Re: NST power rating -- another perspective

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

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

John

I absolutely agree with you.  Power transfer is NOT a simple match the NST
output impedance I think some have questioned.  This simple matching, I
believe,
does work for linear loads for the case where you match the load resistance
to the transformers equivalent resistance and operate at resonance.  Clearly
would stress the transformer.  If you disagree with this, I would be
interested in your reasons.
I have not done the analysis for other than resonance, but I could.  Please
suggest the case that you measured and we can see how close I come
analytically (I don't have all the instrumentation to repeat your
measurements).  I do believe that linear problems can be dealt with
analytically.

In any case, when we add a spark gap, the load is non linear with time.
That was the point of my posting (the first part dealt with linear match at
resonance, and the second dealt with the non linear affects of the spark
gap).  The spark gap is one area that I feel weak on, and I do know it mucks
things up analytically.  My main point was that the VA rating of the
transformer is limited in its usefullness and that the true power (watts not
VA) varies with the operating point.

The transformer model in my posting was intended to be as simple as possible
and yet allow discussion of linear and non linear loads.  My NST model was a
4 component model (vs a more accurate 6 component model) and was based on
actual characterization data.  The transformer impedance that most talk
about (calculated by taking Vs (oc) and dividing by Is (ss) is a value (I
believe) that represents all four components in this model (Rp, Lp, Rs, Ls).
That is why I moved the primary resistance to the secondary so I could
decompose this impedance into its L and R.

I assumed ideal tank cap C and ideal SG only for the purpose of the
discussion.  If a simulation is wanted, this assumption would not apply.

Richie Burnett has done simulations (posted in April 1999) on this that
shows
(with spark gap setting at max spec output and left unchanged) that maximum
power transfered thru the spark gap is fairly broad banded that goes from a
simulation start point of 25% of Cres to about 1.5 times Cres.  Power
changes somewhat but not much and is approximately half of the VA rating of
his NST.  The main thing that changes significantly in this range is the
BPS.  He clearly points out that you can get higher VA's than rating with
this spark gap setting and even higher power output than what his simulation
showed if you set the spark gap to overvolt the transformer and run some
sort of resonant (clearly something we want to avoid with NST's.

Terry Fritz has a derivation of optimum Cp with a static spark gap that sets
Cp to pi/2 * Cres.  His derivation is based on a power (watts) of VA/2 and a
BPS of 120.  His simulations that this derivation is based on seems to agree
with Richie's.

My current experiment is the real problem I want to focus on.  I want to
verify Terry's pi/2 * Cres derivation (at least to my satisfaction) in a
real TC system.  The experiment is to vary Cp and measure spark length in a
controlled way.  The safety SG is set to fire at predetermined value and is
left unchanged during the measurements.  At the Cp being tested, the main
gap is wided until the safety SG fires occasionally.  The spark gap voltage
and quench time is measured with a scope.  The secondary streamer lenght to
free space, distance between topload and grounded rod when corona begins to
occur from the rod, and distance between rod and topload when a power arc
occurs are measured.  I don't have a good way of measureing BPS so i can't
even infer the power being delivered.  I don't have a watt meter so I can't
measure the wall power.  I'm trying to make the best use of the resources I
do have.

Have a good day,

Gerry R
Ft. Collins, CO



 > Original poster: "John H. Couture" <couturejh-at-mgte-dot-com>
 >
 >
 > Gerry -
 >
 > I believe if you do the tests with graphs (resistance vs wattage) of a NST
 > that you will find exactly what I found. Some of your comments below will
 > apply. You will need for the primary a voltmeter, ammeter, and wattmeter.
 > For the secondary you will need a HV voltmeter, ma meter, and a bunch of
 > power resistors and HV capacitors. Record all of the data, at least 9
 > columns, and make a graph of resistor load (x) and watts (y) to find the
 > maximum input and output power (watts). The curve will be a hump type at a
 > certain resistive load. The ma meter in the sec circuit will give you the
 > current. You can then find the sec power output by the equation
 >           Output watts = R x I^2
 >
 >
 > You will find that this is not the standard power transfer problem. The
 > problem is what NST voltage times current will give you the maximum power
 > output? The power output refers to real power output not VA output.
 >
 > You may also be interested as I was in making graphs for resistance load
vs
 > sec volts, resistance load vs pri amps, pri VA vs sec VA, etc. Then add
 > capacitors to the loads and you will become an expert on NST operation
with
 > Tesla coils.
 >
 > John Couture
 >
 > ----------------------------------
 >
 >