Re: NST power rating -- another perspective

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "Gerry Reynolds" <gerryreynolds-at-earthlink-dot-net> 


Hi Terry,

I really appreciate your in depth comments and think the discussion adds a
lot to the readers.  My orignal intent was to keep the model simple with the
purpose of focusing on the primary side of things.  I was not trying to
model the entire system and maybe I should have stated that better.  I agree
with your comments in real life and will respond to how I think I handled
some of that in the simplified model.  BTW I tried an experiment with symbol
fonts for ohms so everywhere you see resistance that appears to have W or kW
for units this was meant to be ohms or kohms..



 > Original poster: Terry Fritz <teslalist-at-twfpowerelectronics-dot-com>
 >
 >
 > BTW - A trust you have seen the papers at:
 >
 >
http://hot-streamer-dot-com/TeslaCoils/OtherPapers/TeroRanta/CurrentLimitedTransformers/NSTModel.htm
 >
 >
http://hot-streamer-dot-com/TeslaCoils/OtherPapers/TeroRanta/NSTCapMatching/ResonantCapacitorMatch.htm
 >

Yes, Great papers.


 > The ESR of "real" Tesla coil primary caps is a critical thing.  But does
 > not mater for the analysis here.

Yes. Beyond the purpose here.

 > >Now comes the complexity.  We add an ideal sparkgap that we can control
 > >"the when" and "how long" it fires.
 >
 > I would submit that there is no such thing as an "ideal spark"
 > gap.  Consider an ideal cap charged to 20,000 volts and an ideal switch
 > across it in an ideal circuit with no resistance.  If you close the
switch,
 > ideally no power is expended. :o)))  You can ask our quarter crushing
 > friends to verify that :-D  You can only push the ideal stuff so far.  In
 > this case, the ideal current is infinite and the analysis fails on
highbrow
 > theoretical grounds.  But again, for the purpose of your analysis and
since
 > you do have Rs which should take into account gap loss too (IMHO) I will
 > let it go ;-))  As long as no currents or voltages approach infinity, you
 > should be fine.

This question was presented in school and is indeed interesting
theoretically.  I think the answer is the energy gets radiated away.  Since
my model is a partial model (just for charging purposes).  I'm not trying to
put a switch directly across the cap even though the partial model says
that.   Behind the scenes, I am assuming that there is a primary coil with
primary inductance to limit the initial discharge current. I am assuming a
zero resistant switch that of course does not exist and is not important for
the intent of this discussion, and I'm not trying to imply infinite current
for these purposes.


 > >We remove Rl and neglect the TC primary inductance for charging
 > >purposes.  The only components for this consideration is Vs, Ls, Rs, C,
 > >and the sparkgap (standard topology).  The TC primary in reality, will
 > >control the discharge rate of C and affect the energy transfer time.

 > >This is a non linear circuit that often results in a lot of confusion
 > >(myself included so don't fret).  First, we realize that a charging
 > >interval (at 60 Hz) is 8.3ms, so we will fire the ideal sparkgap every
 > >8.3ms when Vc reaches peak.

 > >For my coil, the energy transfer time is 18us (to 1st primary notch) and
 > >we realize this is much less than a percent of the charging time.  Lets
 > >assume all of the 1/2 CV^^2 energy goes thru the sparkgap and into the
 > >secondary never to be seen again.
 >
 > Watch out!  If you store all the energy in the primary coil (lossless with
 > no R) and open the switch, the voltage across it goes to infinity hitting
 > my ideal warning condition above!   That can invalidate the rest...

See below,  I'm opening the ideal sparkgap when no energy exist in the
primary after it has all been transfered to the secondary so I'm trying to
avoid the infinities in this discussion.

 > >Otherwords, lets open the sparkgap 18us after it fires.  Now we have a
 > >pseudo linear circiut that would normally not have any real power
 > >delivered (remember we removed the R in the load) but now we are removing
 > >real energy at a rate of 1/2 CV^^2 times the break rate (BPS).
 >
 > With infinite voltage across the primary coil, it may not be right to go
 > further.  Maybe try modeling this.  Or try adding 0.0001 ohms of
resistance
 > and running models on "almost ideal" circuits.  You can't have an ideal
 > switch switching infinite voltages without such an analysis going
 > invalid...  I think you need to add the secondary L-C just to store the
 > energy so the energy stored in the primary coil at gap opening is zero and
 > the voltage at opening is also zero so as to keep things happy.  Then you
 > need to discharge the secondary energy "magically" to keep the circuit
sane
 > (if you add resistance, that messes up you primary side model.  But it
 > should be ok to assume the secondary energy is "somehow" dissipated).

It was not my intent to model the secondary, I'm just assuming it is there
and properly dispatches the energy before the next bang.


 > >This real energy transfer rate is REAL POWER and has to be replaced by
the
 > >charging circuit.  The hard question is how much is this REAL POWER, how
 > >do you optimize for it, and what other constraints do you need to
consider
 > >(like not overvolting the transformer at resonance).
 >
 > One big one (30% of a coil's power loss) is that nasty old spark gap.  All
 > that bright light and noise can't be neglected.  Solid state spark gaps
 > also run into areas and conditions where they are going to "spark"
 > regardless of how wonderful they are.  MY OLTC plays many tricks to be
able
 > to open it's solid state gap at "bad" times...  Mostly by burning off
 > energy like mad...

This is true and ultimately needs to be considered.

 > >I'm still learning the answer to this but believe the best way is by
 > >simulation.
 >
 > Yes!! :-))))  But try to think of and have us do experiments to keep the
 > models on the right track.

I think you wont have a problem with my model if you understand its intent
which admittedly wasn't stated very clearly.

I do have experiments designed for this very purpose, however, and will
report on them when finished.

 >
 > >I DO believe the real metrix for spark length is the REAL POWER
transfered
 > >thru the sparkgap and not the VA rating for the transformer (at least NST
 > >types).  This REAL POWER will certainly be porportional to VA (everything
 > >else being the effectively the same), will vary with Cp, the sparkgap
 > >setting, and the resultant BPS (static gaps).
 >
 > I too think this.  John's formula suggests streamer length is directly
 > proportional to the square root of streamer power.  You have to optimize
 > all the impedances (from wall plug to streamer) and then reduce all the
 > losses except those of the streamer.  One things that really gets messy is
 > that spark gaps and especially streamer impedance is very dynamic through
 > the firing cycle.  I "sneak" around that by using equivalent resistances
 > assuming they are "one number"  But the range of the "right number" varies
 > over many orders of magnitude and I think I loose a lot there in modeling.
 >
 > >It will probably be very close to the POWER measured in the line cord
 > >using a WATT meter (I^^2 R loses in the transformer would need to be
 > >factored out).  The actual line cord VA could be significantly larger
than
 > >the VA rating of the transformer and will ultimately depend on your
chosen
 > >operating point.
 >
 > If you get into the frightening SLTR mode, you could probably pull a few
 > thousand watts out of an NST until it blows the secondary windings.

I do plan on getting into the frightening SLTR mode, but it will be
controlled and I will report on the results (good or bad)  I'm willing to
sacrafice this small transformer if I can learn from it.  I do not know its
design application, just the characterization data I took (it is current
limited).  The xformer is over 40 years old and I have probably abused it in
the pass without knowing it.

 >
 > Neat that you are working on all this!!  There are many many unanswered
 > questions.  Try to think up real experiments that can be performed that
 > could help verify or test things.  I found that real experiments and data
 > were crucial to keeping models on the right track.  Goodness knows we have
 > the stuff to do such testing.

I agree and the test are underway.

Take care,

Gerry R
Ft. Collins, CO