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Re: How to make cheap big cores?



Original poster: "Dr. Duncan Cadd by way of Terry Fritz <twftesla-at-uswest-dot-net>" <dunckx-at-freeuk-dot-com>

Hi Tony!

Date: 19 January 2001 03:30
Subject: How to make cheap big cores?


>Original poster: "Tony Bryant by way of Terry Fritz
<twftesla-at-uswest-dot-net>" <brd-at-paradise-dot-net.nz>
>
>Hi, I'm looking for a way to get a big HF transformer
cores.
>

Interesting experiment . . .

>Next experiment will be to make an iron powder core.
>I intend to get some powder from a metal spraying supply
place,
>dunk it in H3PO4 to get a black oxide layer, and then cast
into
>a big toroid with a small amount of epoxy.
>
>Is this feasable?


Well, technically yes.  Shellac, mica, kaolin and a variety
of other stuff can be used as the insulator.  I doubt you
even need to go to the trouble of passivating the iron (it's
likely to disappear into the acid if the particle size is
small enough for it to be any good for what you want) *but*
the manufacturers of cores generally use around 100 tons per
square inch to form the material.  We are talking serious
hydraulics here.  Half this pressure will drop the effective
mu (permeability) down to maybe 60%.  The problem is that
you want the minimum of insulation - 10% by volume would be
a good aiming point - and a lot of umph (TM) is required to
pack the particles tightly enough.  At this level, the
magnetic quality of the iron is not terribly important, and
the magnetic permeability of the bulk composite will be
given by:

Effective mu = {(a^2 mu) + 1 - a^2} / {mu - (a mu) + a}

where mu is the permeability of the iron particles, a is a
factor depending on what volume of the composite is actually
iron (more below) and effective mu is the permeability of
the composite.  The factor a is derived from considering
that a unit cube of composite contains a volume a^3 of iron,
hence if the insulator occupies 10% of the volume (equals
0,1 of the unit cube) a^3 is 0,9 from which a = 0,9655 (cube
root of 0,9) and thus a^2 = 0,9322

For example, if the permeability of the iron is 100 and
there is 10% insulator, effective mu = 21,7 ; if the mu of
the iron was infinite, effective mu would still only be 28,
and only 58 with 5% insulator. Hence the magnetic quality of
the iron is not terribly important.

The particle size is, however, as this determines the eddy
current loss at a given frequency and the particle form may
well be (round, oblong etc) and particle geometry is likely
to be the thing which ends up determining the success of
your experiment. The shape of the particles generally
determines two things - how well they pack and how much
pressure they will stand before contact points break through
the resin coat (the physical hardness of the iron is
important here too). Modern magnetic core manufacture is a
complex business and it will be very interesting to see
whether a good core can be made in "home workshop"
conditions.  Things must have started out that way back in
1915 when the Western Electric Company made the first dust
cores pressed under 100 tons per square inch (Speed and
Elmen, Transactions AIEE, vol 40, p596, 1921.) I'm sure
there will be more than one person on this list who will be
keen to know the outcome.

If you really want to try, any particles will do for a quick
test in making a miniature core, but you're likely to get
best results (assuming the trial run gives cause for
optimism :-) by using a mixture of two different particle
sizes, as one will fill the voids in between the other.  100
mesh and 240 mesh is an example.  You can get fine mesh
metal sieves from pottery supply companies, as pottery
enthusiasts use them to sieve the ingredients used in making
enamels, frits and glazes.  Geologists and biologists use
them for all manner of things so if you get that far it
shouldn't be hard to find sieves - though it may be harder
to find iron particles which will pass through them ;-)

Good luck!

Dunckx