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Re: Capacitor Calculations
Tesla List wrote:
>
> Subscriber: Rodney.Davies-at-anu.edu.au Wed Jan 15 21:05:51 1997
> Date: Wed, 15 Jan 1997 15:08:18 +1100 (EST)
> From: Rodney Graham Davies <Rodney.Davies-at-anu.edu.au>
> To: tesla-at-pupman-dot-com
> Subject: Capacitor Calculations
>
> Hi All!
>
> I've amazingly been able to get hold of a 15KVA 22KV Single Wire Earth
> Return transformer from the local electricity board.
> It was a decommissioned xformer which lived its life on a power pole for
> several years. The core is still in good condition, but was "too old" for
> power distribution purposes, so when I spotted it in the scrap yard, I
> asked and recieved!
>
> Anyway, I'm trying to work out exactly what value caps to use with this
> xformer.
>
> With Neon's I use the formula -
> 1
> c = ----------
> 2*Pi*F*R
>
> Where F = 50Hz (Aussie Freq.)
> R = V/I
> c is in Farads
>
> Since the power output of the xformer is 15KVA and the voltage is 22KV,
> one calculates an approx. current of 0.6A.
>
> When I plug all of this into the formula, it gives me 0.09uF.
>
> Strange thing is that I've seen other coils that use this sort of power
> level and transformer, and they use 2 0.5uF caps in series to yeild 0.25uF.
>
> Which ones do I go for, and is there a particular formula I should be
> using for these xformers other than the above?
>
> The next question is, how does one calculate what value an inductor RF
> choke should be for this? I don't really feel like coupling 80KHz of RF
> back into our power grid! :-)
>
> Here's the Tesla coil specs that I'll be driving with this xformer -
>
> Secondary -
> 12" diam
> 625 turns of 1mm enamel coated wire spaced at 0.25mm apart
> (I know, tat tat, no spacing, but it's an old secondary from way back)
>
> Primary -
> 5 turns of 3/8" copper pipe in Pancake Archimedes formation,
> spaces at 3/8" between windings
> ie
> __ __ __
> ---( )---( )---( )--
> ~~ ~~ ~~
> | |
> ^--------- 3/8"
>
> Spark Gap -
> Rotating 12 point gap.
> 1440RPM (asynchronous) and does splutter occassionally.
> Wheel is 10" diam
>
> Not the best designed coil, but should work with 10KW of power, and
> should hopefully resonate somewhere between 80-100Khz.
>
> Thanks for your help guys!
>
> Happy Sparking!
>
> Rod
>
> Rodney.Davies-at-anu.edu.au
EXCELLENT score on the transformer!
Re: Cap size - The amount of power you'll be able to transfer to the
secondary is a function of the tank circuit's capacitor size, gap
voltage, and true firing rate (breaks per second). A 22 KV transformer
is a little higher that most coilers would use, but it should work very
well. Most other coilers use 14.4 KV, and a only a few venture into the
20+ KV range.
Because the maximum voltage is significantly higher in your system, the
capacitor size will be smaller to get the same amount of energy "per
bang". For example, a fully charged tank cap in your system would see
31.1 kV versus 20.4 kV for a 14.4 kV pig. The amount of energy stored in
the tank cap in each case (1/2 CV^2) would be about the same. For
example, assume 0.1 uF and 0.25 uF tank caps in the respective systems
will deliver 48 and 52 Joules respectively (BTW, this is LOTS of energy
for a Tesla Coil!).
A suitably ballasted pig can deliver much more current to recharge the
tank cap, while a neon's output voltage will start collapsing once you
approach its maximum current rating. As a result the capacitor
calculation used for sizing a cap to a neon is overly conservative when
using a pig as a power source. Also, the pig can withstand significant
current overloads with no problem, as long as your mains are suitably
sized. Your pig will easily drive a 0.1 uF - 0.15 uF cap, but the cap
will have to be rated at least for 90+ KV.
You may need to increase the primary conductor diameter as well as the
inter-turn spacing to handle the power and reduce inter-turn flashover
problems. The small nuber of turns combined with the large tank energy
(>40 Joules) will generate high inter-turn voltages, particularly if
you're tapping at less than 5 turns. You may want to go with at least
0.5" or 0.75" copper tubing with 0.75" inter-turn space. Also, you may
need to beef up your rotary to handle the higher voltages.
Assuming you're running the rotary across the pig's secondary, you could
opt to run without chokes, and use just safety gaps to safely shunt any
coil strikes to the primary to ground. Pig's are tough beasts... the
insulation capability of yours is probably in excess of 100 kV. If you
do decide to use chokes, they must be able to handle the short-circuit
and cap recharge current from the pig (plan for at least 500 MA current
handling), should be at least 1+ mH, and must be able to withstand very
high peak voltages. A 10" long single-layer winding of 24 AWG on a 4 "
PCV form should do it. The HV chokes are not designed to keep out the
80-100 KHz from the transformer, but to block the high-frequency (VHF)
parasitics generated by the spark gap exciting stray wiring capacitances
and inductances. In addition, you'll need to use high current EMI
filters on the primary side of the pig to keep out conducted "garbage".
Safe coilin' to you!
--Bert --