re: 20 joules at 100 bps vs 4 joules at 500 bps

["Tesla list" <tesla@xxxxxxxxxx>]

Original poster: FutureT@xxxxxxx 

repost of my bps comparisons posting from
Date: Mon, 22 Feb 1999 18:01:18 -0700
All,

I did some tests at a higher power level.  First I installed a .0148uFcap 
with the 120 bps sync gap, then compared with the 400 bps rotary gap.  At 
1370 watts, the spark reached 54" at 120 bps, but at 400 bps, the spark 
could not reach 54" even at 2000 watts.  But it came close.  However, the 
spark was brighter and more flamelike at the high break-rate, and wanted to 
go upwards towards the ceiling.  The ceiling is 36" above the top of the 
toroid.The sparks looked completely different at 120 bps.At this low break 
rate, the sparks were not as bright, but they tended to surge and grow, and 
emitted from different spots around the toroid.  The sparks floated around 
slowly.  The high break-rate sparks tended to be all about the same length 
and,seemed to want to remain together like a giant blow-torch or flame, and 
reminded me a little of tube coil sparks(high powered tube coil sparks of 
course).  So all of this was the same as at the lower power level, but on a 
larger scale.  I tried a higher break rate of about 600 bps, but the sparks 
got shorter, due to sagging voltage in the power supply.  Of course,in any 
case, the voltage on the caps was always lower at the highbreak-rates, 
because the system was unable to benefit from resonant charging voltage 
build up.  Next I replaced the 1500 watt 14.4kV potential transformer with 
(2)1500 watt 7.2kV potential transformers in series for a "stiffer" power 
supply.  With this set-up I got 54" sparks at 1000 watts at 120 bps.At 400 
bps, the sparks were very bright, but never reached 54" but they hit the 
ceiling a lot and seemed more powerful than with the one transformer.Then I 
replaced the 20" toroid with a 6" by 26" dryer duct toroid, This 
arrangement gave 58" sparks at 1050 watts, this I believe is a record for 
efficiency for my coils, based on my formula:spark length (inches) = 
1.7*sqrt input power, wallplug (watts).The formula predicts 55", but this 
coil gave 58" sparks.  I attribute the greater efficiency to the use of two 
transformers, which reduces the losses.Note; those with NST powered systems 
are cautioned to never use the NST power rating written on the NST in my 
formula above. Many NST systems use resonant charging and actually draw 
double or more the NST rated power input.  The only way to know the input 
power of any Tesla coil is to measure it with a suitable wattmeter.If a 
wattmeter is not available, at least use an ammeter, and multiply by the 
input voltage to obtain input VA (volt-amps).  This will not correct for 
the power factor error, but will give a ballpark measurement.  In my coils, 
input current distortions and/or high frequency components did not seem to 
affect the meter accuracyby much.  My formula above uses true input 
wallplug power.  I measure the power before any variacs, ballasts, etc.  I 
then tried using 400 bps, but the sparks jumped to the ceiling too often 
and would not reach 58", so this made it impossible to properly measure 
them accurately, although I did get a general idea.  At 600 bps, the sparks 
did not get shorter, due to the use of the stiffer power supply.  The 
sparks seemed bright and strong at these higher break-rates.I used the same 
wattmeter as my tests from last weekend, which should give reasonably 
accurate results.Table of results:

Cap (uF)     bps     watts      toroid (inches)     spark (inches)

.007         120      620                5 x 20       42

.007         400      1000               5 x 20       43

.0148        120      1370               5 x 20       54

.0148        400      2200               5 x 20       less than 54

.0148        600      2200               5 x 20       weaker

.0148        400      3000               5 x 20       59 one time

.0148        120      1000 (2) xfrmers   5 x 20       54

.0148        120      1050 (2) xfrmers   6 x 26       58

.0148        400      2000 (2) xfmers    6 x 26       went to ceilingbut 
approached 58

.0148        600      2000 (2) xfrmers   6 x 26       not weaker

.0148        400      1000 (2) xfrmers   6 x 26       36 (low cap Volts)

.0148        400      1050 (2) xfrmers   5 x 20       41

.007         400      1000 (1) xfrmer    3 x 10       30

In the second to last test above, I of course had to keep the variac turned 
down somewhat to hold the power level at 1050 watts, so the bang size was 
probably the same as in the last test, so it'sreally the larger toroid that 
is increasing the spark length in the second to last test.  It's hard to 
know how much voltage was onthe caps in any of these tests.For a given 
input power, the low break-rates seem to outperform the high break-rates by 
a significant margin in all these tests, as far as input power vs. longest 
spark length is concerned.I found that at high break-rates, I had to use a 
much smaller ballast inductance than at 120 bps.  At high break-rates, I 
used about 2mH to 10mH.  At 120 bps, I used about 15mH to 25mH.The ballast 
was adjusted for best TC efficiency, and smoothes operation in all 
cases.  High break-rate systems have greater losses due to:1.  .  I do not 
know how much of the highbreak-rate under-performance results from losses, 
and how much may be inherent to high break-rates (if any).  I would not be 
surprised if the losses at high break-rates are much lower(relatively 
speaking) in large coils compared to small ones.It is possible that by 
using 120 bps, and scaling up a TC in size,that the spark lengths might 
outperform the predicted lengths given by my formula.  Let's see....around 
.03uF should begood for a 100" spark, at 120 bps, at 2800 watts, using a 
5kVArated 14.4kV pole pig for low loss.  This is assuming the sparklength 
can outperform my formula.  A  7.5" x 35" toroid will be needed. This might 
be too optimistic but would be interesting to try. John Freau