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Output Voltage vs. Firing Rate (fwd)
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From: FutureT-at-aol-dot-com [SMTP:FutureT-at-aol-dot-com]
Sent: Monday, August 10, 1998 5:26 PM
To: tesla-at-pupman-dot-com
Subject: Re: Output Voltage vs. Firing Rate (fwd)
All,
I did some measurements of power usage and capacitor energy at
various break rates, etc. I had some trouble with my scope; the trigger
circuits seem a little damaged, so perhaps others will do some similar
tests to verify. The question too of course is; are these findings specific
only to my set-up, or are they of a more general nature? I used a 14.4kV,
1.5kVA, potential transformer to power the TC. Ballast is adjustable from
2mH to 19mH in steps, toroid is 4" by 17", secondary is 4" by 23", #28
formvar, close wound, primary is 19 turns, #12pvc ins. stranded wire,
close wound at 15 degree inverted cone. Cap is .007uF.
Test 1: Using this 42" spark TC with sync-gap, the sparks easily hit 40",
at 640 watts (wallplug) as measured with a wattmeter. From past tests,
I know that the capacitor charges to 32kV at this power input level. So
the cap energy is 3.58 Joules per bang, X 120 BPS = 429.5 watts
passing through the caps. The difference between 640 watts and 429.5
watts is the energy wasted in the charging system, including the variac,
filters, ballast, transformer, wiring, etc. This amount of loss is not too
bad.
Test 2: I installed a 12 point series quenching non-sync rotary gap in
place of the sync-gap, and ran the coil at about 240 BPS (actual) or
so. There seemed to be two firings per half cycle, and the gaps fired
at about 21kV, I forget the exact voltage, but it came out to 3.27 Joules
per half cycle (actual joules X two). So I was putting a little less energy
into the spark than in test 1, and the sparks did not hit 40", but they
may have reached 37" or so. The immediate suggestion here is that
the spark length is determined roughly by the total energy applied to
the tank. This is very significant and will require more precise testing
to pin down the exact relationship. The sparks at this break rate did
not grow and waver slowly as they do with the sync gap, rather they
seemed to flare out in a fan-like manner with occasional bolts. But the
sync-gaps sparks fan out too...so the differences are subtle. The spark
action was faster and more frantic. This is a well known phenomenon
at high break rates, especially at these low power levels. The sparks
were not too frantic, since the break-rate was still actually quite low.
The conclusion here is that the higher break-rate does not seem to give
any benefit over the low break-rate for a given amount of energy applied
to the spark, from the caps, and in fact the low break rate seemed better.
But because of possible measurement errors, I can't say the low break
rate is definitely better with equal cap energy throughput. Also, toroid
size and texture, and the overall TC size, may affect the spark
appearance.
What was alarming in this test, was that the input wattmeter showed
1000 watts, versus 620 watts in the sync-gap test, even though the
sync gap (120BPS) sparks were longer. So a lot more power was
being wasted. I don't know yet where this waste was occuring; but
transformer saturation, generally higher losses in components, gap
losses, power arcing, gap "re-firing", etc., come to mind. The main
variac setting for this test was similar to that used in test #1. Also,
for best results I had to re-adjust my ballast to a lower inductance.
I'm considering the installation of a more robust transformer to see if
this reduces the losses. But the sync-gap 120BPS operation seems
to give a smoother and lower-loss charging, as would be expected.
Test 3: Next I tried an even higher break rate that gave about 4
sparks per half cycle. Now the caps charged up only to about 14kV,
I don't have my notes here, but anyway the total energy came to about
2.9 joules per half cycle (joules per bang X 4), but the coil may have
given 5 breaks per half cycle sometimes, so this would raise the joule
figure. In any case, the input power was similar to the other tests,
maybe a little lower, and spark was about the same length as in test 2.
The joule calcs suggest that the caps are only drawing about 400 watts,
not counting losses. Larger or smaller toroids didn't help in any of
these tests, because I'm using a proper size to start with.
Again, the spark length seemed to depend mostly on the total energy
put into the sparks, more or less. However, my charging system losses
became even greater at this higher break rate. The system drew about
1800 watts. I had to use even less inductance in the ballast choke for
best results. I also tried adding some resistive ballast in the primary,
and also in the transformer secondary, but this didn't help. The losses
are about 1200 watts which is extreme.
In both tests 2 and 3, the safety gap tended to fire, even though the
scope showed a relatively low voltage on the caps. Either the high
break-rate was promoting corona on the safety gaps, or some sort
of kickback or RF was affecting the safety gap, I suppose.
Some questions are: Can these losses be reduced easily, where are the
losses occuring, and do typical high break-rate Tesla coils show these
high losses? The key is to measure the voltage on the caps during TC
operation, calculate the joules and watts, and compare this with the
actual measured input power (wallplug), as a starting point. If higher
charging circuit and/or other losses are typical of high break-rate TC's,
then this must be taken into account for general TC design work.
Certain non-traditional charging systems may give better results at
the high break-rates, since losses may be lower in these designs.
These tests suggest that given suitable charging efficiencies, either
method, low break-rate with large bang, or higher break-rate with
smaller bang size, may ultimately be roughly equivalent. If true,
this will provide for a greater flexibility to the Tesla coil design art,
and may permit a tailoring of the spark appearance to suit various
needs.
John Freau