Re: [TCML] SSTC full bridge control system question

[Steve Ward <steve.ward@xxxxxxxxx>]

Adjust the primary tuning for best energy transfer to the secondary.  This
can be somewhat pointless to do in simulation, other than just getting a
feel for how a 4th order resonance behaves to various inputs. The reason
modeling it is almost pointless is that no one has a really good model of
the streamer load (i have some models that are closer than others ive seen)
which really determines largely the behavior of things.  Also, 1MV is a HUGE
tesla coil output and likely unrealistic.  My big coils are about 700kV peak
i estimate from base current and other simulations, and i need about 1600Apk
primary current to get there.

Anyway, im not sure if your analysis is right.  The double peak comes from
the mutual inductance between the coils, where the M either adds or
subtracts from the apparent resonant inductance, which means there are 2
peaks, one just below Fres and one above.  Increasing the coupling (hence
more M) causes these peaks to move further apart as the adding/canceling of
the M term is more dramatic.  Generally, the most *efficient* tuning method
is to tune the primary to the secondary which results in primary current
notches.  The primary current notch is indicative of complete energy
transfer to the secondary (secondary I and V should be peak during primary
I/V minimum).  The transfer time is essentially controlled by the coupling
coefficient, where it should take 1/k cycles for the transfer to take place
(so k=0.1 needs 10 cycles, k = 0.125 needs 8 cycles etc..).  This places
some upper bound on the energy you can deliver to the spark within one
energy transfer cycle, for small coils it often ends up you cant get enough
energy transferred within 8 cycles or so.  So the other trick is to tune the
primary to excite just one of the resonant "poles" which means the primary
current should not notch, and the currents/voltages grow consistently over
time (until the streamer starts to consume all of the energy in the
system).  For my "transient" systems i find tuning to the lower pole
frequency works well because streamers tend to detune in that direction
anyway, which seems to make the system happy.  Tuning the primary really low
can allow you to effectively increase the "bang" energy to really large
amounts, allowing you to make really long sparks, provided your silicon and
capacitors can stand it.

Steve

On Sat, May 1, 2010 at 12:59 PM, Michael Twieg <mdt24@xxxxxxxx> wrote:

> Hello, this is my first post, please be kind,
> I'm working with a team of engineers to build a large solid state musical
> tesla coil, and my task in to project is to build control and telemetry
> hardware.  We've chosen a full bridge design, a two coil configuration
> (flat
> spiral primary), and we're using an interrupter based loosely on steve
> ward's design to control its switching.  Like his, it senses primary
> current
> and switches on zero crossing.  I've built the hardware and verified that
> it
> works in small scale testing.
>
> When simulating the two-coil system, I'm observing the peak splitting in
> resonance, and noticing that of the two peaks, one gives a higher gain in
> voltage on the secondary (the resonance of the secondary coil), while the
> other gives a low primary impedance and thus a high primary current (this
> is
> the resonance of the primary coil).  It seems that it would be preferable
> to
> operate at the secondary resonance, since it gives higher voltages with
> less
> primary current.  However, I've found that in simulation, it is impossible
> to get the interrupter controller to operate at the desired secondary
> resonance, and it instead operates at the primary resonance.  This results
> in excessive currents in our full bridge (much more than our IGBTs will
> like) for modest secondary voltages (we're aiming for between one and two
> MV).  I'm unable to find any combination of coil parameters which allows
> the
> controller to operate at the secondary resonance.
>
> So my question is, is there any way to get a controller to operate at the
> secondary resonant frequency while switching at zero current on the
> primary?  If not, then how are people like Steve able to attain high
> secondary voltages without requiring enormous (like several KA) peak
> primary
> currents?  Is there something wrong with the way I'm characterizing the
> coil
> system or the controller?
>
> Thanks in advance,
> -Mike
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