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Re: Toroid Size?
Tesla List wrote:
>
> Subscriber: rwstephens-at-headwaters-dot-com Mon Jan 6 22:25:48 1997
> Date: Mon, 6 Jan 1997 20:07:27 -0500
> From: "Robert W. Stephens" <rwstephens-at-headwaters-dot-com>
> To: tesla-at-pupman-dot-com
> Subject: Toroid Size?
>
> All, and Chuck Curran,
>
> Here's one to put in your collective pipes and smoke for a while.
> Chuck, you should pay special attention to this post. It, and any
> enlightening replies that it might generate from the experts on this
> list may be just in time to save you some grief. I hope you haven't
> built your topload yet.
>
> I recall another list member recently posted a question (sorry friend ,
> could't re-find your post) about sparks racing up and down his secondary and
> specifically asked as a possible solution to his problem, "Is it because my
> topload is too big for this coil?" In answer to his question I now
> think the answer could well be "yes!".
>
> Over the two years of my limited coiling experience I too have noted
> that a condition can be reached whereby a coil just goes out of
> control after a certain amount of topload is added. Reducing K does
> not seem to help at this point. I've seen this on my terrific performer, the
> single 15K-at-60 powered STC once I pile on too much volumetric aluminum
> upstairs. I've also seen it occur also on small coils on the bench during attempts
> to gain maximum streamer length. It was not a big deal or concern to
> me until now. I just encountered it on my MTC unit! : (
>
> Topload comes in two somewhat inseperable quantities. A larger
> toroid will have a higher isotropic capacitance value, period. This
> is strictly a function of surface area. The way you make this extra
> surface area is a variable somewhat under your control as the
> designer however. If you merely take the same cross sectional toroid
> size and make it into a larger circle you will raise C without
> increasing the breakaway voltage. If you also increase the cross sectional
> diameter of your toroid at the same time as making the overall
> diameter larger, you also increase the holdoff voltage it can
> withstand before corona and streamers can be formed.
>
> I would like to know, is the wall we hit by increasing topload and
> then running into racing secondary breakdown related to this extra
> holdoff voltage, as opposed to the electrical phase angle now
> being some value increasingly less than 90 degrees that is strictly a
> function of L and top C?
>
> Facts are, the more C we place on top, the LOWER the theoretical 1/4
> lambda voltage available from the correspondingly foreshortened
> resonator. I don't see this as a function that can have a sharp 'Good-Bad'
> threshold like a yesteryear drugstore tube tester. C and L are continuously
> adjustable in a balance. One extreme is all L with a needle point top terminal,
> and the other a huge topload C on a straight vertical section of wire
> (minimum L). As the first example is reduced to the second, the voltage will
> decrease in a linear relashionship from maximum to zero (ignoring voltage
> clamping due to real corona).
>
> In attempts to squeeze a bit more streamer length out of my MTC coil
> I have thought of increasing the toroid size. The very successful 12
> inch by 48 inch toroid got crushed in the recent move and although
> I managed to get it puffed back into shape with compressed air, it is
> no longer as pretty and presentable as before. Rather than exactly
> duplicating it, I thought this would be a good opportunity to increase
> its size, if that would boost performance.
>
> Before I took a chance and spent (possibly wasted) big bucks on a custom made
> piece of 14 inch diameter stainless air duct I took my considerably larger topload
> from my largest LTC system which is 15.5 inch cross section by 67 inch O.D.
> and placed it atop MTC the other evening for experimental testing. I was luckily
> just able to retune MTC's primary at the very end of the last turn available to
> accomodate the full 15 kHz lower Fo that this bigger topload made my 50 mH
> secondary now operate at. Operating happily before at 78 kHz it was
> about to try to do something for the first time at 75 kHz.
>
> Low power testing with a bleeder point on the toroid confirmed tune.
> At about 7-8 kVA, the very first time I applied power without a
> test/bleeder point on the toroid, I was pleased to note that my
> system could indeed _punch out_ from a 15.5 inch diameter smooth
> terminal. The ~11 foot arc that hit the earth was noticeably hotter and more
> brilliantly white than those I am accustomed to from this coil. After
> about only one full second, perhaps less (it all happened so fast) of appetite
> wetting fun, the secondary started to break down all over itself with interturn as
> well as full length coil surface arcs. There was much interturn arcing in the top
> 40% of the secondary. I immediately shut the system down each time
> this unwanted secondary arcing occured and would investigate the
> damage to the secondary. I tried reducing K by raising the secondary
> as much as 3/4 inch higher relative to the flat spiral primary than I
> used to run safely. The old coupling was recently and successfully
> increased to K=0.22.
