Re: Smaller than resonance?

["Tesla list" <tesla@xxxxxxxxxx>]

Original poster: "Barton B. Anderson" <bartb@xxxxxxxxxxxxxxxx>

Hi Justin,

Tesla list 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.

Yes.

> 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.  Not sure if
> the primary side inductance has an effect.

Again, yes. And yes, anything extra C or L on the primary side will 
be reflected to the secondary side (i.e., ballasts which most of the 
larger transformers use for current limiting).

> 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.

There are methods to find out everything you want to know about the 
transformer. Open circuit and short circuit testing is required to 
find current and voltage values and then a little math will help find 
the rest. This is even true for smaller transformers if you really 
want to be accurate. It really helps to have a means of measuring the 
high voltage side (such as a hv AC probe) so you can perform accurate 
hv side testing.

> 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.

Yes, absolutely.

>   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.

LTR and STR will prevent the cap from being overcharged as result of 
transformer resonance. I should mention software simulation in this 
condition. If a simulation model does not use voltage dependence for 
gap breakdown, the simulation would actually show the cap voltage 
rising extremely high for STR cases. In reality, the gap does 
breakdown and limit the voltage at the cap and bps increases as a 
result. But also, heat results with higher bps and breakdown voltage 
changes. A lot there to contemplate (almost impossible to accurately model).

For the LTR case, charge time is increased, so there is a limit of 
bps based on the time required for the gap voltage to reach it's 
breakdown. For the STR case, charge time is decreased, and high bps 
values are easily achieved. However, in both cases, the cap size 
should be suited to the coil, not the transformer. NST type 
transformers are better off with LTR because the caps energy release 
is increased with larger cap sizes up to the point where bps slowing 
hinders performance. As a matter of fact, the LTR sizes for NST's as 
shown in programs and documents are based on staying out of resonance 
and still performing well. For larger transformers, cap sizes are 
simply suited to the coil parameters and 9 out of 10 cases will be STR.

I am coming to prefer a 1.3 x cap resonance value which Dr. Resonance 
has expressed a few times. It moves the gap near resonance, but not 
to the point of killing the NST. It's sort of a max performance value 
for NST's. This is a slight resonant charging mode where you run just 
outside resonance. Resonant charging would usually be within the 1.0 
to 1.2 x Cres arena. But remember, the closer you are to 1.0, the 
higher the voltage will be seen at the transformer. Too high, 
especially with an NST, the more risk you are to an overvoltage 
breakdown of the NST secondary windings.

A pig running at resonance or even slightly LTR would result in a 
very large cap energy release (bang size) and the coil must be 
designed large enough to handle that energy. Typical pig garage 
coilers will end up with an STR sized cap. Thankfully to Malcolm and 
Bert, my first coil was a success. If it were not for them, my cap 
size would have been massive and the word "fire" is what actually 
caught my attention in those older TCML postings. Now I understand, 
then I didn't. Although my first coil sucked, it was still a success: 
30" sparks and no fire!!! Funny, after enough TCML time, that same 
coil did 108" sparks (lots of gap changes, 12" added to the 
secondary, and top load changes).

I've used my pig for even small coils running rather extreme STR 
without problems (limiting current). However, when running small 
coils with high current secondary transformers, don't go overboard. A 
coil can be overcurrented and turn into a blaze of fire. The biggest 
problem there is that each coil is different (wire sizes, parameters, 
etc.). Hard to say "when" a coil will burst into flames. Pushing a 
coil to it's limit is the only way to find out (and that's the end of 
that coil).

I prefer using a small transformer with a small coil to prevent 
overindulgence. I think in most cases of overindulgence, the gap will 
heat and limit current before too much "bad" occurs (maybe), but if 
the gap is designed for high power and used on a small coil, then it 
is very possible to burn up the coil. Every now and then, someone 
will post such an event.

Take care,
Bart

> Thanks for any input,
>
> Justin in Austin