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,

Thanks for running the test! Why was the DC voltage so low? Why did you 
chose 40 bps? Do you feel like runing a high bps test to see if streamer 
growth affects anything? It sucks that testing different frequencies is 
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?

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.

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?

Tesla list <tesla-at-pupman-dot-com> wrote:
Original poster: "Stephen Conner by way of Terry Fritz "

Dear list,

After the recent discussion on efficiency of pulsed SSTCs etc, I decided to
do those experiments that Jimmy Hynes suggested.


.oO METHOD Oo.
I ran the mini OLTC three times, once with the secondary in normal
position, once with it raised by around 1", and again raised by about 2".
In each case I measured the output with an antenna made from a metal cup on
an insulating base 18" from the topload, and connected to a x10 scope
probe. I adjusted the DC link voltage so that the peak-peak antenna voltage
(and hence hopefully the topload voltage) was the same on each run. I
adjusted the quenching for first notch in each case, and measured the
maximum spark length between a breakout point and a grounded point. All
tests were c! onducted at the same rep rate of around 40bps. More details of
apparatus and method on request.

.oO RESULTS Oo.
Here are the scope traces I got. The top trace is antenna voltage
(500V/div) and the bottom one is primary capacitor voltage (100V/div)

For high coupling http://www.scopeboy-dot-com/tesla/scopehighk.jpg
DC Link voltage was 100V and maximum spark length 41mm (1.6")

For medium coupling http://www.scopeboy-dot-com/tesla/scopemediumk.jpg
DC Link voltage was 115V and maximum spark length 40mm (1.6")

For low coupling http://www.scopeboy-dot-com/tesla/scopelowk.jpg
DC Link voltage was 133V and maximum spark length 40mm (1.6")

Accuracy/repeatability of spark length measurements +/-2mm (+/- 0.08")

.oO DISCUSSION Oo.
First of all it's obvious that the spark lengths are the same to within the
measurement uncertainty. However, looking at the scope traces, the envelope
of antenna voltage does not show the waveform on! e would expect. There is an
obvious dip, and the peak of the secon dary envelope does not match the
notch in the primary envelope. This may be due to corona losses dragging
the topload voltage down. It is observed that moving a grounded point close
to the breakout causes the dip to get bigger as streamer corona forms. Or
alternatively it may be due to energy from the secondary sucking back
through the anti-parallel diodes after quenching.

Secondly, as the coupling is decreased, it takes a good deal more primary
capacitor voltage to produce the same topload voltage. With high coupling,
around 150V is required, but with low coupling it has to charge to more
than 200V, i.e. at least double the energy. Other experiments I conducted
suggest that the Mini OLTC system is around 55% efficient (From wall plug
to topload :) ) with high coupling, so with low coupling it would be a
miserable 20%.

.oO CONCLUSIONS Oo.
Rise time of the topload voltage envelope does not seem to affect the spark 
length between points to any significant degree. For all of the coupling
factors investigated, the spark length was about the same as would be
expected for a DC voltage of the same magnitude as the peak topload
voltage. This suggests that impulse-related breakdown effects are not
significant in this scenario.

On the other hand, low coupling makes the system much less efficient. More
power is wasted as I^2R heating, corona, radiated RF, etc. during the
ringup period, simply because it takes longer. Hence low coupling would
reduce spark length for a given input power, and make components run
hotter. So practical coils should still aim for the highest coupling
possible, with due consideration to racing sparks or poor quenching.

.oO FURTHER WORK Oo.
Maybe the rise time of the envelope does not affect spark growth, but there
is still a possibility that the rise time of each individual cycle, i.e.
the resonant freque! ncy of the coil, might. Someone else can investigate
this one beca use I don't really feel like making three more mini OLTCs
identical except for different gauges of wire.

Also, the impulse characteristics of a gap are dependent on the shape of
the electrodes. Maybe the results without a breakout point would be quite
different. The Mini OLTC won't do anything without a breakout point, but
the next one sure will

Finally, the rep rate used was too low for streamer growth. If we had been
trying to grow a streamer between points using multiple bangs, the results
might have been different.

any comments?

Steve C.




Jimmy