Re: [TCML] QCW sparks

[Steve Ward <steve.ward@xxxxxxxxx>]

Hi Jim,

Is that actually a "resonant frequency" or the frequency at which you're
> driving the system (more Fop than Fres) without concern for where the
> 1/sqrt(LC) happens to be?


For reasons im not exactly sure of, the system performs best when operating
at the "upper pole" frequency.  To explain further, the primary is
self-resonant via primary current feedback to control the H-bridge switching
(circuit delays are compensated out for "true" zero-current switching and
tracking).  So the oscillatory frequency is primarily controlled by primary
inductance.  If i tune the primary for secondary Fres, i can observe a few
ripples/notches in the voltage/current waveforms before streamer loading (or
other loss) has taken over, i believe this is somewhere around 290khz.  In
this mode, the system eventually settles out to operating at either a lower
frequency or higher frequency, determined by the coupling coefficient (the
usual "frequency splitting" stuff).  If it settles out at the lower freq
(about 265khz as i recall), then the spark performance is diminished
significantly, and the sparks do not grow straight.  If it settles out at
the higher freq, then the sparks can grow straight if i want them to, or i
can make them branched if i increase the voltage rise time.  So to make sure
it always settles out at the upper pole operation i reduced the primary
inductance just enough so that its more stable there.

Another thought i had about this condition (Fop = Fupper) (aside from it
merely being a higher operating frequency), is that the primary and
secondary fields should be cancelling (the amp-turns are opposite sense).
It makes sense to me then that perhaps the stored system energy can be
significantly less (less energy stored in the field, more energy going out
to streamers), than if the pri/sec were in phase with fields adding.  I
think the spark performance was on the order of 2X length for the same
primary drive current when considering tuning for Fop being at Fupper vs
Flower.

I can still see the oscillation frequency drop as the streamer grows, i
should make a quick measurement of what that range is next time i test it.


>
> Is that the actual base current (measured how?)
>

Pearson 411 CT on the lead coming off the secondary.


>
>
> I don't know that the spark impedance is going down, but certainly, the
> capacitance of the spark is increasing (and inductance, to a lesser extent),
> so it's reasonable to think that you can push more current into it.
>

Right, and my thought that Z is going down is probably just the increase in
C, i'll run some numbers and see if thats consistent.


>
> I think you might think of your source as more a stiff AC source than a
> traditional LC primary tank that's coupled.


Well, i suppose, but there is still issues of impedance matching to deal
with (determined by k, and the L's and C's), so you just have to look at it
in the steady-state sense rather than the transient sense that we normally
think about for tesla coils.  It really does look more like a special power
supply than a tesla coil.


>
>
>> Another interesting point is that the top voltage is a lot lower than V =
>> IZ
>> would suggest for a lumped model of my secondary coil.  Basically, the
>> secondary is ~25mH, and with 3Apk current through 25mH at 325khz, id
>> expect
>> a peak voltage of 153kV.  The only way i can account for this large
>> discrepancy in apparent impedance of the coil is that there must be
>> significant capacitance from the secondary to primary, so the base current
>> looks much larger compared to say, the toroid current.  I'd li
>>
>
> I'd guess more that the spark is almost doubling the capacitance of the
> secondary system. Capacitance (for "thin" wires) is more dependent on the
> length than the diameter.  You've got a long spark..
>

Basically im confused over what the secondary impedance is and how i could
possibly force 3A through a coil that should have Zo = 50k without having
150kV across it.  Saying that "there is a lot of streamer C" doesn't clarify
anything for me.


>
>
> One test you could do is to just hook a wire onto your system in the same
> place as the spark, and run it at low voltage (below breakout) and see what
> the waveforms look like.  Something like a long coathanger, supported with
> monofilament fishing line.
>
> Or, if you want to get fancy, some thing like a string of 1-10k 1/8W
> resistors..


Might have to try that and see if i can verify the streamer C's that Bert
listed before.

Steve


>
>
>
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