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Re: Best cap size for a sync gap



Original poster: Kurt Schraner <k.schraner-at-datacomm.ch> 

Hi Gary, Terry, all,

the subject of best cap size for a sync gap and SLTR is stimulating, and
I like to add my SFr.0.01 to the interesting thread. Seeking a well
sized LTR value for my little coil UBTT (Uni-Bern-Tesla-Twin) and a
reply to Dan McCauley's overvolting question, I performed 29 SPICE
simulations, mentioned in my TCML posting of last September:

http://www.pupman-dot-com/listarchives/2003/September/msg00311.html

The sim's were for just for the _linear case_, where the NST parameters
(measured before) would not dynamically change. Meanwhile the coil has
been realized, and the calculated results, though in nice agreement with
reality otherwise, have taught me a different lesson: The NST is indeed
a more complex animal, than many of us might have thougt before ;-). I
experienced, and fully agree with those, assigning magnetic shunt/core
saturation in NST's a significant role in TC use.

The experimental results, compared to the simulation, were as follows:
In simulation a 230V/50Hz line power draw of ~880W / 1700VA (PF=0.52)at
an ~optimal setting of the rotary firing time of 4.2ms was obtained. The
experiment was showing 1300W / 1610VA (PF=0.807) at a rotary setting of
3.6ms. No humming of the NST was heard, because of the 1.5m-spark noise.
The operation is stable and reproducible.

The TC is powered by a 15/60 NST of Italian company FART. The SRSG motor
is fed via a phase-shifter of the John Freau type. The phase-shifter can
also be used to smoothly adjust the power input to the coil. However I
was scared adjusting the phase angle to the predicted optimal 4.2ms,
because of beginning safety-gap firing (2 brass balls of 0.5cm, adjusted
~10mm). A scope-shot of the charging cycle at lower power may be had at:

http://home.tiscalinet.ch/m.schraner/UpriC_oszi1304.jpg

The charging circuit resonance, assuming linear behavior of the 15/60
NST (L.leak ~795H) and a 33nF primary cap would be about 31Hz. If
saturation brings the leak inductance down to 307H (38% of linear), the
cap would again be resonant! My 1300W operation of the NST might imply a
     leak inductance of about 551H (69% of linear), leading to 37Hz
resonance - yet away from 50Hz, but the primary LTR cap value only being
~1.8*Cresonant. Sooo... this alone shows SLTR experimenting implies some
risk, and should be performed cautiously. There certainly are other
effects, interwoven with the magnetically nonlinear behavior of the NST.
More thoughts, or even better: experimental info, would be very welcomed
to be seen appearing on this great list ;-).

Data of the little UBTT may be supplied in a later posting, if of
interest. Two spark snapshots are here:

http://home.tiscalinet.ch/m.schraner/1.06m_1332kl.jpg
http://home.tiscalinet.ch/m.schraner/TeslaAnVorlesung3kl.jpg

The first pic shows operating the TC in single mode, in my lab; the
second in twin-mode at the Physics Institute of the Bern University,
during a lecture (sparklenghts: 1.06m and 1.3m respectively).

Best regards,
                  Kurt