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,
> 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.
Sorry to hear about your flashover problems, Robert! From the sounds of
it, I'd agree with your assessment that you're overvoltaging the
secondary. The giveaway is the inability to rectify the problem by
reducing k. With the higher breakout voltage from the bigger top-load,
decreasing k doesn't markedly reduce the peak secondary voltage, just
increases the number of RF cycles to get there once the gap fires. It
could also be that the thicker, and lossier, PVC insulation is
contributing to lower voltage withstand capability versus a similarly
wound secondary wound with double-insulated magnet wire (like Chuck's).
Also, a smooth conformal overcoat should significantly help "smooth out"
the E-field, further reducing corona and inter-turn arcing.
> 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
Yeah... just think if one hit you! O =:^O
> 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!!!!!!
Obviously! You also need more power, more power... its
"gotta'-get-a-pole-pig" time!! :^)
> Comments, ideas, theories, envelopes stuffed with large bills, all welcomed.
I'd bet that Chuck's going to be fine with his design using magnet wire
and Build50 - this should give him excellent withstand capability.
During tuning and power ramp up testing, he may want to attach some foil
"breakout" bumps along the side of the toroid just to make sure he
doesn't overstress the coil until he's got everything dialed in. If he
starts overstressing the secondary, he can simple leave the bumps on to
Safe coilin' to you, Robert!
-- Bert --