Re: design questions

Tesla List wrote:
> Original Poster: "Mad Coiler" <tesla_coiler-at-hotmail-dot-com>
> List members,
>   I am [still] in the design phase of my first distribution-xfmr powered
> coil. Only a few variables are fixed, and many are still undetermined. I
> have many questions to ask, and I'll limit myself to just a few in this
> post. Feel free to respond in private if you wish
> [tesla_coiler-at-hotmail-dot-com]
> Here is what I have to work with:
> - 19,920V 10kVA pig
> - 10.5" x 44" coil form
> - .04uF 40kV GE pulse cap

With a 19.9 kVRMS Pig running at full power, you may overvolt your 40 kV
cap. The reason is that the voltage swing when the gap fires can be as
much as the initial peak voltage (1.41*19.9kV) multiplied by 1.8
(assumes that the voltage reversal is 80% of the initial cap voltage),
or over 50,000 volts. And, the cap will see even higher voltage stresses
if you use resonant charging and a rotary spark gap. Unfortunately, many
coilers simply don't understand the voltage stresses that these caps
actually see under TC use. More experienced coilers understand - usually
by hard-won (and sometimes expensive) experience. Most pulse caps have
faceplate ratings in volts DC. A tank cap driven off a 15 kV (RMS)
supply should be rated at no less than 45 kVDC, and more conservatively
at 60 kVDC to have at least some margin. Under full steam, your 19.9 kV
system really should have a 60 - 80 kV pulse cap. In ther interim,
you'll need to limit applied primary voltage (via the variac) and use a
static gap across your RSG to insure that you don't overvolt and destroy
your tank cap.

> Here is what is proposed so far:
> - #16 AWG magnet wire close wound 42" ???
> - 5/16" copper tubing primary(flat), 1/4"-3/8" spacing, 12-20 turns
> - hopefully a saturable reactor ballast
> - RSG w/ 1/8" tungsten contacts

I'd recommend using at least 3/8" copper tubing on the primary with 3/4"
ctr-ctr spacing to reduce corona and keep primary losses down. Go with a
welder for ballast - it's relatively cheap, adjustable, and robust. You
don't ever want your ballast to saturate, since you'll pop your mains
breaker in an instant when this occurs - a short-circuited pig will
easily attempt to draw 1000-2000 amps off the low voltage side when your
ballast saturates. Also as you increase output power towards 10 kVA, the
1/8" electrodes will overheat - you'll probably want to use at least

> Here are a few questions:
> If my toroid is, for example, 6" aluminum ducting wrapped around the
> outside of a plywood circle, about 48" diameter, what effect would
> covering the plywood with al foil have to the capacitance? WinTesla
> gives me 50pF for my proposed toroid capacitance, and I assume the
> actual will be higher with foil covered center. But how much higher? Any
> ball-park figures?

The toroid capacitance calculator already assumes that the center
portion of the toroid is conducting. However, the predicted value is the
"free-space" value - when you mount yours on top of your secondary, it
will be less than this, since the secondary "shields" part of the
overall usable surface area versus ground (the other "plate" of the
topload capacitor). With a 10" coil and a 50" diameter toroid, you'll
most likely see about no more than 80-85% of the predicted free-space
capacitance, or about 40-45 pF. You will need to go to largr Radius of
Curvature (ROC) on the toroid so that you can hold off the breakout
voltage - use at least 8 - 10" diameter ducting.

> Also, the matched cap for my pig is .066uF. I am hoping that my .04uF
> will not be to great of a potential loss? I am pretty tight on my budget
> right know and buying a cap isnt what I want to do.

Using a smaller tank cap is not a problem, and does not add to system
losses  - however, you may need to INCREASE the amount of ballasting
inductance on the pig primary side to reduce the capacitor recharing
current (an adjustable welder ballast makes this easy to do). If your
pig delivers too much recharging current to your tank cap, once the main
gap extinguishes the primary charging voltage may rise too quickly,
causing your main gap to prematurely reignite before the electrodes have
seperated sufficiently. Under thias condition, you'll begin getting a
"ring of fire" as the RSG valiantly tried to break the primary arc
that's being driven from the pig - and you'll burn up your gap

> Finally, assuming that a will run the coil up to 10kVA(dont know if this
> will actually happen) I am not sure if I may need more pri-sec coil
> clearence. Is 2" sufficient between the coils or do I need more space?

2-3" All around should be sufficient. BTW, assuming the paramters you've
provided above, and using 2" cleaance and 3/4" ctr-ctr turn spacing,
you'd come into tune at around turn 9. You could increase the inductance
of the secondary a bit by going to say #18 AWG - this would get you to
around turn 11. You probably don't need more than a 14 or 15 turn
primary - remember that the open outside turns will behave like a
(step-up) autotransformer. Too many unused turns, and you may actually
get flashovers from the outermost primary turn to the grounded stikerail

> I would really appreciate comments!
> Tristan Stewart, KC2EBM
> ______________________________________________________
> Get Your Private, Free Email at http://www.hotmail-dot-com

Some of the above information is not readily apparent when cranking
through the numbers using a design tool. Hope you find it useful in
preventing what could be an expensive problem.  Good luck, and safe
coilin' to you, Tristan! 

-- Bert --