Re: PT Ballast Question

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "Mike" <induction-at-comcast-dot-net> 

Hello all,
               I'll tell you what we do for current limiting, works great. 
A little background to let you know the harsh environment this is used in. 
Transformer bank is three plate transformers, primaries all in parallel, 
from induction heaters, to get the current I wanted for continuos usage, I 
had to go to three. Rectified (full wave bridge) output is 2600 volts at 10 
amps, each secondary has it's own bridge, all plus dump together, all minus 
dump together . Filtering is a VERY heavy 25 Hy choke on the high voltage 
side, caps, etc. The load is a 2 feet wide, 6 feet tall glow discharge tube 
at vacuum. When going from the glow mode to the arc mode, it will try to 
pull infinite current without limiting resistors. Problem is what 
resistance you use at the higher currents (back in the glow mode) changes 
the vertical range of the cathode glow. Using 4 one hundred ohm 1 kW 
resistors in series, the cathode glow goes within a few inches of the anode 
and you end up with no positive column. Higher values ( ~ 25 -75 K ) give 
normal ~75 percent positive column (of the 6 feet electrode spacing) but 
the current and optical output drops way down. But without limiting, it 
will after ~60 to 100 ma drift into the arc mode and then into as many amps 
as it can pull.
So it was decided to take out the resistors and control the primary 
current. Primary line voltage is 208 volts using 2 of the three phase legs. 
A 50 amp variac was reconfigured so that rather than the conventional 
variac wiring for input and output. it was set up so that for minimum 
current in the load, the wiper and one end terminal had the most inductance 
and as you turned the variac toward less windings on the core, reducing 
inductance, greater current resulted in the load.
Very much like your adding iron slugs to the wire cores. But this is one 
shaft and using a plastic 55 gallon oil drum cut down to a few feet tall, 
the variac is placed in an oil bath for cooling, the shaft was extended and 
an old car steering wheel was fixed to the shaft so easy turning is there. 
We vary the load from the 10 amps on the high voltage linear down to 400 
microamps (below that, the tube goes out of glow mode). With the windings 
wound on the variac core such as they are, there can be a lot of inductance 
and nice control. If the current is really high on the primary, you want to 
keep at least a few turns of inductance in there to stop run away.
Or, if you want full voltage, turn it all the way to no inductance, as long 
as the experiment is not pulling too much current.
You think arcs at sea level get current hungry, it's even wilder under 
vacuum where it takes so little to break down a given distance. At 100 
militorr (sea level is 760 Torr or 760 mm of mercury) I can get the 6 feet 
distance to ignite to glow at ~1100 volts DC and as coilers, you know how 
much at sea level it would take to break down 6 feet. In the tube, the arc 
can be the whole 22.5 inches ID wide for high current or large cap dumps. 
In the air, the arcs are thinner; Climbing towers I've seen lots of old 
lightning strike burns only dime sized.
So, even with such a beating, the variac as a variable inductor on one leg 
in series works great and smooth. Less voltage drop across so much reels of 
wire, to. We wanted inductive control, not series resistor, which the lots 
of wire becomes.
Anyway, that's how we control the current. You dial it up and there it 
stays, no more. We of course have overloads on the high voltage side that 
over 10 amps, crash the control system and the large primary contactor.
Regards,
             Mike



----- Original Message ----- From: "Tesla list" <tesla-at-pupman-dot-com>
To: <tesla-at-pupman-dot-com>
Sent: Wednesday, October 13, 2004 7:15 PM
Subject: Re: PT Ballast Question


>Original poster: "Dr. Resonance" <resonance-at-jvlnet-dot-com>
>
>I presume you are using some steel rods in the center?  This will increase
>inductanc and limit current even more.
>
>You could unwind 250 ft of wire and rewind this onto another spool.  Then
>use both spools in series with steel rods inside each spool.
>
>This should increase the inductance even more and perhaps hit your target.
>If necessary, this procedure could be repeated using 3-4 spools in series.
>You could also rewind the wire onto a carpet tube (free at carpet store) and
>make a steel rod core (1/4 inch rods) that could be moved in and out to
>adjust the inductance as required.
>
>Dr. Resonance
>
> >
> > I recently picked up a nice GE 1.5kva 100:1 pt and
> > have a question or two about limiting the current it
> > pulls from the line.
> >
> > I set up a jacobs ladder using a 500ft spool of 12ga.
> > wire for ballast (as suggested by Terry Blake) and
> > found that it did indeed limit the current to around
> > 26 amps.
> >
> > I'd like to reduce the current further (to around 12
> > amps or so) before I incorporate this transformer into
> > my coil.  Would it be feasible to add another 500ft
> > spool of wire in series with the first?  Any guesses
> > how much additional current limiting this might give
> > me?
> >
> > One other related question... If I were to get a MOT
> > or two to use as ballast, would this be more effective
> > than the wire spool method?  I'm keeping my eyes open
> > for an old arc welder to use as ballast, but in the
> > meantime...
> >
> > All replies appreciated!
> >
> > Thanks,
> > Greg