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Re: The "second pig" ballast: Questions.

Original poster: Finn Hammer <f-h@xxxx>

Aaron,

I cannot explain the reason for an airgap in an AC-Choke, only vouch for the need for it.
However, about the cores:
From your description of them, they are most certainly standard C-cores. This particular type of core is wound on a mandrel with a rectangular cross-section, and the silicon steel lamination is glued in the process. These cores are cut afterwards, the mating surfaces are ground, perhaps even lapped, to a high standard of flatness, then assembled around the vindings on the bobbin, and clamped together with straps.
You can therefore take them apart without the laminations separating from each other, provided that you exersize the necessary care.
Since the transformer is an under oil type, it probably wasn`t varnished, so the core halfs should fall apart right as the strips are cut open.
If the transformer has been varnished, the cores might stick together, so you have to clamp one half of the core in a vise, with strips of plywood between the jaws and the core. Then with a mallet, hit the other core sharply, but also here a piece of plywood inbetween. This is to break the seal of any varnish.

The cores should come apart nicely.

Reassembling them, use stainless hose clamps, to clamp the cores with an appropriately thick shim in each side. I would imagine 1/16" thick shim would be fine.

Hope this helps.

Cheers, Finn Hammer


Tesla list wrote:

Original poster: "J. Aaron Holmes" <jaholmes@xxxxxxxxxxxxxxxxxx>

Thanks, Steve.

Well, the core *is* in two pieces already :)  The two
spools are separatly bound with steel straps, then
bound together side-by-side with another pair of
straps.  I'd be happy with a 3-5kVA ballast to start
with, so perhaps I only need to mess with half of the
core anyway.  Unfortunately, I'm still a bit unsure as
to how I'd go about introducing an air gap without
either letting the thing fly into pieces or without
the laminations simply filling the cut as I go.

So, a quick physics lesson on why the air gap is so
critical would be interesting (at least to me).  My
mental model of an inductor is obviously
over-simplified.

On the other hand, perhaps I should *let* it fly
apart, then just use the laminations as core material
for a "sliding choke"-type ballast.  There's certainly
an abundance of good steel here...

Aaron

--- Tesla list <tesla@xxxxxxxxxx> wrote:

> Original poster: "Steve Conner"
> <steve.conner@xxxxxxxxxxx>
>
>  >The core consists of two separate "spools" of
> steel (not
>  >E's and I's, although the whole core has two
> "windows"
>  >like an "EI" core), and won't come apart in any
> way
>  >which might make winding easy.  Bummer.
>
>
> Bummer is the word. If you can't take the core
> apart, then you can't
> introduce an air gap to it, so you'll find it
> impossible to make an
> efficient ballast no matter what number of turns you
> use.
>
> If it were me I would be thinking of sawing the core
> in half (after binding
> it in a suitable way so it won't spring apart
> violently)
>
>
> Steve C.
>
>
>