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Re: Smaller than resonance?



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.