Re: Over-voltage at Synchronous Gap ? ? ?

["Tesla list" <tesla-at-pupman-dot-com>]

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

Hi Dan,

I did a series of 29 microsim8-pspice simulations on that. Malcolm is
very right, the question is: HOW MUCH LTR? My example: 15/60 NST with
LTR caps of 33nF, 30nF and 25nF (at line f=50Hz), and a classic coil I'm
currently building (fairly detailed simulation). The varied variable was
the time delay of the SRSG, relative to the zero crossing voltage of the
50Hz line, recording the peak voltage on the cap, the peak secondary
voltage and the line-power draw (a PFC capacitor of 80uF kept the power
factor in a acceptable range, without disturbing the sim otherwise). The
main results are as follows:

LTRcap=33nF, Vcap.max=24.2kV(-at-5.5ms delay), Vsec.max=411kV(-at-4.5ms delay)
LTRcap=30nF, Vcap.max=28.4kV(-at-5.5ms delay), Vsec.max=453kV(-at-4.5ms delay)
LTRcap=25nF, Vcap.max=39.9kV(-at-5.5ms delay), Vsec.max=558kV(-at-4.5ms delay)

A summary PDF of the results can be had under
http://home.tiscalinet.ch/m.schraner/LTR_Sim1.pdf   (484kB)
...and a more appropriate result in excel-form at
http://home.tiscalinet.ch/m.schraner/LTR_Simu.xls   (171kB)

Considering maximum cap-voltage (with inductive kick), this voltage is
NOT identical with the SRSG time delay, obtained for max. secondary
voltage ( and also not identical with the firing voltage of the main
SRSG gap).
It appears, that peak primary cap voltage is quite sensitive, on how
much the LTR cap value deviates from the "ideal" value of 33.2nF. The
recommended value for 15/60 NST's, of 30nF, in the table under:

http://hot-streamer-dot-com/temp/MMCcapSales.gif

is just ~3nF below the "ideal" value, but brings more than 4kV more
stress on the cap. Thinking, 2 more 150nF caps on each of the 2 MMC
strings would make it "safer", in fact make it worse, because the
capacitance is then reduced to 25nF (instead of 33nF), which leads to
~16kV more possible stress to the primary capacitor, than "ideal" 33nF.

This investigation has shown me to be careful about selecting a
deviating value for SRSG LTR-caps, relative to the "ideal" value (in
fact I decided to make an MMC of 2 x 12 150nF CD's + 1 x 12 100nF CD's
for my 15/60 NST). The little new coil is not yet completely buildt and
tested, but I'll report the results.

Cheers,  Kurt


Tesla list schrieb:
 > Original poster: dhmccauley-at-spacecatlighting-dot-com
 > I am having some interesting phenomenon occur at the safety gap I have
 > at my
 > SRSG gap.
 >
 > If I adjust my SRSG to the point of which (should yield 75% to 100% maximum
 > voltage at primary capacitor), i get firing of my safety gap with
 > loud bangs (which are likely because the MMC is discharging into the safety
 > gap)  The safety gap is adjusted slightly larger than the no-load voltage
 > on the NST.  I am confused to why this gap is firing as I am using a LTR
 > type capacitor and didn't think i could get over-voltages using a SRSG.
 >
 > Any thoughts??
 >
 > Specs on my small coil are:
 >
 > 15kV, 60MA NST
 > 0.0257uF, 24kV (MMC - LTR sized)
 > Standard secondary and primary coil
 >
 > The Captain