Sounds like you are taking on a considerable task, having to fabricate your
own high voltage transformer and ballast. I envy your 1800 square meter
underground bomb shelter as your "play ground", though.
I would say that you have already received some good advice regarding your
questions. Like Steve White and John Freau, I concur that you should not
worry yourself with the issues of going synchronous with your rotary spark
gap for such a large and powerful system, nor should you be concerned with
the "losses" of the material of your secondary coil form, so long as it is a
good insulator. And yes, if you do not coat your secondary coil with
varnish, urethane, epoxy, ect, you will almost certainly suffer a
catastrophic failure of the coil falling off of the secondary form when the
ambient temperature changes significantly. This would be especially true if
the ambient temperature were to significazntly drop after the winding
operation is completed. Ask me how I know this. ;^o
I would further emphasize what Steve stated about using large enough
diameter tungsten roundstock for your rotary and stationary spark gap
electrodes. My large coil routinely processes >15 kVA when I fire it up and
I use a relatively simple aynchronous rotary gap consisting of (2)
stationary and (8) flying electrodes. However, my stationary electrodes are
solid 1/2" diameter x 3" long pure tungsten rods, mounted in a soild brass
1.25" diameter x 2.5" long brass square stock, while the flying electrodes
are each 3/8" diameter x 1.75" long solid tungsten rods. The flying
electrodes are set in a 1/2" thick x 11.5" diameter garolite (G-10) epoxy
fiberglass rotary disc, which is driven by a 130 volt DC, 3 HP treadmill
duty motor. I vary the speed of the motor with a simple variac inside of my
control panel and running the control voltage through a FWB rectiier brick
to give the motor it's required DC and have found the "sweet spot" to be
where there is about 300 -350 breaks per second (BPS). As for the primary
wiring between the capacitor, the spark gap and the primary coil, I use some
very thick tinned copper mesh conductor - about 2" wide and about 1/4"
thick, probably equivalent to #2 AWG wiring!
For a Tesla coil of this magnitude, many coilers find it more practical (and
often cheaper) to opt for large commercial HV pulse capacitors for the tank
circuit capacitor as opposed to purchasing (and soldering together!)
hundreds of small snubber caps for a large MMC unit. I am currently using
(2) GE 13.8 kV(AC) rated, 0.25 uFd (measured C is actually right at 0.27
uFd each) protective power capacitors in series - the welded rectangular
hermetically sealed and oil-filled metal canister, with a single bushing
"hot" terminal and the metal casing as the "other" terminal. This gives me a
0.135 uFd unit that's good for at least 27.6 kV at 60 hz AC.
Good luck,
David
----- Original Message -----
From: "Jan Ohlsson" <jan@xxxxxxxx>
To: <tesla@xxxxxxxxxx>
Sent: Friday, June 29, 2018 2:49 PM
Subject: [TCML] Please review my coil plan
HI, i am new on this list and in the early stages of designing a big coil.
A few years ago I built a medium size coil, but want to go bigger now. I
have access to at least 40 A at 400 V. Pigs are hard to come by in my
country, so I am in the final stages of fabricating my own, 55 kg UI core,
isolated with paper and in a tank under transformer oil, in the old
fashioned way. It should be capable of delivering up to 16 kVA
intermittently at 13 kV,I think.
But now I would like to test a few ideas in this forum, before I commit.
Please correct me if they seem crazy!
1. The ballast should have a low flux in the core at max current,
otherwise the current limitation will be unstable due to the unlinearity
of the iron cores permeability. I am thinking of a UI core weighing 110 kg
with a good sized air gap, would that be excessive? I am planning on
several taps to be able to change the current from 10 to 40 A.
2. The SRSG should have a high rotational speed for good quenching, I am
planning 3000 rpm and 400 mm diameter, 4 electrodes.
3. The secondary should be wound on a tube with very low losses, I am
thinking of polyethylene, and varnished as well with low loss material,
epoxy resin. Nothing lossy inside the coil form, low dielectric loss is
very important.
4. Secondary with large diameter, 315 mm, and not to long, 900 mm. Cu wire
1,2 mm, low losses are more important then many turns.
5. Very large thoroid, minor diameter 300 mm, major 1,5 m.
6. A small thoroid under the big, to be able to raise the main thoroid and
still avoid downwards strikes.
7. No possibility for a normal earth, the site is a big underground bomb
shelter with a very well reinforced concrete floor. I plan on using
several sheet metal plates on the floor to get a counterpoise type ground
by coupling capacitively with the metal sheets to the rebar system in the
concrete floor. The floor is very big, 1800 square meters, so it is a big
electrical mass with more or less conductive rebar.
8. An isolating plastic sheet above the primary, to avoid strikes to the
primary.
9. MMC capacitor bank, probably up to 200 nF, will have to be tested.
What do you think? Please feel free to critizise!
Jan, Stockholm, Sweden
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