Preliminary Result, Current limiting inductor

To: tesla-at-pupman-dot-com
Subject: Preliminary Result, Current limiting inductor
From: "Coiler" <mycroft-at-access1-dot-net> (by way of Terry Fritz <twftesla-at-uswest-dot-net>)
Date: Thu, 19 Aug 1999 05:56:29 -0600
Approved: twftesla-at-uswest-dot-net
Delivered-To: fixup-tesla-at-pupman-dot-com-at-fixme
As some of you remember, I've got this thing for "building my own" even
when I can buy the parts. I wanted a way of limiting my pig, and while I
have
been using my lincon welder, it just wasn't quite what I wanted. Mostly
because
I had 3 choices, 1.2KVA, 2.8KVA, and "Oh my god that's a lot of power"
(6KVA).

So.. to make a long story short I decided to "wind my own". Here are the
preliminary
results of that effort:

Materials:
45lbs of 3/8" rebar cut in 20" lengths
4" PVC, 20" long
2 4" PVC endcaps
2 1/8"NPT to 1/4" compression fittings.
25" of 1/4" tubing
600' (!) of #10AWG stranded wire, with PVC insulation
Total cost: around $85.00
weight assembled: 75lbs.

Construction:
I took the 20" length of PVC and started winding, leaving 2" unwound at each
end.
This gave me approximately 100turns per pass. After 200 turns, I filled with
the
rebar and tried the current, removing turns till I hit my target of around
30A.
This was at approximatly 130 turns, Addtional taps were then placed at
intervals
of about 30-40 turns, for a total of 8 tap points (total of approximatly 400
turns).
 Measured currents, at 240V for the various taps were 32, 24, 17, 14, 12,
10, 9,
and 6A.
Measurements were done with a clamp on Ampmeter, using a 240V 6KVA Variac to
control
the power input.


The gotcha:
Rebar is not exactly thin silicon steel laminate, so I was losing some power
as eddy currents in the iron. This caused some substantial heating while I
was
doing my testing. The thermal mass of 45lbs of iron loses heat *Very*
slowly.
As in 10 hours after my testing, the rebar was still hot to the touch (the
PVC
 of the pipe, and the insulation on the wire most likely did not help)
After some thinking, I came up with a solution for removing the heat: I
installed a thermal siphon. Thus the compression fittings and pipe.
basically I
capped the pipe at one end, and installed one compression fitting just above
the base.
I installed the other fitting just above the windings on the top
(about 1.5" below the top of the rebar) and then put the pipe between the
two.
Filled the whole mess with water (will replace with 50% antifreeze
tomorrow!) and
made certain the copper pipe was full of water as well. Sure enough, as the
iron
heated, the hot water rose, entered into the copper, and cooled, sinking to
the
bottom. I figure if I really need it later, I can switch to a holding tank
to increase
the volume of liquid and radiating surface, or even use the pump and
radiator
approach with the same fittings, but this seems to work well.

The unit is compact, compared to my welder. It looks kinda cool (spiderish,
with
all the taps in temporary "fly in the air" mode), and gives me finer control
than
my welder did. For those that wonder, with those taps, I now have:
1.4KVA, 2.2KVA, 2.4KVA, 2.9KVA, 3.4KVA, 4KVA, 5.8KVA, and 7.7KVA
Far more choices than before, and a somewhat more mobile solution. Now I can
use my welder for welding :) And I fully admit, it is nowhere near as
efficient as the welder.. but I can point at it and say "I did that!", which
has a definite geek factor missing from the welder. [Same for my caps]
I'll try to get some pictures up on my web page in the next week or so.

Of course, this will now require that I build a *real* control panel, rather
than
the mess I use now. *shucks*

Michael Baumann
Coiler, Homebrewer, Nerd...mycroft-at-access1-dot-net
http://www.access1-dot-net/mycroft