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[FEMM] About the modelization of small coils
Greetings to all
First of all I'd wish to thank David Meeker for FEMM, I've been using it
troughout this last month, and it has been a very good experience,
completely outside my usual field of activity. I've been browsing through
the archive to find elements to help me, and while I found many interesting
things, I remain with some doubts.
I've been designing and constructing a probe for ac magnetic susceptibility
measurements. It consists in two counterwinded secondaries coils with about
1020 turns of AWG40 wire, in series, with a inner coil radius of 4.5mm,
each secondary being long 10mm, thus having 10 layers of about 100 turns.
The primary is winded on top of it. It is made of AWG32 wire, over 60mm,
with an inner radius of about 5.78mm, for a total of 873 turns, in 3 layers
of 264turns and more compensating turns in order to have the greatest field
homogeneity over the sample space. Attached I send a model of the real
probe, where some of those compensating turns have been moved in order to
cancel the residual signal due to differences between the two secondaries.
The probe should be used in the range 50-10000Hz, thus avoiding any eddy
currents problem, and I worked satisfactorily imposing a constant current
density, with 1mA corresponding for AWG32 wire to 30900A/m^2.
I used FEMM to study the effect on field homogeneity of the compensating
turns' position, and on the value of the field generated at the sample
positions.
My questions, after perusing the mailing list archives, are
1) I tried to use the circuit constraint in order to impose the current,
but I didn't understand how to do it in that case (if it is at all
possible, since I haven't really a closed circuit). That point is more for
my own curiosity.
2) I modelized my primary coil (the only one of interest in fact) by a
solid conductor, and individual turns by perfectly circular square wires,
with conductivity zero and a current density corresponding to the one in
the individual wires. But then when I integrate the current over all this
conductor, I obtain a value that is quite far from current per turn *
number of turns. That is, I obtain 1.384A instead of around 870mA.
I saw the very interesting posting by Finlay Evans (see at end) about the
filling factor, and thus confirmed my misgivings about the lost space
occupied by the wire varnish and air inside the windings.
Integrating the primary area, I obtain 4.479280e-005 meter^2. With a wire
of 0.228mm diameter with insulation (0.203 without), I have thus a ratio
primary area/wire area of 1097. I get then 873/1097=0.796, a ratio very
much near the one cited by Evans.
My point is that I would like to use FEMM to KNOW the field produced by a
given current in the coil. Measuring with a Gaussmeter could prove
difficult (fields are in the 0.1-2G range) and more time-expensive than a
calculated estimation. But I don't know at this point whether the block
model is precise enough, or if I can correct simply by decreasing the
current density to have a current integration over the primary
corresponding to current per turn * number of turns. In that case what
would be the approximation made on the real value of the field?
Any similar experience would be welcomed. If someone had to draw individual
coils and wrote some routine to do it, that would also be welcomed, I don't
feel much like drawing one-by-one 800 and plus circular wires...
Warmest greetings
P.S. I didn't suscribe to the mailing list, please send any answer to the
mailing list as well as to me personally
"[...]
I know that I cannot design a coil to have a packing factor of more than
about 0.78 and expect to actually build it reliably, and this is true only
if I leave a small clearance on the outside surface of the coil to allow for
messy overlaps. This number actually is very close to a theoretical
winding that has a square lay pattern (i.e. one turn directly on top of
another = (pi/4)). In reality a coil tends to form in a hexagonal
close-packed lay which has a theoretical packing factor of about 0.9,
however the lead-in turn messes things up and the coil generally begins to
get messy towards the outer turn, hence the 0.78 figure. This 0.78 figure
is pretty handy though since you can easily guess how many turns you can
expect to get in a rectangular enclosed space (e.g. a coil using 0.5mm
diameter wire will have 10 x 20 turns in a space of 5mm x 10mm).
[...]
Finlay Evans"
Dr. Patrick Rosa
Laboratorio di Magnetismo Molecolare
Dipartimento di Chimica
Università degli Studi di Firenze
Via della Lastruccia 3
50019 Sesto Fiorentino (FI) Italia
Tél. +39-0554573338
Fax. +39-0554573372
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