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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