# RE: ballast formula

```Original poster: "David Dean by way of Terry Fritz <twftesla-at-uswest-dot-net>" <deano-at-corridor-dot-net>

Hi James,

I have an old text book with some of that stuff in it.  It says:

L = N^2*mu*A/10^8*l

(L equals N squared mu A over ten to the eighth l)

where L is in henries,
and   N = number of turns
mu = permeability of core (in English units)
A = area of core in square inches
l = mean length of core in inches

The mean length would be {twice the length of the center leg (with the I
part on there) plus twice half the width of the core} plus [the perimeter of
one window} / 2.

Trying to draw pictures with words is not my strong point. If you want, I
can scan the pages and email them to you.

later
deano

> -----Original Message-----
> From: Tesla list [mailto:tesla-at-pupman-dot-com]
> Sent: Wednesday, February 21, 2001 5:48 PM
> To: tesla-at-pupman-dot-com
> Subject: ballast formula
>
>
> Original poster: "James T by way of Terry Fritz
> <twftesla-at-uswest-dot-net>" <jamest2000-at-att-dot-net>
>
> Hi All,
>  I was wondering if anyone had some math that may help.
> I would like to use a transformer core for a ballast. Given the
> dimensions, magnetic properties, frequency, current ect. , is there a
> way to calculate turns = inductance? Lets assume an ungapped core, say
> an EI type.
>  I have the formulas to calculate inductance = reactance.  From that I
> can calculate current. How do you calculate inductance without trial and
> error?
>  If you have the answer, please speak slowly, as my math skill are
> lacking. :-)
> James T
>
>
>
>

```