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Re: SISG: OLTC-ish coil design: Help?
Original poster: Vardan <vardan01@xxxxxxxxxxxxxxxxxxxxxxx>
Hi Aaron,
At 09:45 PM 5/29/2006, you wrote:
Hi, all!
I'm crunching numbers on a SISG-based coil I'm getting
ready to build, and wondering how to make best use of
the parts on hand. Specifically: I've got a 2400V
200VA (yes, little!) potential transformer and a 14"
sphere for a topload. Additionally, I've set the
somewhat arbitrary goal of putting 200W into the coil.
If I use three SISG sections, my firing voltage will
be 2700V, so at 120BPS I'll need about 450nF in the
primary cap.
120 BPS x 1/2 x 450e-9 x 2700^2 = 197 watts ;-))
That's a big cap, and drags the primary
frequency down under 100kHz. This has a bunch of
seemingly-negative side effects; it suggests that I
either drastically reduce the number of turns in the
primary down to, e.g., *one* (like in a OLTC), thereby
drastically *increasing* my peak primary current (bad
for the little IGBTs, methinks)
Yeah! The primary peak current is given by:
IPpeak = Vfire * SQRT(Cp/Lp)
So if Lp is real small IPpeak might be real high!
, --OR-- I could put
2000+ turns on the secondary (gulp), thereby making it
pretty lossy but lowering its frequency to the point
where my primary can actually have a few turns in it
(and keeping the primary current from running away).
My secondary is 6 in dia. x 20 inches long and has about 1363 turns
of #28 wire on it. With the top terminal it runs at 105KHz and I can
get 7 turns on the primary.
Any other ways out of this? As the coil will be
low-powered (200W), I want it to be physically small,
and hence I don't want to go berserk on adding
topload. The 14" sphere ought to look nice atop a
6"x24" secondary.
My coil "loves" to fire much higher than 120 BPS!!! So you could use
a smaller cap and much more BPS to get to the 200W power
level. However... That increases IGBT heating (not a real big
deal). But it seems to me that high BPS just does not help much with
streamer length.... I have not gone back and worked on trying to get
120 BPS yet... Been playing with the new version ScanTesla...
Alternatively, I could go for a smaller cap and shoot
for a higher break rate on the SISGs, but Terry
indicated that he didn't get much bang for the buck
(or would that be *buck* for the *bang* in this case?)
when he pushed things above 120BPS. Hmmm...
:-))
If I fairly arbitrarily choose 2000 turns of #30 wire
on a 6"x24" secondary with the 14" sphere topload, the
frequency works out to about 90kHz. To match that in
the primary circuit, I'd need little more than four
turns (!) which yields little more than 6uH. That, in
turn, suggests a peak primary current of almost 700A!
Be a bit careful of lots of turns of thin wire. My secondary Rac is
burning almost as much heat as the IGBTs!!! MandK-3.1 seems to give
the best numbers for Rac compared to my measured values:
http://hot-streamer.com/TeslaCoils/Programs/Programs.htm
It says 420 ohms and I measure 460 at 105kHz. About 50 watts to the
IGBTs and about 50 watts burning in the secondary!! Fortunately, the
secondary is big with lots of high emissivity surface area... Those
losses might be less significant with "better" streamer modeling...
The new version of ScanTesla will also give IsecRMS now... The AC
resistance of the secondary is a "new thing" that we always knew
would "get us" one day ;-)) That day is here!! Rsec is now a major
(50% of the loss) coil power loss with the SISG!!! I was going to
wind a new secondary with aluminum wire... I may have to rethink that...
700 amps is a lot and it take the very best "little" (TO-247 style)
IGBTs to handle that without getting real hot. Isotops and bricks
don't care though. The big worry is heat on the IGBTs.... Hard to
say how hot a random IGBT will get... I get the feeling that a
current spike alone will not kill them, but "heat" will...
The data out there is of no use for running IGBTs at say 20X there
ratings :o)) Thus, I have no answer for Brian Foley's IGBT question either...
Maybe I should just concede defeat on the 200W and
shoot for 150W to be safe ;-) I don't want the
performance to "suck", but I also don't want to be
red-zoning the little IGBTs. I suppose if I can
duplicate Terry's success and get >24" streamers out
of the thing with well under 200W, I ought to be
pretty darn happy! ;-)
The bleeding edge of technology is always tricky... The bleeding
edge of Tesla coil technology is "super" tricky :o)))
There are not a lot of good answers yet....
BTW - The latest document (V1.6) is here:
http://drsstc.com/~sisg/
I just loaded the "latest" ScanTesla stuff up there too just now:
http://drsstc.com/~sisg/files/scantesla/
Cheers,
Terry
Regards,
Aaron, N7OE