Homebrew variac paralleling choke

To: tesla-at-pupman-dot-com
Subject: Homebrew variac paralleling choke
From: "Paul Eugene Kidwell" <tmb-at-ieee-dot-org> (by way of Terry Fritz <twftesla-at-uswest-dot-net>)
Date: Wed, 22 Mar 2000 21:51:12 -0700
Approved: twftesla-at-uswest-dot-net
Delivered-To: fixup-tesla-at-pupman-dot-com-at-fixme

Hi Everybody,

Some time ago I posted a question about paralleling a 520V, 30A, 3 phase 
variac.

The commercial solution was just a bit too pricey at $570.00 US (Ouch!
:)

There were a couple of suggestions on alternatives, the most interesting
suggested winding my own.

So.....

I went scrounging through my collection of *stuff* (that my wife calls 
junk :) and came up with a set of 3 custom wound transformers. Careful
inspection revieled that the cores had almost all of the E's facing 
the same way. Fifteen minutes work and the sacrifice of 4 lamanations
(2 on each side) yielded a rather beefy EI core just waiting to be 
made into a paralleling choke :)

Core dimensions: 5.25" x 5.75" x 1.41" overall, 2 windows -at- 4" x 1.5"
center leg of "E" 1.5" x 1.41" (original thickness was 1.5" before
removing the 4 laminations)

My intentions are to wind 3 seprate 10 gauge windings on the core with
as many turns as will fit through the windows. I have no idea how to
calculate how many turns I need. A current limiting inductor for a
30A limit would be 21mH. (I think) I'll wind one winding and measure
the inductance to see if I've overshot, then remove windings to get 
to 21mH, then wind the other two windings with the same number of 
turns. In the event that a winding using 1/3 the available window
doesn't give me 21mH, I'm planning on using 2 of the cores together
to give me 8" x 1.5" windows.

If this works, I'll wind up with a variac rated at 90 amps. My present
design plan calls for a maximum of 60 amps, so this will give me a 50%
margin (I like to "over-engineer" :) 

The best part is that I have a friend donating the 10 gauge wire to 
wrap this thing, so it's going to cost me exactly, *NOTHING*!!!!!!!

So..... Think it'll work????

Any thoughts on what I'm planning here would be greatly appreciated.

Paul
tmb