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Re: Smaller than resonance?



Original poster: Justin <rocketfuel@xxxxxxxxxxxxx>

Hi Gerry,

Thanks for your informative reply!

Just to clarify, you're talking about making these measurements on a
ballasted transformer, yes?  This would seem to dovetail nicely with
John's comments about the resonant value changing with the changes in
the inductive ballast.  So, I should be able to take several
measurements with the adjustable ballast in different positions and
calculate the resonant cap value for each, possibly even graphing
the results for extra credit.


Justin




On Wed, Apr 04, 2007 at 11:04:43PM -0600, Tesla list wrote:
> 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.
>
>
>
>