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Re: The "second pig" ballast: Questions.



Original poster: "Gerald  Reynolds" <gerryreynolds@xxxxxxxxxxxxx>

Hi Steve,

This is the very subject Ive been trying to learn about recently and I'm a little confused as to what you mean by "it impossible to make an efficient ballast no matter what number of turns you use". Following example will use a toroid cause it is easy to wrap your mind around and the math is simplier.

If one has a toroid using a core with permability (u), with a cross sectional area (A), with number of turns (N), and a magnetic path (length Len) (I'm choosing the center of the cross section and following it around the toroid for the path) and evaluate:

closed integral {H.dl}= NI      around the path (constant H)

(I=current thru one turn so NI is the amp-turns or total amps enclosed by the path)

therefore:

H*Len = NI,   so H= NI/Len,   B=u*NI/Len (B=flux density)

assuming H is constant within the core of the toroid (no air gap) (close enough), then BA will be the total flux (phi) flowing around the magnetic path. So the inductance (L) of one turn is:

L = phi/I  = BA/I  =  u * NA / Len

The inductance for N turns (Ln) is:

Ln = u * N^2 * A / Len

Now as the number of turns increases, the inductance goes up squared and the ballast current goes down (inverse squared). So the amp-turns goes down as N goes up and B goes down as the turns go up and with enough turns the core should not saturate.

If the core did saturate with a given number of turns, adding an air gap somewhere in the magnetic path would reduced both H and B (assuming the current didnt change) to keep the core from satuating. The air gap increases the reluctance of the magnetic path.

Since B has to be continous crossing the gap and the permability of the gap is uo and the permability of the core is u, then:

Hcore * u = Hgap *uo

therefore Hgap = u/uo  *  Hcore = ur * Hcore  (ur is relative permability)

When the H.dl integral is evaluated, one now gets:

Hcore * (Len - Dgap) + Hgap * Dgap = NI    where Dgap is the gap distance

For high permability core:

Hcore * Len + ur * Hcore * Dgap = NI

Hcore = NI/(LEN + ur*Dgap)

This is what I think I understand to date (please corrent any errors). What I dont understand is this:
Lets say that with a given N that results in a L and a current I (assuming no saturation), a gap is introduced to reduce the field intensity and corresponding flux density in the core. If the flux density is reduced (for the current I), the inductance goes down. This would result in the current going up and seems to undue the benefit of the air gap. I must be missing something.


Gerry R.


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.