Re: A few questions about coil designing

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

Original poster: "Dave Leddon" <leddon1-at-llnl.gov> 

At 08:02 PM 11/18/2004, you wrote:
>Original poster: "David Rieben" <drieben-at-midsouth.rr-dot-com>
>Hi All,
>
>I am in the process of constructing a fairly big coil with a 12 3/4" x
>44" long secondary wound with 897 turns of #17 heavy build Essex
>magnet wire. That makes for just over 3000 feet of wire.
>The primary is 12 spiralled turns of 5/8" OD soft copper
>tubing with the outer most turn at nearly 40" diameter. There is an
>approximate 1/2" spacing between each primary turn.
>
>For the primary capacitor I will be using either one or two paralleled
>.1 uFd, 50 kV Hipotronics commercial pulse capacitor(s). These are
>Tesla coil specific custom capacitors that Jeff Parisse was using for his
>Model 10 coil. I'm thinking that .1 uFd will be sufficient primary
>capacitance for my design.?

I think that 100 nf would be ideal.  I think that the rules that dictate 
larger than resonant capacitor values do not apply to pig-powered 
coils.  The resonant value for a 10 kva pig is about 120 nf which is what I 
started with.  The coil worked fine with this value but the spark gap 
current seem excessive.  I use the 1/2-inch tungsten electrode tips sold by 
Dr. Resonance ($13.50 each) and the wear on these brutes was 
significant.  During operation I could actually see incandescent bits of 
tungsten flying from the electrode tips.  I then went to the other extreme, 
running with 30 nf and found the performance of the coil to be generally 
poor.  I just couldn't push any power through the system unless I cranked 
the arsg up to insane speeds.  I finally settled on 92 nf as a good 
compromise.  With this I can push 60 amps out of a 10 kva pig at 300 bps 
generating 14-foot arcs which is the same length that I got with 120 nf.  I 
know that Dr. Resonance advocates larger capacitance as the best method to 
increase pig coil performance but it's just not been my experience.  Try it 
both ways and report your results to the list.

>The power supply will be either a 10 or 15 kVA, 14,400 volt pole trans-
>former (have one of each), controlled and ballasted with (2) paralleled 1256D
>Powerstats and a Lincoln 225 amp arc welder core set for the maximum welding
>amperage. The arc welder's core guts have removed from the factory steel
>box and placed in a tank of transformer oil to increase the duty cycle.



I also use a continuously variable 225-amp welder core for my ballast which 
I crank up to the maximum current setting once I'm sure the coil is 
correctly turned.  I've never found over-heating to be a problem with this 
thing but then I'm only running 280 volts at 60 amps as my maximum limit 
and I never run at that level for more than 60 seconds (I live in a 
residential area).

>The rotary gap will most likely be ASYNCH G-10 and tungsten
>with a variable speed DC or universal drive motor. I may have to
>break down and pay someone to do this part for me as i am
>no machinist and I do want it to be right :^))))
>I have designed and built big coils before but I want to make sure
>that I get this one right the FIRST time ;^)

I make my own hubs and rotors using ordinary tools (you do need a good 
drill press and a few precision reamers) and a homemade lathe setup.  Total 
cost for the lathe was about $100.  I can provide details if you're interested.

>My questions are: Will there be a problem of turn/turn flashover
>on the outer turns of the primary coil since there is often less than
>1/2" spacing between the adjacent turns? If it's tapped at say the 7th
>turn then there's 5 additional unused turns that can develop an auto-
>transformer effect. I may just remove most of the unused turns if
>this proves to be a problem once tuned in.

I like your coil design (it's very similar to my largest coil). I 
particularly like your large primary.  I went through three primary designs 
on my 12-inch coil, each larger than the previous and finally ended up at 
34" in diameter with about 12 turns.  I'm using 1/2-inch tubing spaced at 
3/8 inches which is tighter than your spacing and I've experienced no 
problem with turn-to-turn flash-overs in the unused portion.  I've tried 
several capacitor sizes with the largest being 120 nf which resulted in 
about 6 unused turns and I was also concerned about the auto transformer 
effect.  I'm not sure, but I believe that the turn-to-turn voltage on the 
unused portion is exactly the same as the turn-to-turn voltage of the 
active portion so, given that the spacing is constant and adequate to begin 
with, you should never have a problem.  My biggest problem was preventing 
arcing from adjacent turns to the tapping clamp.  I finally came up with a 
clamp design having rounded edges and a 1/4-inch clearance between the 
clamp and the adjacent turns which has proven to be very reliable.

>Also, my Hipotronics pulse caps are the single bushing, steel
>can type with the steel case grounded to the terminal opposite
>of the bushing. I'm thinking that I'll need to place the capacitor
>in parallel with the transformer instead of the spark gap in paral-
>lel with the same, otherwise I'll end up with the entire exterior
>case of the capactior floating at 14,400 volts ;^O. I've heard that
>this wiring variation of the primary circuit places more strain on
>the transformer but since I'm using a pole transformer, I don't feel
>that this will be an issue in this case.


I think the issue here is that in a parallel configuration your safety gap, 
which is also parallel to the transformer,  will short circuit the cap when 
it fires.  If your pig has two output terminals you can wire your coil such 
that whole primary circuit floats with respect to ground so making the 
capacitor case as part of the circuit presents no hazard.  And if you have 
the single output variety you can still wire the circuit with the case of 
the cap connected to the case of the transformer which is, in turn, 
grounded.  The only problem with wiring it this way is that the neither end 
of the primary winding is at ground potential.  And why are you concerned 
anyway?  It's not like you're going to be stroking the capacitor with your 
hand while the coil is operating, at least I hope not.


>I plan on building a 12" minor diameter toroid from aluminum flex-
>duct tubing from McMaster-Carr, with a major diameter of 48" to
>54". I'll probably also use a smaller toroid (4" minor diameter x
>16 to 20" major diameter) for a corona shield of the topmost
>turns of the secondary. With a toroid this large, will .1 uFd be
>sufficiently large for the primary capacitor? It seems that my
>power supply combo may not have suffiecient power to drive
>.2 uFd of primary capacitance. Any comments welcome ;^)
>
>David Rieben


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David H. Leddon
Lawrence Livermore National Lab
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Livermore, CA 94550-9516
leddon1-at-llnl.gov
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(925) 422-1796 (FAX)
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