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Re: [TCML] Re: Pros and cons

Tuning DRSSTCs:

Most all tesla coils are 4th order systems.  This means that there are 2
resonant peaks due to the coupling of the 2 resonant circuits.  The DRSSTC
can excite the system at a lower or higher frequency than the natural
resonance, which means that 1 resonant peak (or "pole") is excited more than
the other.  This produces primary currents that never notch back down to 0A
due to energy transfer to the secondary coil (there may be just a partial
notch in primary current).  In some instances of spark production, it can be
useful to detune the primary coil to a lower resonant frequency than the
secondary 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
would not want to detune the system as mentioned.  You would exploit the
classical and well-behaved ideal of tuning both primary and secondary LC's
to be the same Fo.  In this case, i see absolutely no reason why a "DRSSTC
is more sensitive".  Im not sure i understand that comment anyway, but
perhaps what was meant was that "DRSSTCs can work with different tunings".

Another question about your load... How will you suppress sparks from
forming?  Is the load completely under oil?  Or are all the radii something
much larger than say 10"?  Last time i checked, 800kV is about the order of
some very large tesla coils that hurl sparks in the 15-25 foot range (though
not on a single-shot basis).  Also, given the extra capacitance of your
load, im assuming you will need to re-tune to compensate?


On Mon, Dec 7, 2009 at 2:51 PM, <uhvsystems@xxxxxxxx> wrote:

> Steve,
> Thanks for your advice and nice online program you provided link
> to.Yes,forced excitation mode improves the situation here and reduces the
> stress of IGBT components.In the same time it introduces more complex
> topology .I will try to explore first every possibility of the simple IGBT
> coil topology before I *switch to* DRSSTC.Some other things about DRSSTCs
> concern me.Several coilers on this mailing mentioned DRSSTC are more
> sensitive to tunning than SGTCs.Is that true,and why?I can't figure out it.
> Dex,
> Yes,it is challenging.More than I expected to be.I must be prepared to
> challenges though.Many engineers I know actually like challenges.
> Haven't seen the link of 4 kW IGBT coil you provided link to.Look rather
> risky to me.
> As of 4 HVIGBTs in parallel,each overloaded by peak currents about 3500 A,I
> am not sure.Maybe IGBTs will survive,maybe not.Mr.Maybe isn't always a
> gentleman.
> Rereading 3rd Q/A from my previous post reveals a terrible error in it.My
> appology.
> The correct question should be:"What is the output LOAD of the tesla
> coil?".
> Not "the output impedance" like I wrote mistakenly.
> Than the answer that it is (predominantly) the effective capacitance of all
> attached objects makes sense.
> In a real set up that capacitance will be a very changeable parameter
> varying from test to test.
> Regards,
> Fez Zaev
> Steve Ward wrote:
> I just remembered something about the DRSSTC topology that might be
> beneficial to you.  Theoretically, because the energy transfer is spread
> out
> over time, you can achieve 4X the peak energy in the secondary as you ever
> see in the primary (im talking peak instantaneous energy level).  This
> occurs after 1 complete energy cycle of the double-resonant system.  The
> second benefit of the DRSSTC is that the primary characteristic impedance
> can be significantly higher than the single-shot pulse method considered
> before (so likely more than 1 turn primary).  Now of course the down-side
> is
> that you need 2 or 4 switches to make a half or full bridge inverter to
> drive the thing, and now you are also more subject to turn-off losses in
> the
> IGBT.  Luckily the DRSSTC exploits zero current switching.
> Anyway, it may be worth a quick design to see if the DRSSTC has anything to
> offer you (i usually find its the best silicon utilization of any other
> topology i've looked at).  You have the unusual case that your load is
> fairly well defined (compared to a spark), and because you intend on
> suppressing spark formation, the factor of 4X can actually be exploited
> (minus losses of course).  The 4X thing comes at a cost... it takes
> comparatively longer for the secondary voltage to ring up (this can be
> important for spark production, but probably not too critical for you).
> For really quick design iterations, Antonio has a really handy design tool
> for the DRSSTC:
> http://www.coe.ufrj.br/~acmq/programs/<http://www.coe.ufrj.br/%7Eacmq/programs/>
> Antonio may want to comment more on this, and how you might set up the
> simulation for your situation.
> Steve
> Dex Dexter wrote:
> Constructing a low impedance primary with
> a large bang size in the same time presents
> a real challenge,doesn't it?
> I don't know if you saw it ,but if you didn't
> ,here is the link where IGBTs are heavily
> abused by 4 kw tesla transformer:
> http://scopeboy.com/tesla/t4spec.html
> Not the example of a very efficient coil,but
> it works.The point is maybe it's not entirely
> impossible you could drive your coil with
> only 4 IGBT modules in parallel!
> And I am curious too.Can you elaborate bit more
> on the output impedance and the secondary circuit
> test set-up (Q/A #3)?Sounds intersting but I am
> not sure I quite understand.
> Dex
> --- uhvsystems@xxxxxxxx wrote:
> From: uhvsystems@xxxxxxxx
> To: tesla@xxxxxxxxxx
> Subject: [TCML] Re: Tesla Digest, Vol 26, Issue 7
> Date: Sun, 06 Dec 2009 12:11:58 -0500
> Steve,
> Your reply confirmed my fears.
> The same thing about IGBT probable destruction by too high peak currents
> induced
> saturation I was told by my colleague Mayer.No need for the test of this
> sort.It
> would only unnecessarily destroy one IGBT module.If the limit of 1200 A is
> given
> by manufacturer one should respect that and stay within safe margins.Back
> to the
> drawing board..You asked some questions and it is my turn to answer.
> Q: Why not to parallel these HVIGBT modules?
> A:Paralleling 12 of them requires space and increases total parasitic
> inductance
> of the primary circuit.That is not a desirable feature having on mind 1
> turn
> primary coil design.Another thing is that it looks we have only 4 of these
> modules in a stack,and would need ordering 8 more.Not so big problem for
> us,but
> anyway..
> Q:What is OSIV?
> A: OSIV= Oscillatory switching impulse (over)voltage.   Such transients
> occur as
> the consequence of HV switchgear (de-)energization activities,faults
> clearing,distant lightning events etc.GIS monitoring over years clearly
> shows
> OSIVs have high the potency of damaging insulation of HV equipment
> ,triggering
> restrikes during operative sessions of HV circuit breakers,and causing
> various
> sorts of other problems in  power networks.
> Q:What is the impedance of the tesla coil concept under consideration?
> A:The output impedance is the effective capacitance of tesla coil screening
> electrode and the tested objects attached to it.In the cases without
> insulation
> breakdown the energy gets dissipated in the form of the joule heat in
> secondary
> circuits,at decay rates dictated by secondary circuits' Q.
> Regards,
> Fez Zaev
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