Re: A few questions about coil designing

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

Original poster: DRIEBEN-at-midsouth.rr-dot-com 

Hi Dave,

Thanks for your most informative reply :&) First of all, both of my
pole pigs are of the double bushing type. The reason that I was try-
ing to avoid having to "float" the case of the capacitor was that
it will be sitting on a wooden deck and here in the south, even dry
wood tends to become somewhat conductive when exposed to multi-kVs.
That means   I would have to place some sort of insulating plastic
between the cap's body case and the wooden deck. I certainly can do
that if I have to, but I was hoping to avaoid that extra compli-
caton in my design,if possible.

As far as turn/turn spacing, mine is actually between 3/8" and 1/2"
as it's vitually impossible to maintain perfectly even turn/turn
spacing with even the best made primary coils. Anyway, it appears
that turn/turn flashover won't be a problem for me :^) I hope so
anyway.

I can attest to the very neat, professional appearance of your
RSG Dave, as I've seen some pics of it that you sent me. Very
impressive work for relatively simple machining tools. I'm
afraid that I couldn't make mine look that good even with a full
complement of machine shop tooling :^P so I may have to "bite the
bullet" and pay someone that knows what they're doing to handle
that part of it for me ;^/

BTW, what sized toroid and secondary coil length were you using
to get 14 ft. arcs? That's about the spark size that I'm hoping
to achieve with my finished coil.

David Rieben


----- Original Message -----
From: Tesla list <tesla-at-pupman-dot-com>
Date: Friday, November 19, 2004 2:40 pm
Subject: Re: A few questions about  coil designing

 > 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
 >
 >
 > ***************************************************************************
 > David H. Leddon
 > Lawrence Livermore National Lab
 > 7000 East Ave. L-495
 > Livermore, CA 94550-9516
 > leddon1-at-llnl.gov
 > (925) 422-1532 (VOICE)
 > (925) 422-1796 (FAX)
 > ***************************************************************************
 >
 >
 >
 >