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Re: OLTC update
Original poster: "Antonio Carlos M. de Queiroz by way of Terry Fritz <twftesla-at-qwest-dot-net>" <acmq-at-compuland-dot-com.br>
Tesla list wrote:
> Original poster: "Terry Fritz" <twftesla-at-qwest-dot-net>
First of all, congratulations for the nice work.
> I have found that I set the secondary capacitance to 37.3pF in error last
> night. It should have been 30.5pF. The secondary frequency was 34.7kHz
> instead of 38.3 like it should have been. So the tuning was 10% off in
> frequency resulting in only 25% power to an air streamer. That explains
> the short streamers to air but big arcs to ground.
k=0.095 is mode 10-11, with 5.5 cycles for energy transfer. Tuning
starts to be critical, but 10% of error would make little difference.
An interesting question is how the tuning affects the voltage gain.
The tuning becomes more critical when the coupling is low.
Note that the presence of the other terminal, that receives the
arcs, causes significant mistuning, and even the presence of the
breakout wire can make a little change.
> I may have to pull a
> few turns off the secondary eventually. The primary inductance is only
> like 410nH! That is how I am getting such a nice high Fo frequency. I
> think the coupling needs to be around 0.10. The input power was about 225
> watts. The IGBT heatsink got just slightly warm (30C) with a long run. If
> the coil is not breaking out, I really have no idea where the power goes.
> Perhaps into secondary heating.
I like to see this by observing the primary voltage without quenching.
The presence of deep notches shows that the tuning is correct, and the
peak of the first beat maximum after the first notch indicates how much
energy was transferred back from the secondary. Interpolate between the
initial energy and this energy and you have an approximation of how
much energy was transferred to the secondary at the first notch.
Here for example: http://www.coe.ufrj.br/~acmq/tesla/vc1910.jpg
The voltage at the second beat peak is approximately 3/4 of the
intial voltage, indicating that 9/16 = 0.56 of the initial energy
returned. The energy that was transferred at the first notch is then
((1+3/4)/2)^2 = 0.77 of the initial energy, approximately.
> So I'll tune things up and work on it some more to see if the streamers
> perform more as expected. I will be writting all this up too. I will
> probably get everything running at 120VAC and do destructive IGBT testing
> before going up to 240VAC. Now that the coil is working, I find I would
> like to keep it that way ;-))
It's easy to tune a coil at low power by observing the primary voltage.
Just adjust the top load until you see deep notches. Approach your
hand to the terminal to see if you need more or less capacitance. At
-low- power, of course. Losses don't affect much the tuning.
Antonio Carlos M. de Queiroz