[TCML] Re: Pros and cons

[uhvsystems@xxxxxxxx]

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/

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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