
    to get group1 plus the shared section. When you do multiple
    selectgroups, the previously selected group doesn't get unselected.

    > The other option is to build up the entire problem in the lua script,
    > bypassing the GUI, creating keypoints with coordinates calculated by
    > functions of defined parameters. That way individual nodes
    > coordinates can be a function of many parameters. This is the most
    > powerful and general way of parameterising, and the way I used to
    > work with Ansys becuase its GUI is such a dog compared to femm's, but
    > really.... life is too short!
    >
    > BTW this same nested loop technique can be used to generate lookup
    > tables of force and inductance at a range of currents and armature
    > positions, for export to dynamic simulation packages (AmeSim, Easy5,
    > Simulink) This allows the complete dynamic simulation of a solenoid
    > taking into account damping, spring, inertia, colomb friction,
    > electric circuit switching... I have heard that transient phenomena
    > eg. eddy currents can be taken into account by running a frequency
    > analysis in FEMM and inserting parasitic lumped parameters into the
    > dynamic model.
    >
    > Niall

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    - From: "niallcaldwell" <niall.caldwell@ed.ac.uk>

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