Hello Jan,
Sounds like a fun project. I haven't built anything as big as what you are contemplating but my coil is probably classified as "big". The secondary is 39" tall with a diameter of 8.6" and 1420 turns of 22 AWG wire. Capacitance is 45 nF implemented with 6 Maxwell pulse capacitors in a series-parallel configuration. It is powered by a 10 KVA pole pig. I am presently using only 4.8 KVA. Maximum spark length is about 8 feet.
Regarding the ballast, just make a variable air gap ballast and forget the adjustable taps unless you have a specific reason to use variable taps. I made my own variable air gap ballast. It weighs 70 pounds or more. I can't remember exactly. You can look up my specs on a recent posting. My ballast is designed to handle up to 40 amps at 240 volts without saturation. Since I think you are at 50 Hz, you will have to use more iron and more turns than mine which is designed for 60 Hz. It sounds like you have plenty of iron to make it as big as you need which is fortunate. You can get the design equations from any transformer design textbook.
My RSG sounds similar to what you are contemplating. It has a 10" rotor with 4 flying electrodes spinning at 3600 RPM. Considering the power you are contemplating, I would use at least 0.25 inch tungsten on the flying electrodes. Mine uses 1/8 inch flying electrodes mounted in brass holders and shows almost no erosion at 4.8 KVA due to the excellent cooling provided by the 3600 RPM spin. I would also use at least 1/4 inch tungsten on the stationary electrodes and maybe bigger. They will erode more quickly because they are firing 4 times as often and are not getting cooled as well as the flying electrodes. To help with the cooling I made some small aluminum auxiliary heat sinks that mount on the 2 stationary electrodes about 1/2 inch away from the firing gaps. They look similar to the anode and grid heat sinks you see used on some 833A vacuum tubes. I found out that when these heat sinks are that close to the firing point, the radiant heat will begin to erode the aluminum quite rapidly. To prevent this, I mounted a small aluminum nitride ceramic heat shield (from E-Bay) on the surface of each heat sink facing the spark gap. The melting point of aluminum nitride is 2200 degrees C (4000 degrees F). Here is a link to the ceramic heat shields if you are interested.
https://www.ebay.com/itm/Coors-Aluminum-Nitride-CERAMIC-Plate-Sheet-99-ALUMINA-1-5-X1-5-X1mm-2PC-New/152138870666?ssPageName=STRK%3AMEBIDX%3AIT&_trksid=p2055119.m1438.l2649
Considering the power level that you are contemplating, for your primary circuit I would not use wire. I would use either copper tubing or copper strap for all "wiring". That is what I did. I did the primary tuning with thick flexible wire. After I found the proper tap point I replaced the thick wire with a custom-bent piece of copper tubing.
I have no experience using a counterpoise in place of an earth ground.
I have seen that some people have used acrylic sheets to cover the primary in an attempt to prevent primary strikes. From what I have read, they don't seem to help that much. Since you are asking a 1/4 inch plastic sheet to withstand greater than 500 KV it doesn't seem like it would help that much. A proper strike rail should divert the vast majority. Occasionally I will get a strike to the primary but it doesn't seem to cause any ill effects.
I assume that you either have or will use JAVATC to design your coil. Don't buy or build anything until you have a complete design in hand and know how you are going to build it. If you start before the details are in hand, you may end up just wasting time and money on parts and ideas that will not work. This is especially important on a large coil because the parts are very expensive for high power.
Steve
----- Original Message -----
From: "Jan Ohlsson" <jan@xxxxxxxx>
To: tesla@xxxxxxxxxx
Sent: Friday, June 29, 2018 2:49:20 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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