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Re: Pool Pig...



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

Hi Chip,

As Greg mentions, it's very normal for pole pig sized transformers to use a cap size that is smaller than resonance. The point is to try to stay away from the resonant cap size. Sometimes, coilers will actually use some resonant charging, thus, their caps are straying near the resonant cap size. The resonant capacitance of the transformer has been a NST killer in the past and is why we have tackled the issue and found that preventing the NST secondary's from overvolting is key to NST longevity. Moving the cap size away from resonance is one important part of keeping the NST alive as is ensuring proper gap spacing, safety gaps, Terry filter, etc..

High current transformers can also be used which are far more robust than NST's but they are heavy, bulky, and can be expensive in both initial purchase and shipping. Pole distribution transformers were ideal for a high voltage robust transformer not prone to the easy deaths NST's had experienced. Applying NST cap size rules to pole pigs is not practical. For one, the cap would be huge, and second, the coil itself would have to be designed to accept the large cap. So, we simply run a small cap size (comparative to the transformer resonant cap size), but still usually much larger than NST cap sizes. The cap size is usually not sized to be within the resonant mode and does not exhibit resonant voltages. So, just like NST's using LTR cap sizes (preventing resonant voltages), STR cap sizes will prevent the same situation. I speak here only for resonant voltages (there are still high voltage transients that can damage or degrade the transformer).

Take care,
Bart

Tesla list wrote:

Original poster: G Hunter <dogbrain_39560@xxxxxxxxx>

Hi Chip,

The transformer / cap matching formulas often break
down when pole pigs are involved for precisely the
reasons you've just illuminated.  An optimal cap for
7200vac at 2 amps is just too big.  It's not a big
deal.  Smaller-than-resonant caps are the norm at such
power levels.  Start off with about 50nF and
incrementally add capacitance until you get decent
sparks.  Maxwell Labs 30nF 30KV plastic caps go for
about $100 apiece on e-bay these days.  Try one or two
of those to start with.

Cheers,

Greg


--- Tesla list <tesla@xxxxxxxxxx> wrote:

> Original poster: "Chip Ford" <chipford@xxxxxxxxxxxx>
>
> Ok...there are many kinds of transformers out there.
> Seems that the
> really BIG coils use a pole pig. Such as "The green
> monster". His is
> a 15kva @ 14400 volts. Using the math KVA = ((VOLTS
> X AMPS) / 1000),
> I came up with an amperage of @ 1.042 Amps (15,000
> watts). Since .001
> Amps is equal to 1ma, he has 1042ma. According to my
> calculations,
> the resonant cap is @ .1919 uf. I think he said his
> cap was
> .1uf...close but not real close..Now, I think alot
> of the line
> transformers have a selector switch on them. I think
> they are like
> 7200, 6400, 5600 volts for the ones around here. So,
> if I have 15 kva
> transformer @ 7200 volts, it will have an amperage
> of @ 2 amps
> (2000ma)(15,000 Watts). At this rate, my cap is
> gonna be around .75
> uf. This is really high and from a few calculations
> that I have made,
> A secondary coil 12" 81/2" dia. 48" tall will need
> over 10000 turns
> of #39 awg. wire on it. This pretty much makes this
> transformer
> unusable. Besides that, where is a hobbest ever
> gonna find a cap? Am
> I missing something here? Are my calculations
> incorrect? Am I looking
> at the transformer incorrectly? Please
> comment...Chip Ford
>
>
>