RE: The "second pig" ballast: Questions.

["Tesla list" <tesla@xxxxxxxxxx>]

Original poster: "Malcolm Watts" <m.j.watts@xxxxxxxxxxxx>

On 7 Feb 2005, at 11:56, 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.

In order to do useful things (such as limiting and/or maintaining a
current) a choke must have the ability to store energy. Core material
has a fixed number of magnetic domains that can align with the
magnetic field and as these line up, the inductance of the coil is
progressively reduced until it becomes that of air (saturation).
Since air (space actually) has no domains, any amount of energy can
be stored in it, hence the need for an airgap. The core material
concentrates the magnetic field to the vicinity of the airgap and
most of the energy is stored in the form of magnetic flux in the gap.
Inserting a gap in a core greatly increases the reluctance in the
magnetic circuit and greatly decreases the inductance given the same
core and number of turns wound on it.
    I'm not sure your double core is suitable for your intended use
of it. Just one of the sections with a gap in it sounds to be a
better proposition to me. I don't know about handling a coil of
spring steel though - I don't think I would. I think it would be best
used as a transformer core.

Malcolm

> 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.
>  >
>  >
>  >
>
>
>