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Re: Resonant charging (in the days of King Spark)
Original poster: "Ed Phillips by way of Terry Fritz <twftesla-at-qwest-dot-net>" <evp-at-pacbell-dot-net>
Tesla list wrote:
> Original poster: "Jim Lux by way of Terry Fritz <twftesla-at-qwest-dot-net>"
> >Weazle wrote:
> > The power transformers they used had a turns ratio of 1:300, whereas
> > our NSTs and OBITs are 1:100, meaning they must have been getting a
> > very high secondary voltage in a resonant condition. I'm surprised
> > that the plate glass caps that they where using in those days could
> > stand that sort of strain.
> Unless they overdesigned the plate glass cap... Sure, the glass is lossy,
> and the breakdown voltage characteristics not as desirable as modern
> plastics, but, in 1920, what else did you have? (Ruby Mica, I suppose)...
And compressed air. Marconi used atmospheric pressure AIR dielectric
for at least his Glace Bay installation. As I recall the capacitance
was around 0.2 mfd, and the capacitor occupied a large building!!!!!!!
There were a lot of pictures of that station published on the web on the
occasion of the Marconia centennial, and I saved a lot of them in case
anyone is interested.
> And, of course, this is for a ship or shore station, where weight isn't as
> big an issue (you don't have to drag that ton of glass around). I think
> that they would have just used healthy design margins, both for dissipation
> heating and for dielectric breakdown.
> Assuming you can deal with the other breakdown related issues (corona,
> e.g.), efficiencies are generally better with higher voltages (less IR
> loss, for one). The typical hobby tesla coil works around 10-20 kV mostly
> because the components that are readily available inexpensively are in that
> voltage range (i.e. NSTs, MOTs, polepigs, etc.). Start getting up above 30
> kV, and HV design becomes a bit trickier. Also, their transformers and
> other components may have been lossier, so a 300:1 ratio was needed to keep
> the voltage up.
The typical "big" ham spark station used as much as 25 or 35 kV. The
little guys usually operated around 15 kV, as the transformers and
capacitors were cheaper at that voltage. I have JPEG images of some
typical ads with prices which I intend to post with Terry as soon as I
can get the ambition.
> On the other hand, if you are buying things new, for a specific purpose,
> then you can pick your design point differently. A 10 kVA transformer for
> 30kV costs about the same as a 10 kVA transformer for 10 kV, if you're
> buying it brand new (as opposed to surplus, or mass market (pole pig)).
That's more nearly true now than it was 90 years ago, when insulation
breakdown was a bigger deal.