Re: [TCML] Re:Pros and Cons

[Steve Ward <steve.ward@xxxxxxxxx>]

Sounds like a reasonable solution.  Its good to have someone with the
experience of driving series HV switches.

About the 25 foot discharges (maybe off by a factor of 2, maybe not), this
occurs because of an effect we call "streamer growth".  Above some
repetition frequency, the previous spark event leaves a heated air channel
for the next spark event to travel along, and further extend its reach.
Measurements of large tesla coils revealed that the peak top voltage was
significantly less than the spark length suggested.  Anyway, the spark can
typically grow to perhaps 3X the "single shot" length (the single shot
length follows the generic kV/cm type figures you see quoted).  So on a
single shot basis, 800kV is in the 1 meter range, but as you exceed about
60pps, this streamer growth phenomena starts to show up and you might see 2
meter long sparks.  At 100pps you might see 2.5 meter sparks, and at say 300
pps it might be a full 3 meters long.  So essentially, once a spark
*starts*, and you provide sufficient power (in pulse per second) it *will*
grow much longer!

Anyway, i seem to recall DC Cox claiming that his "Big Bruiser" Tesla coil
(which makes about a 25 foot spark at peak) only generated about 850kV.  If
DC could supply the specs on his primary/secondary L/C and peak charge
voltage, we could figure out the theoretical maximum secondary voltage.

Greg Leyh also has some big coils and perhaps has some secondary voltage
measurements?

So i know pretty much nothing about the DUT, but if it has any exposed radii
of less than maybe 6", its likely a spark will break out and grow to very
huge lengths if operating at 100pps.  To me it sounds like a fairly good
challenge to suppress spark formation at 800kVAC!

Steve



On Wed, Dec 9, 2009 at 12:59 PM, <uhvsystems@xxxxxxxx> wrote:

>
> Steve,
>
> Thanks for the explanation of DRSSTC tuning.I was wondering...
> Good news is that we will not have to construct DRSSTC which fullfils our
> requirements.Yesterday,I talked to Mayer and ,in mere 10 min of a coffee
> break,we found the solution to the problem of tesla coil primary circuit
> :-)Looking back it is kind of funny that what I disliked the most now turns
> out to be the best option.
> We are going to use 8 IGBT modules arranged in 2 paralleled stacks in
> series connected 4 IGBTs.Mayer knows pretty much everything of the safe
> driving of such configurations (with or wihout auxillary circuits).He
> designed 12 MVA AC/DC step-down transformerless converter for powering 3000
> VDC railway network where HVIGBTs were connected in series stacks.
> Thus,only 1.8 uF charged to about 14000 V will be enough to enable primary
> energy of about 180 J.Obtained benefits  are clear.Primary impedance and
> inductance increase,tesla coil efficiency increases,peak IGBT current drops
> below 1900 A,tuning by varying primary inductance becomes possible.The only
> downside we will need to get 4 more CM 600HB-90H modules (relatively
> acceptable).
>
> I am not convinced I get your question about load and oil.Are you curious
> about E-field control techniques and electrostatic grading structures,or you
> are asking what we have in plan to test?
> Regarding 800 kV and 25 feet discharges I must say I can hardly believe
> this. 25 feet is aproximately 8 meters.
>
> And you're correct,we will need to retune to compensate.
>
> Regards,
>
> Fez Zaev
>
>
>
>
>
>
>
> ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> Tuning DRSSTCs:
> Most all tesla coils are 4th order systems.  This means that there are 2
> esonant peaks due to the coupling of the 2 resonant circuits.  The DRSSTC
> an excite the system at a lower or higher frequency than the natural
> esonance, which means that 1 resonant peak (or "pole") is excited more than
> he other.  This produces primary currents that never notch back down to 0A
> ue to energy transfer to the secondary coil (there may be just a partial
> otch in primary current).  In some instances of spark production, it can be
> seful to detune the primary coil to a lower resonant frequency than the
> econdary coil (i could explain why, but its getting off subject).
> For your case, you are NOT trying to pump more energy into a spark, so you
> ould not want to detune the system as mentioned.  You would exploit the
> lassical and well-behaved ideal of tuning both primary and secondary LC's
> o be the same Fo.  In this case, i see absolutely no reason why a "DRSSTC
> s more sensitive".  Im not sure i understand that comment anyway, but
> erhaps what was meant was that "DRSSTCs can work with different tunings".
> Another question about your load... How will you suppress sparks from
> orming?  Is the load completely under oil?  Or are all the radii something
> uch larger than say 10"?  Last time i checked, 800kV is about the order of
> ome very large tesla coils that hurl sparks in the 15-25 foot range (though
> ot on a single-shot basis).  Also, given the extra capacitance of your
> oad, im assuming you will need to re-tune to compensate?
> Steve
> _______________________________________________
> Tesla mailing list
> Tesla@xxxxxxxxxx
> http://www.pupman.com/mailman/listinfo/tesla
>
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