>
> I must have lowered K to around 0.1(?) but my helper was in a hurry to
> leave and an actual K measurement was not therefore possible. At any rate,
> this reduction in K did not help at all. I also attempted running
> with the toroid at several different heights above the secondary top
> winding, and also introduced an extra 3 inch by 18 inch smooth
> commercial toroid immediately adjacent to the top winding, placed
> a foot or so below the big donut for extra field control. all to no helpful effect.
>
> I'm going to guess that the assumed significantly higher breakout
> voltage that the bigger topload created, plain and simply
> OVERVOLTAGED MY SECONDARY COIL.
>
> I sure wish I had my HV probe built now so I could investigate this finding
> meaningfully through actual voltage measurement, however, I expect
> to be able to do so within several months.
>
> I was interested to note the evident violence of the damage caused
> by the interturn coil sparks. In the past I have observed a brown
> spot on the white PVC insulation of the #18 GA. secondary wire
> wherever interturn arcs occurred. I had stabilized my design where
> interturn arcing NEVER occured. This time (dinking with perfection) there were
> several clean craters, with no browning of the surrounding PVC, where a hole had
> been literally blown out of the sidewall insulation on the wire, and clean tinned
> copper wire was exposed!
>
> I think this is pretty clear evidence of massive pulse currents
> created by the bigger topload C. I don't know what the self C of
> this secondary is, but it plus the large topload worked out to close
> to 100 pF! Greg Leigh's large system employed a toroid of very nearly
> identical dimentions for which he has posted a determined breakaway
> voltage of approximately 500 kV. That voltage stored in 100 pF
> represents an impressive 12.5 Joules. Discharged in a half sinewave period at
> 75 kHz (6.65 mSec, but undercalculating the real power by calculating as a square
> wave) nets 1.88 megawatts of peak pulse power. Can you imagine what a
> pinpoint of PVC insulation might do when assaulted by that much pulse
> energy?!
>
> So what does one do if they find they have built a beautiful topload
> toroid that causes such coil breakdown? The only solution I think
> is to wind a bigger secondary to stuff underneath!!!!!!
>
> Comments, ideas, theories, envelopes stuffed with large bills, all welcomed.
>
> rwstephens
Robert & All:
Sometimes being slow/patient might be a benefit. Your post is of great
interest to me, specifically in how it relates to what I observed with
the coil design by Resonance Research, Inc. or D.C. Cox. The last few
weeks I have been considering whether that big toroid I mentioned in
early December would ever be built. Seeing that 19" dia by 39" wound
length coil spit out sparks in the 12 foot range with a 30" diameter
toroid has at least made me think there is at least two ways to skin
that poor abused cat!
No Robert, I haven't even purchased the material yet for the toroid.
It was planned for the end of this month or early February. I was
planning to start up the new coil with my present 8" x 48" toroid and
get things roughed in and then switch to a bigger toroid--now I might
change those plans somewhat. My present coil, 8" diameter by 28" wound
length using #20 wire ran with the 8"x48" toroid. The big difference in
performance between my coil and the Cox coil was I had 5'-7' sparks that
were not continuous. The Milwaukee Museum coil had continuous sparks
that reached out and just sorta happily sat there two or three at a
time, always one up to 12 feet long the others much smaller. They were
inside on a stage, I was out in the driveway with the normal 9:00 PM
summer night breeze. That pretty little spun aluminum toroid seemed to
be plenty for that coil, so your comments Robert, are hitting a target
over here by me. I already down-sized my cap, which I mentioned
yesterday, to .05 mfd-at-30KVAC, for two reasons, resonant frequency
dictated that was the perfect size, plus cost factors!
Hope the damage done to your secondary wasn't too bad. A couple of
questions on your coil--you mentioned the PVC coated #18 wire, I don't
recall if you have a layer of varnish over it or anything elsed? Most of
the standard hook up wire has about a 600 volt rating guess it wasn't
too hard for that MTC of yours to pop chunks like it did--would of
probably damaged any combination of materials under the cicumstances you
described.
I will be ordering the 12" diameter flexible aluminum duct no matter
what. The unknown at this point will be what exact size to make the
whole thing. Since I don't have a clear path but alot of curiousity
right now, I think I will build a slightly bigger toroid and learn what
I can. I am anxious to check out the k of the new system, but that will
be some time away--8 week lead time on the cap. I should have the
primary coil system done by 1/18/97 and then I will finish my static
gap. I have several photo's and I will post them when the primary is
completed. Any constructive criticism would be appreciated from all.
Tune for minimum smoke is always the goal.
Let us know about that "bigger toroid to stuff underneath"--maybe 30" x
90"?? :-) Getting late here time to go to bed!
Chuck Curran