RE: NST power rating -- another perspective

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

Original poster: "John H. Couture" <couturejh-at-mgte-dot-com> 


Gerry -

If one watt second (one joule) can produce an output spark length of 8.5
inches can two joules produce 17 inches? That is a good question because we
can also ask what would a 10 KVA (10 KW) Tesla coil produce if we assumed
the square root law?
   sqrt(10000/120 BPS) = 9.13 watts/break x 8.5 inches = 77.6 inches spark or
6.5 feet?
This would be a controlled spark.

John Freau's equation gives
   1.7 sqrt(10000) = 170/12 = 14.17 feet
This is a random spark with unknown input joules.

It is obvious that the one joule TC input for 8.5 inches spark output may
not be extended for higher TC inputs. However, the one joule per 8.5 inch
test is a fact and is a simple test that can be easily repeated by any
coiler. This test makes more sense from an engineering standpoint compared
to the typical random spark test where the actual joule input per spark is
an unknown.

Until more coilers do the test and study the problem the answers to the many
new TC operating questions will have to wait. It will now be possible to
look at TC operation in a more informed light. The BPS or breaks per second
are also a part of TC operation that needs to be studied in more detail.

At present the real world design parameters for optimum TC operating
conditions are unknown. However, if you want to increase the spark length
increasing the input watts, secondary inductance, or TC income voltage will
help. Because of the losses the size of the TC system will limit these
possibilities.

The best way to show the TC "power" relationships is to draw the VA, Watts,
and RVA values as a right (Pythagorean) triangle. The watts as the x axis
and the RVA as the y axis. With this triangle you can easily show how a VA
can be greater than Watts. The secret is in the power factor.

In your tests you should use 100 Watts instead of the 100 VA, etc.    Watt
seconds is energy and work done while VA seconds includes circulating energy
that is returned to the source by the reactive components. To do proper
input metering you need a voltmeter, ammeter and wattmeter. This will also
give you the power factor as a decimal
   PF = watts/VA
The TC input circuit is similar to the NST test with a load that includes a
resistance and a capacitor.

When a coiler tests a NST with different resistance loads he will find that
the maximum wattage output that he will get from the NST will be about 25%
of the nameplate rating on the NST. The reason for this is that as the
resistance is reduced the NST current  increases but the output voltage
decreases (to zero). The output power (watts) will be the product of current
times voltage and will never be more than about 25% of the nameplate rating.
Coilers who say that their NST is producing 50% of nameplate watts output
are not metering their systems correctly. It is obvious the maximum power
theorem does not apply to NST's. The power  transformers that are not
current limited are another story.

In my tests of a 7.5KV, 30 ma, 225 watt NST with various resistor loads the
maximum power output was 59.3 watts (26.4% of nameplate 225 rating). The
output watts was the same as the output VA. The input watts and VA were
never the same. The input watts will always be greater than the output
watts. When I added HV  capacitors to the load the input and output
conditions changed drastically.

John Couture

----------------------------------


-----Original Message-----
From: Tesla list [mailto:tesla-at-pupman-dot-com]
Sent: Sunday, October 12, 2003 3:20 AM
To: tesla-at-pupman-dot-com
Subject: Re: NST power rating -- another perspective


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

Hi John,

I can see I need to stop making serious posts at 2:00am.  I think I have now
confused three or four of the great ones in this group and to each of you,
my apologies.

My original post was in response to a thread where someone was thinking is
was a "con" that you could not get the VA rating out of an NST in terms of
watts.  Someone in the thread was trying to match to the NST output
impedance and were saying that only 25% of the VA rating in watts was
possible (if I read it correctly).

I tried to give an example of where you could get even more than the VA
rating out of an NST in terms of real power, but the method is wholly
inappropriate for a TC application and it probably would result in the NST's
death.  I don't think I have ever advocated doing that kind of matching for
a TC.  I agree with you that a TC is a totally different beast and simple
impedance matching does not apply.

I would like to put a silver stake into any controversy that I may have
stirred up.


  > Original poster: "John H. Couture" <couturejh-at-mgte-dot-com>
  >
  >
  > Gerry -
  >
  > The problem with matching the output of the NST with the TC system is
that
  > the NST optimum and TC optimum conditions are never the same during TC
  > operation. My tests indicated that the NST power output was in the shape
of
  > a humped curve and depended on the amount of the output resistance load.
  >
  > The point of maximum NST power output would almost never equal the point
of
  > optimum TC input power conditions. In the tests when a capacitor is
  > introduced and resonance is a factor the situation becomes very
complicated.
  > To my knowledge no coiler has ever attempted to find these unique NST and
TC
  > operating conditions.

My view on this is that we don't worry about maximum NST power output and
instead optimize for what we want the most (this may differ from one person
to the next).  For me, it is optimum TC output (maximum spark with a
reasonable frequency of occurrence).  I think (and again I'm still learning)
this corresponds to optimum TC input power conditions and optimum BPS (there
may be some trade off in this respect). For static sparkgaps there is
certainly a tradeoff between bang energy and BPS and the optimum combination
may be different for different reasons. I have already found two combination
I like but for different reasons.

  >
  > In my TC Design Manual I indicate that the matching of the power
transformer
  > to the primary capacitor should be done by matching the transformer
wattage
  > to the amount of capacitance of the primary capacitor. This would
guarantee
  > that there would be the correct amount of energy available to properly
  > charge the capacitor. The resonant charging problem is another issue.
  >
  > With your tests of pri cap (a axis) vs Spark length (y axis) you should
get
  > a humped curve if Terry is correct. The maximum spark length for a
15KV/60ma
  > NST would be at
  >    pi/2 x Cres = 1.57 * .0106 = .01664
  > if I understand you correctly. I have never tried this test.

That would be the implication.

I did get a humped curve and the peak was at Terry's pi/2 x Cres (defining
peak as longest spark).  I also found a point at a slightly smaller Cp that
did not compromise spark length that much and the voracity of the spark
frequency was noticibly higher.  I still don't know which I like better so
maybe I'll note the two Cp values with my yet to be built larger coil and
program it for the affect I wish to achieve at that time.  So far my
experiments (and experience) are at 100VA.  My next stop is 200VA.

  >
  > To do my NST/TC testing I use a voltmeter,ammeter,and wattmeter with
  > controlled sparks output. Unlike the typical random streamer spark tests
  > this arrangement can give me a known energy input for each spark output.
For
  > the tests I use a fixed 120 watt second (energy) input to produce
controlled
  > sparks. The breaks per second are then one per output spark and the
output
  > is spark length per watt seconds. As an example in my tests the maximum
  > spark length is 8.5 inches per watt second of energy. The watt seconds
  > include the losses so it does not take much energy to produce an 8 inch
  > spark. This length will vary depending on the size of the TC system.

Does this mean that 2 Joules of energy with produce a 17 inch spark or does
it follow a sqrt relationship.

  >
  > It may be possible to find the NST/TC unique conditions by finding the
  > intersection of two curves. One curve would be the NST power output vs
input
  > resistance, a humped curve. The other curve would be the TC input
  > resistance/impedance vs the TC output power or spark length, another
  > non-linear curve. I have never been able to do this because of the
  > difficulties in determining the TC input curves. My preliminary tests
  > indicate the input TC sqrt(L/C) impedance calcs are of little use.
  >
  > Coilers have made great progress in finding the parameter values of the
  > Tesla coil system but we have a long way to go in finding the details of
the
  > Tesla coil operations.
  >
  > John Couture
  >

You folks have so much experience.  We all benefit from your sharing and I
for one thank you.

Have a great day,

Gerry R.
Ft Collins, CO