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

[Jeremy Scott <supertux1@xxxxxxxxx>]

--- DC Cox <resonance@xxxxxxxxxxxx> wrote:

> 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.

That sounds interesting, and I may just take you up on
it. I have another option as well that I'd like to
explore too. 

I've got 225 amp lincoln arc welder (240VAC@50A input)
which I was going to use but I have some safety
concerns. 1) What kind of current limiting will it
provide at 120V as opposed to 240V and 2) Does this
work if the welding leads are shorted vs. not? (The
switching mechanism temporarily breaks the secondary
welding circuit and I am worried that this will
translated to 'full blast' if I am running the coil 
in between clicks.) 

I may settle for it being adjustable only when the
main power is off, but I really like the idea of
ramping up slowly by pulling a core. 
 
> 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.

Okay now that's interesting. I originally wanted a 
small 5kV pole pig. But I settled for a PT as an
intermediate stepping stone from the NST. 

I take it these things are much more robust than the
typical NST? (Fried my first one, like everyone else
:) ) Are RC protection circuits on the secondary a
good thing to have for it? 

> 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 will probably not run that high a kVA on my coils,
as I haven't sized the other components for it. (The
spark gap I'm planning on using is a 120bps 1/4"
electrode SRSG 'propeller' style with tungsten rods
and the secondary is a 25"x6.5" form. Caps are going
to be MMC or a series of one, two or three Maxwell
.03uf units.)

I have to play with javatc a bit more to figure out
the specs exactly, and to start that I need to know
what I can put into it. 

> 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.

Do you have any issues balancing them? For example,
can one core be left in while the other is out or do
you have to move them in parallel?

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

I may do that, thanks.

> 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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