Re: Variable current controls & chokes

Tesla List wrote:
> Original Poster: "Michael J. Veach" <veachm-at-sunyit.edu>
> I am working on a new driver and was wondering about using large
> variacs as variable current chokes. I want to be able to adjust my
> current between a minimum value and a maxinum set value. I was thinking
> of a large variac in series with a choke made from another variac, and
> these in series with the transformer. Since I have never made a current
> limiter before (I usually use neons), I'd like to get the advice and
> imput of the list members, and maybe some help with the design for this
> current limiter. Any hints and tips, or instructions on how to modify
> and rewind the variacs, would be a big help. I know they need to have a
> slot cut into the core, but should I make it a certain size? I am
> planning on using a 14.4 kV pig in case you need to know.
> Thanks in advance,

Using a variac connected as a choke (one end and the wiper) works
great.  For everyone except me, apparently!  That's what we've been
discussing in another thread.  A lot of people make it work just
fine without cutting the core but mine's acting real funny.

Anyway, look at these links.


Terry is hosting these photos for a bit since I don't have a web
site yet.

Chokes.jpg is a snappy frame grab of a couple of chokes.  In the
foreground is a choke I hand-wound to be used with a variac for
current limiting feeding a 20KVA, 14.4 pig.  It is 175 turns of
0.200" magnet wire around 7 lbs of mild steel welding rods cut in
half to make 18".  Butcher paper is wound around the rods to pad the
wire and it is dipped in motor varnish and baked afterward.  This is
desigined to work with my 25 amp variac in the 0-60 amp range.  In
the rear of the photo is my miller welder-based choke.  This thing
regulates extremely smoothly but makes some noise from the sliding
core and it takes about 10 turns on the crank to go from one end to
the other.  This was a junky old Miller buzzbox I found in a pawn
shop.  The type with the heat crank on top.  Simply short the three
secondary (welding) leads together and use the primary as a choke. 
You can even leave the welder assembled and install your relaying
and metering in the welder case if you like.  The only disadvantage
with the welder is that at high current it is a low duty factor
device.  Nothing a tank of oil wouldn't fix :-)

If you go the variac route, I'd highly recommend your making a fixed
choke instead of sacrificing another variac to the cause.  It only
took me about an hour to make the thing and when I got through, I
had forearms like Popeye!  0.200 wire is stiff!  The welding rods
were kinda expensive but it was the only thing I could get to on a
moment's notice when I decided to build it.  A cheap alternative is
those wires used to hold insulation between joists in a house.  You
can buy 20 lbs of 'em for $20 at Home Depot.  They're nice and soft
so they make good choke core material.  The copper should cost you
under $30.  Less if you can find a spool end at a motor repair
shop.  If I was winding it again, I'd probably go to 200 turns.  Ran
out of wire at 175 and decided it was close enough but some more
turns would spread out the range of the variac a bit.


John De Armond
Neon John's Custom Neon
Cleveland, TN
"Bendin' Glass 'n Passin' Gas"