Re: [TCML] DRSSTC tuning at high power

[Steve Ward <steve.ward@xxxxxxxxx>]

Mike,

On Thu, May 26, 2011 at 10:06 AM, Michael Twieg <mdt24@xxxxxxxx> wrote:

> Yeah I've always been on the watch for detuning as power level changes.
>  The
> weird thing is I observe that as power and streamer length increases, our
> measured operating frequency also increases.  Completely counterintuitive;
> it should decrease as streamers grow.
>

This is peculiar, now that you mention this I will have to see if i get
similar results on my own systems.  Is your measurement just what the
oscilloscope displays for a frequency measurement? Or are you looking at the
period between zero crossings?

I cant think of an explanation for it yet, either way.


>
> Is it really true that a higher K makes the system more immune to detuning?
>

Well maybe?  The energy transfer is faster (less cycles) so the accumulated
phase error would be less by the end of the burst?


> I can't think of why this would be the case, but I think I've observed it
> myself (originally we tried tuning at k=0.1 and had a real trouble at low
> power, but at k=0.12 we were able to nail it).


Interesting.  I still consider K = 0.12 to be "medium-low" coupling, though
i think thats approximately where i operate my larger coils.


>  How would I know if the
> tradeoff between tuning and voltage gain no longer worth it?
>

I used to pay more attention to my "Freau Factor" which was part of John
Freau's spark length (in inches) vs input power (in watts) relationship:

Spark_length = K*sqrt(wall_power)

I think John claimed that his best performing spark gap coils would have a K
= 1.7 or so, at 120BPS.  I found some of my DRSSTCs to get nearer to K =
2.0, but i cant recall if that was with power factor correction or not.
Well lets see, I seem to recall my coil makes approximately 10' sparks with
3500W (DC bus power) at 120BPS, so my K = 2.0.  Of course K goes down as BPS
goes up, since i dont gain enough spark length at higher BPS, but input
power is roughly linear with BPS.  But i personally would say if you get a K
> 1.7 (at 120BPS), then you are probably getting near the best performance
that most tesla coils have achieved.  So until more theories are worked out
as to how to better optimize things, this seems to be the most simple
benchmark to use.

As a side note, whenever i play music through DRSSTCs, i apply a pulse width
truncation vs frequency (the curve was determined mostly experimentally but
follows a sqrt(freq) trend mostly).  That way you can squeak out the high
notes at reduced pulse width (which does reduce the spark length) and save
your power controller from going into current limit and dropping the output
voltage.  Id imagine its more efficient to run shorter pulse widths at full
bus voltage than it would be to run full pulse widths at reduced bus
voltage, but that might vary from system to system.  And on top of this, i
actually boost the pulse width for the lowest notes (anything below 100hz or
so) as they seem to have less benefit from ions/hot air left over from
previous sparks, and could stand to use some more bang energy.  Simple
controls, but extremely useful for this application.

Steve


>
> -Mike
>
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