Re: [TCML] Taming the Beast: Inductive Ballast vs. Variac, PFC

["DC Cox" <resonance@xxxxxxxxxxxx>]

I have some inexpensive laminated cores which you can wind 200 turns
of #8 AWG magnet wire to make a very nice current limiter for PTs and
even pole xmfrs.

I usually clamp the core in such a manner I can use a "lead screw"
mechanism to slowly pull the core in and out of the winding.  In this
fashion a variac is not even necessary --- you start with the core all
in and slowly pull it out to build the current to the value you
require.

Most PTs of the size you have can safely be operated as high as 6 kVA.
 Most PTs are rated 1.5 kVA and I run them at 4.2 kVA all the time
with no problems.

I set the limited at 4.2 kVA to produce a 35 Amp total draw on 120
VAC.  I use two separate 120 breakers (outlets on separate breakers)
so I can pull up to 40 Amps on a 120 Volt input without blowing a
fuse.  I usually use separate power reactors on each of the separate
120 VAC inputs, then parallel them after the two reactors and feed the
120 VAC primary of the PT.

Contact me off-list if you want more info on the cores.

Dr. Resonance



On 5/25/08, Jeremy Scott <supertux1@xxxxxxxxx> wrote:
> Hello Coilers!
>
> I'm getting back into the 'hobby' after a few years of
> being out. A little rusty on my theory and math. I've
> accumulated some stuff since then and would like to
> build another coil using some parts from my old coils.
>
>
> At the heart of my system is a 3KVA GE Potential
> Transformer.
>
> Most of the things I'm confused about is how to build
> the proper controls to tame this thing without blowing
> breakers.
>
> INDUCTIVE BALLAST:
>
> In the past I've used a 500 foot 12 gauge wire spool a
> la Home Depot. The problem with this is that the heat
> it generates almost melts the spool it's on. I think
> this is because there is no cooling for all but the
> outer layer of wire.
>
> I'd like to build a more robust and controllable
> ballast.
>
> My idea is to wrap the 500ft of wire around a PVC pipe
> of a diameter that is the average (half) of the
> spool's diameter and then slide a chunk of metal in
> and out of the pipe to control current. That half
> diameter thing is just a guess. I have no idea what
> the reactance
> of the spool is or how to recreate it in a more
> heat-dissipation user-friendly adjustable version
> using common parts.
>
> I'm also scared of turning my coil on full blast
> instantly. In the past I've used an expensive variac
> to dial the voltage up and look for problems, unwanted
> arcs etc, but I think it would be better to control
> the current instead. Perhaps both.
>
> I guess I'm looking for something that will limit up
> to 20A, so I can get all that the outlet will give
> without tripping a breaker.
>
> POWER FACTOR CORRECTION CAPS:
>
> Making the most of what I can get from the wall, I've
> got 6 oil filled 'can' capacitors wired in parallel
> across the primary. Each is 100uF @ 370VAC. I'm not
> sure if this is correct, I don't know the reactance of
> the transformer's primary to apply a formula. I did 6
> by trial and error, monitoring the current pulled from
> the outlet for the lowest value. It's probably
> completely wrong but it doesn't blow the breaker. :)
>
> Thanks in advance! -Jeremy
>
>
>
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