Hi Jan,
First, a disclaimer - I have not attempted a TC as powerful as what you're
considering - only NST-powered coils.
But, I think most similarly-powered coils use a variable-speed ARSG. Is
there a reason to use a SRSG? I calculate that with a 200 BPS SRSG,
charging voltage of 18.4kV, 200nF cap, the maximum power than can be
processed is 1/2 C V**2 * 200bps or 6.7kW. Serious juice but no way to
turn it up.
I can't comment on ballast considerations. I think you know more than me
on this topic.
I think the concern for low-loss secondary form is unwarranted. Dielectric
losses occur in capacitor dielectrics, but the e-fields in a coil form are
not significant. Polyethylene is fine but uncommon and not easy to glue.
PVC or sonotube won't disappoint as far as losses are concerned.
Good luck winding that transformer, I think that's going to be your biggest
challenge. Coil on!
Regards, Gary Lau
MA, USA
On Fri, Jun 29, 2018 at 3:49 PM, Jan Ohlsson <jan@xxxxxxxx> wrote:
> 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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