Re: ALF: why not DRSSTC?

["Tesla list" <tesla@xxxxxxxxxx>]

Original poster: Steve Ward <steve.ward@xxxxxxxxx>

Hi Greg,

Just a few comments within


The key feature of an RTC is its ability to *quickly* (in less than 2
cycles) return the secondary energy back to the primary after leader
production in the arc channel drops off -- typically about 2-3 cycles
into the envelope.


I notice that on my DRSSTCs, depending on tuning and the mode of 
operation, i can get most of my streamer production in about 3-4 
cycles (which seems to support your findings with Electrum) and my 
primary current falls to zero within maybe another 2-3 cycles after 
driving has ceased.  I need to look at the secondary current to 
verify just how quickly the energy is zapped out of the system.  The 
DRSSTC recycles un-used energy as well, though perhaps not as 
effectively as your scheme.


The ALF 1:12 prototype is actually more distant from the off-line
OLTC concept than the ALF, in that it uses 4 paralleled power
transformers to isolate the primary drive from the AC mains.  Tsk,
transformers... yes, I know.  This prototype unit will also operate
as an RTC to test out basic concepts and determine overall effectiveness.


Im awaiting this test of the RTC!

DRSSTC
The DRSSTC, as I understand it, employs a full H-bridge of switching
elements to drive the primary of a Tesla Coil in a series resonant
configuration.  Secondary feedback controls the operating frequency
in addition to having a tuned primary, hence the term 'double resonant'.
The quasi-CW nature of the DRSSTC results in lower stored primary
energy and RMS currents, easing the stresses on the primary capacitor
and IGBTs.


I use primary feedback now.  This is very benificial, not just from a 
reliability point of view, but it offers some interesting operating 
modes.  One such trick is to detune the primary (tuned maybe 25% 
lower than the secondary Fo).  The primary builds energy for maybe 20 
cycles (on a 220khz system, much less cycles on a larger 
system).  Once the secondary starts to form a little bit of corona, 
the secondary Q drops quickly.  There is relatively high coupling 
between the coils (K=.2).  So now all of this energy built up in the 
primary gets sucked out in maybe as few as 4-5 cycles.  After 
supporting the streamer for maybe 3-4 cycles, the inverter is shut 
down.  The little remaining energy stored in the primary and 
secondary is rectified back to the buss caps through the diodes in 
the H-bridge.  Primary current decays to zero in maybe 2-3 cycles 
after driving has ceased.

IMO there would be several key design challenges in extending the
DRSSTC to >1MW power levels:
A)  Managing the fast commutation dynamics between the IGBTs and
back-diodes, since zero-voltage switching is not possible in this
configuration.
B)  Effectively recovering unused secondary energy, since the
quasi-CW nature of the DRSSTC requires many cycles to transfer energy
between coils.  Typical DRSSTC envelopes appear to require about 10 cycles.


Im not so sure i agree.  While the primary might operate for a long 
period of time, it seems *most* of the energy is transfered very 
quickly right at the end.  Some of my coils can operate with 6 cycles 
or so as well.

C)  Managing circuit reliability and total parts cost, with the
larger number of IGBTs that a full DRSSTC H-bridge requires.


But, you are talking of using a 12kv supply, requiring seriesed 
switches, while you might obtain the same results from an H-bridge 
running at a lower voltage.

Of these, the slower energy transfer characteristic would likely be
the biggest obstacle.  Since the Fo for ALF is unusually low (~6kHz)
there is not sufficient time to operate in a quasi-CW DRSSTC mode, as
10 cycles of 6kHz would use up most of the time available between
pulses at 300PPS


Why operate at such high pulse rates?  It seems as long as you keep 
the streamer channel active, you could get the same spark lengths at 
100bps or maybe even less.  Im sure some might argue that just 
reducing your break rate would have a much higher return (in spark 
length vs input power) than the RTC topology even offers.

To quote from your other post:

" believe that it's not only possible, but essential to determine the
best topology at the beginning.  Simulations can accurately model
much of the complex behaviour exhibited by TC's, and good
physics-level models now exist for the HV IGBTs."

Exactly, this is the only true reason i asked this question to begin 
with.  I wanted to know if the DRSSTC topology (or something similar) 
has been considered.

Steve