Re: Spark length vs. coupling results

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "jimmy hynes by way of Terry Fritz <teslalist-at-qwest-dot-net>" <chunkyboy86-at-yahoo-dot-com>

Hi,

I would have thought that at 40 bps the loss wouldn't really matter, thats 
far less power than at 1200 bps. What I meant when I asked why 40 bps, is 
why don't you want streamer growth to affect it? I would think that it 
would be a more relevant answer, because most TCs run at high enough break 
rates for streamer growth. It wouldn't be hard to do a second test at high 
break rate would it?

Changing the topload would change too many variables. With a smallaer 
topload, you either have less energy, or higher voltage. I think that could 
make as much change as the frequency alone. If you did use the shorted turn 
idea, it might not change the frequency enough with "loose" coupling. If 
the coupling was 1, the frequency would be "high". Maybe the coupling would 
have to be too tight, I dont know. I will look into it.

If you are allowed many cycles to ring up, the DRSSTC isn't much more 
efficient than a SSTC with an untuned primary. At a higher frequency, the 
magnetizing current goes down, and with more cycles, it goes down with 
respect to the in phase current because the in phase current builds up over 
many cycles, and magnetizing current doesn't. The power factor of the 
DRSSTC also increases, because the frequency splitting is less, and the 
driving freq. (the zero) is closer to the resonant freq. (the pole), but 
the change is much less. The only other benefit of having a tuned primary 
is that the average to peak ratio is higher because it has a sinusoidal 
instead of linear shaped envelope. The average to peak ratio is about 0.6 
instead of 0.5. If you can get away with taking enough cycles, it may be 
better to ditch the primary capacitor, because of the cost.

When you say Richie's tuned primary coils work better, is that for the same 
power, number of primary turns, or what? I think the main advantage in 
slower SSTCs is probably increased clearance from pri-sec, like you said.

Although the primary losses are taking up more energy, in order for the 
secondary voltage to be dropping due to primary loss, the primary would 
have to be sucking energy out. The primary voltage is decreasing though, so 
the efficiency at that point would be negative. I think the "weirdness" is 
caused by secondary loss.

  list <tesla-at-pupman-dot-com> wrote:

Original poster: "Steve Conner by way of Terry Fritz "


 > Original poster: "jimmy hynes by way of Terry Fritz "

 >
 > Hi,
 >
 > Thanks for running the test! Why was the DC voltage so low?

Because I knew that as I reduced the coupling the losses in the IGBTs would
get a lot bigger, and I didn't want to overheat them. I overestimated the
losses though and if I did it again I would start with at least 200V.

Why did you
 > chose 40 bps? Do you feel like runing a high bps test to see if streamer
 > growth affects anything?

I chose 40bps because I wanted a frequency high enough that my scope would
show a steady trace but low enough that streamer growth would be out of the
equation. Again there's no reason why I couldn't have used more at least in
short bursts.

It sucks that testing different frequencies i! s
 > hard. If you put a shorted turn around the secondary, so that it is loosly
 > coupled, it might reduce the inductance enough to raise the frequency, as
 > if it were a different secondary. Any thoughts on this idea?

Maybe but I doubt it would raise it by much. Replacing the topload for a
smaller one would probably be a better bet. The bowls that I made the
topload from come in three sizes :)
 >
 > It has been known forever that higher coupling helps spark gap coils (
yes,
 > even before tesla coils were invented ;-)), and it would make sense that
it
 > would also help OLTCs. In the case of DRSSTCs and SSTCs with untuned
 > primaries, longer rise time helps, because it allows more time for
 > transfer, and therefore, the peak current is less.

I'm confused now, in that case what would be the point of using a tuned
primary on a SSTC? I saw Richie Burnett's SSTC both with an untuned primary
and a ! series-tuned one. With the series-tuned primary it produced longer
sparks and didn't need a breakout point.

I think the main advantage is that the series-tuned primary works
efficiently with a looser coupling, so it doesn't need to be as close to the
secondary, and more voltage can be generated before it flashes over. However
the tuning is more critical especially since Richie's coil isn't a feedback
one.

 >
 > The waveforms look a little bit weird at first, because the peak doesn't
 > line up with the notch, but it makes sense. Because there is loss, the
 > point where energy in equals energy out occurs when the primary is still
 > adding energy. It looks like there is very little damping in the system,
 > how big were the streamers?

At the times the scope traces were taken there were no streamers at all,
just a very small bit of corona round the breakout point. I'm puzzled
because I once tried connecting an IGBT switch to my big coil (running at
much reduced voltage obv)! and I got textbook waveforms all the way out to
the 4th notch. I think it is more probably losses in the _primary_ of the
OLTC that mess up the waveform.

Steve C.



Jimmy