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Re: Smaller than resonance?



Original poster: "resonance" <resonance@xxxxxxxxxxxx>




If you plan on using a PT, contact me off-list.

I just finished a neat design for the Univ. of AZ Physics Dept that uses a PT running at 3.8 kVA on two separate 120 VAC circuits, each delivering approx 16 Amps, running in parallel. I modeled it in Javatc and ended up with a .083 uF, 36 kV MMC type pulse cap (9 strings of 18 pcs in parallel). On the pri side it uses two separate power reactors each limiting the two separate 120 VAC outlets to slightly less than the standard 20 Amp breaker trip point.

5-6 ft spark output with a RSG running at 1725 rpm, 10 electrodes.

Dr. Resonance



Original poster: Jim Lux <jimlux@xxxxxxxxxxxxx>

At 10:22 PM 4/3/2007, you wrote:
Original poster: Justin <rocketfuel@xxxxxxxxxxxxx>

Hi All,


I'm trying to figure out the right size tank capacitor bank to use with
my new potential transformers.

After much thinking and poking around on the net, I've come to the
conclusion that pole pigs, potential transformers, and any other
transformer (not just NSTs) can hit a resonant condition with the tank
cap.  Please correct me if this assumption is wrong.  From what I
understand today, it seems that the resonant cap value is a function of
the inductance of the HV secondary and the line frequency.

Pretty much. There's also the parasitic C of the windings, but that is small (pico or nanofarads) in comparison to the typical tank capacitor (tenths of microfarads)

 Not sure if
the primary side inductance has an effect.  It would be nice if I could
simply measure the inductance of my transformer with a meter and work it
all out on paper, but I have a hunch that a reading on a non-energized
core will be different than when current is flowing through the
transformer.

Can't use an inductance meter, but you CAN do it with a volt meter, ammeter, and a suitable resistor. Find a textbook that describes measuring transformer parameters (not just turns ratio, but also the reactances) or, look here:
http://home.earthlink.net/~jimlux/hv/xfmrmeas.htm
It's a bit incomplete..
There also used to be a good page on measuring Neon Sign Transformer Parameters out there, but I can't remember who put it up. Maybe google will find it?



On to my "real" question.  In looking over several other people's coils
online, it seems that many are running much smaller than resonant tank
caps with these larger transformers with no problems.  I say smaller
than resonant as a guess based on results I've seen from JavaTC when
entering the parameters listed on these coil sites.

Is it possible to run STR rather than LTR to protect the transformer and
capacitor bank?  Of course everything will still need to be matched and
tuned.

Sure.. just make sure you have a safety gap across the transformer and cap to protect them against unforeseen resonant rise. And, if you're substantially STR, you'll have no problems. (mind you, the stored energy is smaller, so the sparks will be shorter...)



Thanks for any input,

Justin in Austin