Re: Smaller than resonance?

["Tesla list" <tesla@xxxxxxxxxx>]

Original poster: "Gerry  Reynolds" <gerryreynolds@xxxxxxxxxxxxx>

Hi Justin,

See interspersed:

> Original poster: Justin <rocketfuel@xxxxxxxxxxxxx>
>
> 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.

This is correct.

> 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.

Well, sorta.  You can represent all the current limiting inductance 
of the transformer in the primary, or in the secondary, or both.  Any 
current limiting series inductance in the primary can be transfered 
to the secondary by multiplying it by the n^2 (n is the turns 
ratio).  The converse can also be said.  One method is to measure the 
total current limiting inductance indirectly by measuring the 
Vs_open_circuit and the Is_short_circuit and figuring the limiting impedance:

ZL = Vs_oc / Is_sc = 2*pi*line_frequency*Ls   where Ls is the total 
inductance transferred to the secondary.

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

It is easier to not think of separate primary and secondary 
inductance since these are very difficult to measure when you have 
iron core.  It is hard to magnitize the core with an inductance 
meter.  Think of the total effect of limiting the current.

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.

Yes, difficult to do directly.  Easy to do indirectly.  Measure Vs_oc and Is_sc

> 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?

Yes.  Most use LTR with NST's using a static SG or SRSG at 120 BPS.

When you get into higher power PIG coils, static gaps become 
impractical (heat) and 120 SRSG may require too large of a bang size 
for the coil (racing arcs) and too expensive of a cap.  Many go STR 
but this requires a much higher BPS to keep the voltage 
reasonable.  Think of a ballasted KVA level.  If the bang gets 
smaller (smaller cap), the BPS must get larger to support the 
resulting power level.  If the BPS does not get larger, the voltage 
will grow until the BANG energy * BPS sorta equals the ballasted KVA 
level.  (I'm ignoring the power factor and losses in the transformer 
with this approximation)

Gerry R.