Re: [femm] Shielding effectiveness

["Emiliano" <emiliano.menghi@xxxxxxxxx>]

My goal is to calculate the reduction of B around the shield, in the
external region.
In this situation is importan to calculate the currents induced on the
shield.
.... But the currents depends on the lenght of the wire and the shield ....
So, is correct adjust the sigma of the shield in rtelation of its lenght ?
For example for 10 mt: sigma_alluminium=sigma_alluminium/10.


----- Original Message -----
From: <dcm3c@xxxxxxx>
To: <femm@xxxxxxxxxxxxxxx>
Sent: Thursday, April 26, 2001 5:13 PM
Subject: Re: [femm] Shielding effectiveness


> In a message dated 4/25/01 4:39:26 PM Eastern Daylight Time,
> emiliano.menghi@xxxxxxxxx writes:
>
> > But if i use a second conductor (at distance of 5 meters)
> > for the return, the problem is solved?
>
> It all depends on exactly what problem you are trying to solve and what
> results you are trying to obtain. Cases that I'd guess you could be
> interested in are:
>
> 1) Currents are conserved in the shield (shield is "floating"), wire
carries
> a fixed current. The goal here would probably be to determine the losses
in
> the shield. You'd model this setting A=0 at the outer edge of the shield,
> specifying two circuits: one where total current is zero that is applied
to
> the shield, and one where the total current is 324 A that is applied to
the
> wire. Now, the eddy current distribution in in the shield will be
"right",
> so that the losses will come out OK. However, the impedance that the
voltage
> reports for the wire won't be that meaningful, because it will be a
function
> of where you draw the boundary.
>
> 2) Currents are not conserved in the shield, because the shield is
> "grounded". The wire carries a fixed current. The goal here would be to
> find the impedance of the wire as well as the current and losses in the
> shield. And what I mean by "grounded" isn't just grounded at one
> point--this would be the same as "floating" to femm, since femm doesn't do
> capacitive effects. To get the currents not to be conserved within the
> shield, you'd have to have both ends of the shield connected together, so
> that currents can basically make a big "ground loop". In this case, you
have
> to model both the the wire and a second conductor for its return. You'd
also
> want to model either another shield around the return wire or some other
path
> by which currents through the shield complete and electric circuit path.
> There would be two ways that this might be modeled:
> a) if the return wire has the same dimensions as the first wire and has an
> identical shield, you can use an A=0 line of symmetry on line that bisects
> the circuit so that you only have to model one wire. Then, you'd just
define
> one circuit property for the model: a 324 A circuit that you'd apply to
the
> wire. Because of the symmetry, any current that you induce in the shield
> you'd modeled is returned in the shield that you haven't explicitly
modeled,
> so no additional circuit property is necessary. It would be a good idea
to
> use some sort of "open" boundary condition on the outer edge here.
Multiply
> the impedance that you get from looking at View|Circuit Props to get the
> impedance of the entire wire circuit subject to the shields.
> b) if the return for the shield is just something like a wire that you
have
> alligator-clipped to each end of the shield, this probably precludes the
use
> of symmetry. Then, you'd have to model the wire, its return, the shield,
and
> the shield's return, ideally applying some sort of open boundary
conditions.
> Then you'd make 3 circuits: one with 324 A and one with -324 A, which are
> applied to the wire and its return respectively, and one with a total of
0A,
> which is applied to both the shield and its return. Sum the impedance
> attributed to the wire and the impedance attributed to its return to get
the
> total impedance that you'd have to drive.
>
> 3) "floating" shield with the goal of determining the impedance of the
wire
> and shield losses. Here, you'd model the shield, the wire, and its
return,
> using some sort of open boundary condition for the outer boundary. This
is
> the same as 2b), except that the current=0A circuit property is applied to
> just the shield (since there is no shield return wire).
>
> > 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 tubular shield (ext rad = 19.5 mm)
> > A second wire with R= 7.1 mm at 5 m 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 m
> > Wires and shield are in aluminum
>
> > And the question about the length of the wire (and the shield)?
>
> As long as a 2D model is a good assumption, you can just take the 2D
> per-meter results and multiply them by the length of the wires. Where this
> could go wrong is if you are interested in impedances and the distance
> between wires is on the same order as the length of the wires. Then,
you'd
> really need a 3D solver to get things straight, but you can probably get
the
> results that you are interested in without resorting to this extreme.
>
> Dave Meeker
> --
> http://members.aol.com/_ht_a/dcm3c
>
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