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



Hi Jeremy,

There are welders, variacs, and homemade pipe ballasts used for inductive current limiting in general. Some have also used MOT's or other iron core transformers. I use a variac. I would like to change over to an air gap ballast some day based on what Richie Burnett has done. It's just something to consider if you happen upon a nice core someday. May even spark other ideas.

Mechanically, it's a ballast with with the turns wrapped around the center core. The top of the core is adjustable and this is where Richie sets the air gap for current control. He used high density polystyrene to fill the gap preventing the magnetic field from collapsing the air gap. This is probably the best ballast idea I've seen. Anyway, you can see pics and read about it here. In my opinion, this is really an ideal ballast as it can be set for any TC transformer current (small to large), it's compact, and it's efficient.
http://www.richieburnett.co.uk/parts2.html#ballast

Richie was going to put together a program way back when for inductive ballast design, but I don't think that actually transpired. Regardless, Richie does have a procedure in the TCML archives which helps shed light on this subject.
http://www.pupman.com/listarchives/2001/June/msg00744.html

Richie Burnett's web pages in my opinion are the most informative on the internet.
http://www.richieburnett.co.uk/tesla.shtml

Take care,
Bart


Jeremy Scott 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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