Re: design questions
If you can run some tests with some old 8 or 10 AWG house wiring as your
primary, then you can get your tuning before fabricating your final
primary. These are usually conducted with a variac around 30-40% and will
give you an accurate indication of resonance point before final primary
fabrication, therefore, allowing you to make a primary with only 1 or 2
extra outer turns \
If winding with the magnet wire be sure to sand the tube thoroughly and
then 2-3 coats of glyptal before winding (inside and out) on PVC tube.
Keep the lower edge of toroid approx 1/2 to 3/4 inch above upper most
winding. Space wind the last 5 upper turns of the secondary approx 3/16
inch -- slowly reduces capacitance at top end of hv winding.
It sounds like Jack Wu is getting coil fever --- he keeps asking me the
same questions. Nice of you to help him out.
> From: Tesla List <tesla-at-pupman-dot-com>
> To: tesla-at-pupman-dot-com
> Subject: Re: design questions
> Date: Monday, January 11, 1999 3:55 AM
> Original Poster: Bert Hickman <bert.hickman-at-aquila-dot-com>
> 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
> > coil. Only a few variables are fixed, and many are still undetermined.
> > 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?
> > 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
> > 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
> > 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 --