RE: [TCML] 20KV primary drive voltage tips ?

["Lau, Gary" <Gary.Lau@xxxxxx>]

Hi Chris,

You don't need to resort to charging inductors to achieve a 20kV bang size.  A 15kV NST has a peak voltage higher than that, and the normal NST/cap resonance will boost that even further if the gap allows it.  More comments interspersed...

> -----Original Message-----
> From: tesla-bounces@xxxxxxxxxx [mailto:tesla-bounces@xxxxxxxxxx] On
> Behalf Of Chris Swinson
> Sent: Friday, February 29, 2008 6:41 AM
> To: Tesla Coil Mailing List
> Subject: [TCML] 20KV primary drive voltage tips ?
>
> Hi all,
>
> I am working on a 20KV res-charging design to boost 10KV to 20KVDC. I forget
> the size of the inductor but the break rate will be 100 or 200bps. I plan to
> use a sync motor to keep the timing spot on. So the RSG will have a suitable
> number of electrodes on the disc.
>
> So my first question really is has anyone built a low power 20KV system.
> This will only be 500watt prototype ( don't have room or a NST for anything
> bigger anyway).

15/30 NST's are commonly used with a sync RSG.  So a simple 15kV NST is out of the question?

> I know my 10KV primary is spaced out about 0.5" so I assume a 1" gap between
> turns will be needed ?

No, you should be just fine with 0.25" between turns.  A 1" gap will make the primary needlessly large.

> The 20KV drive should help spark gap quench times ?

I don't know that I've heard that bang voltage correlates with quench time.

> Ideally I want to aim for 1st notch quench and keep the transfer cycle time
> to 3 cycles maximum( so >0.3K needed)

Quenching suffers as K is increased.  It's not even clear that first notch quench is the best thing for streamer production.  And running with a K of 0.3 is simply unrealistic.  You'll get racing sparks long before achieving anything close to that.

> 20KV will probably suffer from corona on the primary, but that will have to
> be delt with if/when it happens.

I've never noticed any.

> I also want to keep the coupling as tight as possible. Going by my
> simulations 0.38K seems to be the maximum before hitting problems. Also
> should a cone or vertical primary be used ?

What simulator predicts coupling problems?  Predicting racing sparks is one of the unsolved mysteries of coiling.  I'm unaware of any method of predicting it beyond noting the experience of many coilers, where coupling much above 0.15-0.20 seems to be unmanageable.

A vertical (helical or solenoid) primary is typically shunned in favor of a flat spiral primary, for the very reason of having TOO high a coupling.

> Onto the key points now, the last system I built was 500khz 10KV, so 20KV
> should allow a 1mhz coil to be used.  Really its half the inductance and
> double the voltage input to get the same spark output. A bit of a long story
> for another day that one!

I don't follow your logic.  The experience of most successful coilers is that larger coils make for bigger sparks, and that larger coils inherently operate at lower frequencies.

> The secondary will be using at least 0.8mm dia cable ( its what I used on my
> 500khz coil ) I want to half the inductance ( half the turns probably)

Nope - inductance follow the square of number of turns.

> which
> will also half the resistance, though now I can afford to use double the dia
> of wire to keep the secondary the same height as my 500khz coil was.
>
> I am aiming for 1ohm DCR on the secondary. the larger dia wire should help
> with the running at 1mhz.

The DC resistance of the secondary is of little importance, given that the load (streamer) impedance is many orders of magnitude higher.  AC resistance will be far higher, especially if you're talking about 1MHz, but I still don't think it's a major consideration.

> The secondary form will be 6.3" dia and about 50-60" tall. A lot of figures
> to crunch up and some compromises no doubt.

Yours would be considerably more candle stick-like than most well performing coils.

> The design is to start at 500watts input power as before, keeping the
> secondary size physically the same size, but just using a larger dia wire.
> Will run at a higher frequency and have less turns/inductance hence the need
> for a 20KV input.

I'm not sure that the secondary geometry cares what the primary bang voltage is.  One typically sizes the secondary by the bang-SIZE.

> Well that's the overall design idea I am aiming to build. I have my RSG disc
> and sync motor built, also have the charging choke, collected a fair few MMC
> caps to get the values I will probably need. So all pretty much ready to
> start work on the primary/secondary design now.
>
> There are probably other factors I've not taken into account yet too.. so
> plugging for ideas and help before I spend hours building something which
> could melt at first light :-)
>
> Cheers,
> Chris

I hope this helps!

Regards, Gary Lau
MA, USA
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