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RE: Power in a TC System


To All - The following is a method for a complete TC coil design that has
never been presented before. Losses are not ignored.

Hi Gavin,

I was happy to hear that you found the graphs in the TC Design Manual to be
helpful to you. All of the graphs in my books were made from data obtained
from real world coils that was available at the time the graphs were made.
Much more TC information is available now but the graphs will still get you
ballpark numbers for your coils. The Fig 14 graph can be used to solve many
TC energy problems when using the equation
   J = .5 C V^2
because it gives you the relationship between two of the variables (J and
C).

The Fig 14 graph is a logarithmic graph that can make nonlinear equations
appear to be linear. The equation  is
  Cp = (6.89^10^-5)(W^0.85)+0.000698
where Cp = pri capacity   W = input watts
This equation was made by entering the collected coil data into Borland's
Eureka program. The equation is empirical so the accuracy is dependent on
the data obtained. The graph was made by entering the equation and other
data into Borland's Quattro program.

The main differrence between output power and output energy is that output
power can be greater than input power but output energy can never be greater
than input energy. Coilers presently use random spark lengths for comparison
outputs only. A method to measure TC output that is acceptable to all
coilers is still in the future.

Finding the optimum value for the primary cap capacity or any other
parameter for a particular Tesla coil can only be roughly determined at
present. The reason is that we do not have enough information on the losses
for TC systems. Terry Fritz is the only coiler I know of who has found the
input to output losses for one of his coils. The primary losses (mostly
spark gap) for his coil were 62% of the input energy leaving 38% for the
primary cap.

For your coil the primary cap capacity using Terry's losses can be found
approximately by
  Cp = 2*A*J/Vp^2
where Cp = pri capacity  J = joules/break  Vp = pri peak volts and A is a
factor representing the losses. The energy available from your 5KV 25 ma 125
VA NST per break is
  J = 125*.38/100 = 0.475 joules/break

Your optimum primary capacitor
  Cp = 2*J/Vp^2 = 2*0.475/(5000*1.4)^2 = .019 uf

This is close to the graph cap value of .005 uf at 15000 volts
   Cp = (.019)(5000*1.4/15000)^2 = .0043

The next step is to design the secondary circuit. This is most easily done
by using the JHCTES Ver 3.1 program that is now available as a free Internet
download. With this program you can design dozens of secondary circuits that
will be in tune with the above primary capacitor or any other capacitor. One
possibility is
  Sec radius = 2 ins   Lgt = 14.7 ins   Turns = 1000
  Turns/inch = 68 (28 AWG)  Sec terminal = 15 pf

The program will give you 17 outputs to complete the design. The terminal
can be a sphere or toroid. All of the parameters can be changed and the
computer will do the necessary recalculations to keep the TC system in tune.

There is much more to designing, building, and adjusting Tesla coils but
that will come only from experience.

Comments are welcomed.

John H. Couture

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

-----Original Message-----
From: Tesla List [mailto:tesla-at-pupman-dot-com]
Sent: Wednesday, May 17, 2000 5:09 PM
To: tesla-at-pupman-dot-com
Subject: Re: Power in a TC System


Original Poster: "Gavin Dingley" <gavin.dingley-at-astra.ukf-dot-net>

Hi John,
I have a copy of the Tesla Coil Design Manual and so know that you take a
practical engineering approach to coil design.
My goal in this question is to find a mathematical, but practical rule for
calculating the primary capacitor value for a given NST knowing it's max
power
rating and rms voltage output. I want to get the maximum power out of the
NST
without it being destroyed/ magnetically saturated etc. One formula states
that
the reactance of the capacitor should be equal to X = V/I, where V is the
NST's
output voltage and I is the maximum current that can be drawn from it. It
goes
on to say that the capacitor value that comes out of this relation can be
doubled due to the low duty cycle of the resulting impulsing current.

I had a look at the graph in the above mentioned book (fig 14) and found the
graph to be linearish (big emphasis on the "ish"). From it I deduced a very
rough formula:

C = POWER (VA) / 45,000      uF

This in not perfect and personally I have got away with more capacitance
than is
lifted from the graph (12nF for a 5kV/125VA NST). Of course, the graph
probably
represents NST voltages in the range of 10kV and so would not give a good
indication for my 5kV set-up.

I can see what you mean regarding energy and power, the streamers from the
secondary isotropic are more to do with energy, mainly due to the very low
currents flowing in them. Put at the primary circuit, we must be talking
power.
At the end of the day average energy over time equals power and this is
fore the
whole process from NST to streamer. But it is get the most from your tank
cap
that matters to me at this time, the rest has too many variables as you have
pointed out.

So, how do you calculate the highest value of primary tank cap given NST
specs,
while taking into account the irregular current wave form drawn from the
NST?

Thanks for your help and look forward to your reply,

Regards,

Gavin, U.K.