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# Re: [femm] Shielding effectiveness

- To: <femm@yahoogroups.com>
- Subject: Re: [femm] Shielding effectiveness
- From: "Emiliano" <emiliano.menghi@inwind.it>
- Date: Wed, 25 Apr 2001 22:41:33 +0200
- References: <9c4l36+uf63@eGroups.com> <3AE726D0.A98BF6FD@foster-miller.com>

    But if i use a second conductor (at distance of 5 meters) for the return,
    the problem is solved?

    ----- Original Message -----
    From: "David Meeker" <dmeeker@foster-miller.com>
    To: <femm@yahoogroups.com>
    Sent: Wednesday, April 25, 2001 9:34 PM
    Subject: Re: [femm] Shielding effectiveness

    Hi Dave, i know the problem of non-zero total current and for this, i used
    this configuration:

    A wire with R= 7.1 mm
    A dielectric (ext rad= 16.5 mm)
    A tubolar shield (ext rad= 19.5 mm)
    A second wire with R= 7.1 mm at 5 mt from the first
    A circuit property with I=324 A for wire 1
    A circuit property with I=-324 A for the return (wire 2).
    A=0 at R=25 mt
    Wires and shield are in alluminium

    And the question about the lenght of the wire (and the shield)?

    >
    > emiliano.menghi@inwind.it wrote:
    >
    > > Hi. I use femm to calculate shielding effectiveness of various
    > > material (such as alluminium, copper, iron, mumetal etc,) at 50 Hz,
    > > but i have some problem.
    > > My configuration is simple:
    > > A alluminium wire with 1000 A 50 Hz (r=7.1 mm)
    > > A dielectric around the wire (external radius r=16.5 mm)
    > > A tubolar shield around (external radius r= 19.5 mm)
    > >
    > > I use A=0 as boundary condition
    > >
    > > If i don't simulate the shield all is OK, but when i use for example
    > > an alluminum shield, i have these problems:
    > >
    > > 1- Shielding effectiveness depends on the position of the boundary
    > > condition
    > > 2- I compared femm results with real measures: in the measure setup
    > > the wire and the shield lenghts were 10 mt. The wire was grounded
    > > (for femm is grounded at infinity, i think)
    > > Results ( distance from wire: 20 cm)
    > >
    > > without shield
    > > B femm= 323 microT B measured=321
    > >
    > > with alluminium shield
    > > B femm= 101 microT B measured= 275 microT
    > > I in the shield femm= 200A measured=35 A
    > > with shield floating B measured= 303 microT
    > > Why?
    > > Is The problem, perhaps, the lenght of the wire (and the shield)
    > > infinite for femm and finite (10 mt) for the measure?
    > > Can i simulate with femm terminations of shield (floating or
    > > grounded)?
    > > Thanks and exuse me for my poor english
    > >
    > > Menghi Emiliano
    > >
    > >
    > >
    > > Your use of Yahoo! Groups is subject to
    http://docs.yahoo.com/info/terms/
    >
    > You have to be extremely careful in interpreting 2-D planar problems in
    which
    > the current is not conserved (all current doesn't add up to zero). This
    is
    > totally independent the way femm solves problems, but is instead a
    > fundamental attribute of the problems themselves.
    >
    > Basically, the trick is that A doesn't go to zero for an exact solution of
    > your problem as the distance from the currents goes to zero (i.e. on an
    > unbounded domain). Instead, A goes to + or - infinity. Consider the
    exact
    > solution for the A resulting from a point current, which is proportional
    to
    > log(r))--that's what's happening here. For the magnetostatic problem,
    things
    > work OK because there are no dynamics and flux distribution just has to
    > satisfy Ampere's loop law--specific values of A aren't important. For the
    > eddy current problem where your shield is "grounded", however, the
    inductance
    > of the wire and of the shield are dependent on the radius at which you
    > declare A=0. The bigger the the radius, the bigger the shield's
    inductance.
    > If the impedance that the shield sees is different, the induced currents
    in
    > the shield will be different as well. It is interesting to note that as
    your
    > radius goes to infinity, so does the inductance. (The case in which the
    total
    > shield current is constrained to be zero ("floating shield") is
    different--it
    > doesn't "talk" to the outside region, so the A=0 location isn't important
    as
    > long as the wire current that drives the shield is constrained to be the
    > "right" value).
    >
    > An interesting way of looking at is that by defining A=0 at some radius R,
    > it's like having a very conductive shell of radius R that carries all of
    the
    > return currents. The inductance that you are driving is associated with
    the
    > flux in the air between your wire, shields, and the shell of radius R.
    > Outside the radius R shell there is no field, since the enclosed currents
    for
    > any loop that lies beyond radius R are zero.
    >
    > One way or another, you have to model how the eddy currents complete their
    > electric circuit in the finite element geometry. (or the program ends up
    > doing it for you by virtue of the equations that it is trying to solve,
    > perhaps returning the currents in a way that you did not intend). You
    can
    > use the "circuit" properties to define constraints on the net current
    carried
    > in a section or group of sections. For example, for your shield, you'd
    > define a circuit property where the total current is constrained to add up
    to
    > zero. Then, you'd apply this property to both the shield and its return
    path
    > to get the "grounded" case. To get the floating case, you'd apply the
    zero
    > total current constraint to just the shield itself. You could also define
    a
    > second circuit property that contains the total current in the wire to
    make
    > sure that the total current is what you want it to be in the presence of
    eddy
    > currents in the wire.
    >
    > Dave Meeker
    >
    >
    >
    >
    > Your use of Yahoo! Groups is subject to http://docs.yahoo.com/info/terms/
    >
    >

- References:
  - Shieldinhg effectiveness
    - From: emiliano.menghi@inwind.it
  - Re: [femm] Shielding effectiveness
    - From: David Meeker <dmeeker@foster-miller.com>

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