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Re: [TCML] large DC coil power supply

Hi Jim,
Sounds as if you are already more qualified than the bulk of
the of the TCML members to take on this proposed
project of yours. ;^) I would suggest talking with Greg Leyh
or Bill Wysock ( I think they are both in California, too), as
they would be the ones that actually have some practical experience with SG driven coils of this magnitude on a pro- fessional level. I think Greg has even built a DC powered coil of this magnitude.

David Rieben

----- Original Message ----- From: "jimlux" <jimlux@xxxxxxxxxxxxx>
To: "Tesla Coil Mailing List" <tesla@xxxxxxxxxx>
Sent: Sunday, August 15, 2010 12:02 PM
Subject: [TCML] large DC coil power supply

I'm looking at various ideas for a power supply for a large-ish DC spark gap coil (let's say, 25kW)... It's an art project, so it's not an income producing widget where you can trade off customer costs. It needs to be portable/transportable without taking days to set up. The basic environment for discussion purposes is a stage or performance setup.. coil on the stage, big heavy stuff (generator, transformers, etc) offstage, separated by some dozens of feet.

This is sort of like the power supply for Electrum, but Electrum was designed for permanent installation, and so some of the design decisions might be different. Technology has also marched forward.

Various issues and alternatives for discussion:

How to get prime power in.. You're not going to use 16 gauge cheap extension cords here.

if you want to run off 240 single phase, that's 100A.. bigger than one normally plugs into the old dryer socket.

In show business, at this kind of power level, you start running 4 wire banded (4 separate wires with what are essentially fancy banana plugs on the end)

Clearly, this is bigger than typical residential loads, so that means probably using a standalone generator, and in this class, that means 3 phase too, which is nice. So, does one start with 240 or 480?

My gut feel is that 240:14400 transformers are more common (cheaper) than 480:14400. And 240V (or 208V) generators are going to be more common too (and so cheaper to rent.. in the $100/weekend category, it appears). If you put the HV power stuff near the generator, then the fact that you're running BIG wires for the lower primary voltage won't be as big a deal, cost wise. (and if someone is *paying* you to run this thing, and they want to put the generator elsewhere, then they can *pay* for the rental of the longer wiring)

You've got to get to HV somehow, and the simple way is just to get a couple or three pole transformers and run them (open) delta. 25 kW @ 15kV is a couple amps, well within solid state rectifier territory, especially in oil.

But, 2 or 3 transformers is a "big" package.. (but then, a 25kW coil is a big pile o' stuff anyway), and they're oil filled which makes transportation a bit more of a hassle. Do you put them in separate road cases? or just one big 1000 pounder?

OR... what about switching power supplies.. Big three phase rectifier off the prime power, driving a big power supply (hmm.. but you'd probably not be able to find such a power supply *surplus/cheap* off the shelf, unlike you could find pole transformers) And designing and building such a thing is a huge project in itself. (compared to a fairly easy 6 pulse rectifier with 3 transformers or a 12 pulse with 6, which one could lash up in a weekend, if you had all the parts)

But maybe there *is* some sort of industry that uses up 20kV 1 amp DC power supplies? (yep.. buy 20 1kW class inverter microwave ovens, work out the non-trivial primary isolation issues, and run them in series/parallel)

And then, once you've got your DC power.. how do you get it to the coil? It's fairly obvious that the power supply is big and bulky, and you're not necessarily going to want to stash it under the coil.. so that means running a few amps at 20kV around.. RG213 coax should do nicely for that.

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