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Tesla Coil toroid Size-Measurements??
Hi John, All,
At 01:00 AM 9/15/98, you wrote:
>
> Terry, Bart, All -
>
> I agree that measuring the TC input energy (power x time) would be
>difficult. However, this test can be greatly simplified if a constant load
>is used (controlled sparks). The advantage of a constant load in testing
>any electrical apparatus is that the input conditions can be easily
>correlated with the output conditions. The reason is because instantaneous
>time does not have to be considered. Both the input and the output
>waveforms are automatically averaged out. The overall losses are also
>averaged out.
>
I think the input signals are too complex to just use their average.
Perhaps a true RMS meter could give a reasonable average RMS value of both
the current and voltage but the power factor jumps around quite a bit. For
the fine measurements we are dealing with here, you need more accuracy.
> If the test conditions are properly set up the power factor will be close
>to 100 percent. The constant load (sparks) will be mostly resistive.
But the input to the transformer driving a capacitor and spark gap will be
far from a resistive load. I have never seen anything close to a unity
power factor. Is there a way I can set such conditions up that I am not
aware of?
>
> For example if the input is 960 watts and the bks are 120 and the sparks
>are 2 feet long then the TC is producing 8 watts per break (including
>losses) and four watts per foot of spark. The effect of an existing
>conductive path in the air and some other considerations like Vp and Ls,
>etc, come into play, but this would at least be a start. This could then be
>compared with the JHCTES computer program which is a watts to controlled
>spark program.
> JHC
>--------------------------------
>
But how do you measure the 960 watts?
I have several scope pictures of the input voltage and current waveforms of
my coil under various conditions. They are posted at:
www.peakpeak-dot-com/~terryf/tesla/misc/
This is a 15kV /60mA driven coil with about 20nF in the primary circuit
(111kHz Fo).
inp002.jpg - These are the voltage and current waveforms without the gap
firing. The top trace is 50 volts/div and the bottom trace is 5 amps/div.
Note that the voltage is leading the current by 3.4 mS (73 degrees) giving
a power factor of 0.285. Hardly 100%!! In this case the RMS voltage is 40
volts and the RMS current is 5 amps. If you just multiply the voltage and
current you get 200 watts. However, if you then multiply by the power
factor, the real power is 57 watts! Without taking the power factor into
account you miss the true input power by 350%!!
inp003.jpg - This shows the coil with the gap firing but without breakout.
The power factor is now 0.49 and the waveforms are close to sine waves.
inp004.jpg - Now with sparks. The PF=0.675 but look at the distortion!! A
true RMS meter could give the correct "average" RMS values but the power
factor is unstable. This is were you need a true power meter. Just taking
the voltage and current values and multiplying them by the power factor may
have significant error. Of course, you would have to have a way of knowing
at least the average power factor. Note that the scale is now 100v/div and
10A/div.
inp005.jpg to inp008.jpg These are more of the above at different time
scales. inp006.jpg shows that the gap fires at somewhat different times
relative to the input voltage further adding to the measurement problems.
The streamers were about a foot long. I kept them short to keep from
blowing anything up. At full power the waveforms would no doubt be even
more difficult to quantify. The voltage was measured by a step down
transformer across the neon's input. The current was measured with a
fiber-optic current probe. The scope's scales were adjusted to calibrate
both against measured values.
I feel that the input power is a poor measure of a coil's performance. I
can dump hundreds of watts into the coil and get anything from no output to
long streamers depending on all kinds of things. So is there a good
measurement to use?? I don't know of any. It is hard to even try to get
the best output from say a 15kV/60mA neon because some can use resonant
charging at extreme limits while others (like me) don't use this effect (I
paid real money for my neon :-)). The best I have ever come up with is the
primary firing energy multiplied by the firing rate. But even that needs
good equipment to measure. I will be working more on measuring the
real-time voltage and current of output sparks in the near future which may
shed some light on how the arcs are dissipating energy. However, the true
measurement of Tesla coil performance is very complex and I am not
convinced simple meters can even begin to provide any real data.
Terry Fritz
References: