RE: Tesla Coil Efficiency Test

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

Original poster: "John H. Couture by way of Terry Fritz <twftesla-at-qwest-dot-net>" <couturejh-at-mgte-dot-com>


Terry,

Defining Tesla coil efficiency can be a problem. Engineering efficiency is
usually defined as
   useful work out/work in
But with a Tesla coil what is useful work out, voltage, watts, a spark,
noise, incandescent lamp, etc? That is why I thought the "black box" method
would simplify finding the TC efficiency. You only need to know the input
and output data of the black box to find the efficiency. An incandescent
lamp is a useful watt load out and the variac adjustment can be used to find
the watts in.

As you are one of the few (maybe only) coilers who has determined the
efficiency of one of his coils by the "internal losses" method it would be
interesting to see how this result compares with the "black box" method.

With your coil the output = 283.5 watts and the input = 892.3 watts. This
gives

            eff = 283.5/892.3 = .3177 or about 32% efficient

I would estimate that with an inductive coupling of about .70 your coil
would light a lamp
            283.5 x .70 = 198.45 watts
or a 200 watt lamp to almost full brightness. You would need a light meter
to do the calibration of the lamp. If the lamp output was dimmer that would
mean the coil efficiency was less than 32%. However, that assumes the .70
coupling is correct. This coupling factor (K) would have to be verified by a
separate test which could be done by the frequency test (Tesla Coil
Construction Guide p 14-10)

       K = sqrt(1-(Fo/Fs)^2)

Maybe you or some other coiler will find the time to do these tests to
advance our understanding of Tesla coil operation. As for accuracy, the
Tesla coil like any other electrical device will have an efficiency that
will vary depending on the type of load. Electrical resistance (incandescent
lamp) is the least complex type of electrical load when properly applied.
With the lamp you can determine the TC output energy very accurately, much
more accurately than for the streamer. You can measure the voltage and
amperes required for a certain lamp brightness to determine the TC output
energy to better than one percent. Any changes you make in the coil will
show up as a change in the lamp brightness which will be detected by the
light meter.

The best way to check the accuracy of the Tesla coil efficiency tests is to
use more than one type of test and compare the results. Losses for motors,
panel lights, etc, can be ignored. The efficiency of small TC's (under 15
watts) runs about 85%. For larger coils (over 15 watts) the efficiency runs
about

       eff = 1/(log(watts))

This is for coils that are of good design and are properly tuned.

Your coil  = 1/(log(892.3)) = .3389 or about 34% efficiency

Would efficiency be a better way to compare Tesla coils than the output
spark method? It would make more sense from an engineering standpoint. It
would also be more fun watching the lamp get bright, like watching a snail
race, rather than watching those boring streamers! (:})

John Couture

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


-----Original Message-----
From: Tesla list [mailto:tesla-at-pupman-dot-com]
Sent: Sunday, June 16, 2002 10:57 PM
To: tesla-at-pupman-dot-com
Subject: RE:Tesla Coil Efficiency Test


Original poster: "Terry Fritz" <twftesla-at-qwest-dot-net>

Hi John,

There are a few problems we run into when we talk of Tesla coil
"efficiency".

First, is how do we define "efficiency"?  My small coil has a rotary gap
whose motor draws a significant amount of power.  Do I count that?  What
about heat loss in the variac windings?  What about the 25 watts of lights
on the power panel.  Then we can get into power in power out issues...  So
we need some definition.

"I" always took the power input as the voltage and current out of the NST.
All the other stuff before that is variable and is not of great interest to
us.  I disregard the motors, lights, and other odd things that are not
really part of the primary/secondary system.

For power out I look at "streamer power".  That is how much heat the
streamer produces (I assume the ground return path is perfect).  This is
the pure integral of the product of streamer voltage and streamer current
over time.

So basically, efficiency is "streamer output power / primary system input
power".  It is really quite simple since the only place other than
streamers for the power to go is into the spark gap.  So all we need to do
is find the 'power into the coil', 'power lost in the gap', and 'power in
the streamers'.  We really only need to find two since the third is easily
calculated.

We then must consider "accuracy".  How accurate does our measurement need
to be?  I could just "say" the efficiency is 25% to 75% and be right :-)
But that is not very accurate.  If I want to get to 10% or accuracy or 1%
accuracy it's a whole new ball game.

The test you describe will "work".  You just need to figure out how you are
defining efficiency and accuracy of the numbers.  I submit that the answer
will be "poor".  Possibly as high as 50% off.  But if that is "good
enough", it does work :-)

We also run into the problem that if we carefully measure one coil, the
results will not be real applicable to another coil.  Tesla coils are just
too different from unit to unit to really blindly compare them.

Unfortunately, really accurate measurements are not easy or cheap.
However, they are easily possible.  There is no reason they cannot be made
to within a few percent accuracy other than the test equipment gets too
expensive.  I would submit that we do have an accurate efficiency
evaluation for at least "my" coil :-)  And now others are getting or have
the same capabilities too.

Input power is actually pretty simple.  Just find the firing voltage of a
sync gap system and the input power is 120 x 1/2 x C x V^2.  Just the
energy stored in the primary cap released 120 times a second.  A Pearson
style wide band current probe and a voltage probe run into a digital scope
can capture the voltage and current waveforms of the NST output too to give
the actual input power to high accuracy.  That method just cost thousands
of dollars more!  But is certainly does work.

Spark gap loss is pretty difficult do to the noise at the gap fouling the
measurement equipment.  But a 50,000 amp Pearson current probe and two 40kV
TEK probes run into a TEK 3014 scope would make it child's play, $8000
latter...  No great technical challenge there other than paying for it.
The scope can subtract one voltage from the other for the true differential
voltage, then it can multiply that by the current with gigahertz sample
rates, do the RMS power calculation, and then display the number...  You
don't even have to write it down since it will store it on a floppy disk
for you :-))

Streamer voltages and currents are a bit harder but more fun.  Streamer
current can be measured with fiber optics to good accuracy and bandwidth.
Terminal voltage can be measured with plain wave antennas to "ok" accuracy
and high bandwidth.  Fortunately, streamers do not load the terminal too
much so that voltage is pretty good, but streamers can affect the fields a
plan wave antenna sees.  So you have to be careful and realize the voltage
may not be very accurate.  I am presently working an a coil that does have
a direct terminal voltage probe built in:

http://hot-streamer-dot-com/temp/HVProbe-2.gif

Of course, all this is assuming one makes no compromises in the
measurements.  By doing things like equivalent loads (such as 220k ohm +1pF
per foot of streamer) we can get a whole lot of good numbers.  We may not
see every blip in streamer power, but the efficiency is still an excellent
number.  Computer models can then crunch the numbers very accurately for
such a purpose.

http://hot-streamer-dot-com/TeslaCoils/MyPapers/modact/modact.html

I think you are looking for data like:

http://www.pupman-dot-com/listarchives/1999/June/msg00323.html

One nice thing about computer calculations is that you can make small
changes and see how they affect things.  Computers are not affected by high
voltage, noise, setup errors, and other problems with "real" testing.  This
is what I did with the k value testing in the above.

So to finally answer you question...

Neon Primary = 30.6 watts
Neon Secondary = 38.87 watts
Filter Resistors = 98.4 watts
Spark Gap = 386.5 watts
Secondary Losses = 54.4 watts
Power to Arc = 283.5 watts
Total Power = 892.3 watts

Power into the coil is 892.3 - 30.6 - 38.87 - 98.4 = 724.43

Efficiency is 283.5 / 724.43 = 39%

Like I have always said, spark gaps eat about 1/2 of a coil's power...  The
other losses are pretty high too but the gap is what really kills
efficiency.  Even at 39%, this LTR coil is a strong performer.  I guess I
would have to say your number of 80% if far too high.

Interestingly, if one were to use a solid state gap and put about 300 more
watts into the streamer, the efficiency would jump to 80% and increase
streamer length 44%.  Nothing else would change but the gap which would run
cold and silent...

Cheers,

	Terry




At 09:27 PM 6/16/2002 -0700, you wrote:
>
>Antonio -
>
>I agree that this test loads the secondary and reduces the output voltage.
>However, don't we want to have a secondary load so the useful output is not
>zero?
>For the TC efficiency shouldn't this be the integrated and RMS
>output/input,  otherwise, the efficiency would be an instantaneous or peak
>type?
>With a high enough inductive coupling to the secondary the inductive
>coupling to the primary would be negligible?
>
>I also agree that your test method is a better one but have you or any
other
>coiler been able to make this kind of test? What were the results? In My
>book I show how Breit, Tuve, Dahl found (1930) their coil to have about 25%
>efficiency using a similar method as yours. The output voltage was
>determined by a special calibrated spark device.
>
>It is interesting that after 100 years the Tesla coil is about the only
>electrical apparatus for which we do not have an accurate efficiency
>evaluation.
>
>John Couture
>
>-----------------------------
>
>
>-----Original Message-----
>From: Tesla list [mailto:tesla-at-pupman-dot-com]
>Sent: Saturday, June 15, 2002 9:06 PM
>To: tesla-at-pupman-dot-com
>Subject: Re: FW: Re: Tesla Coil Efficiency Test
>
>
>Original poster: "Antonio Carlos M. de Queiroz by way of Terry Fritz
><twftesla-at-qwest-dot-net>" <acmq-at-compuland-dot-com.br>
>
>Tesla list wrote:
>>
>> Original poster: "John H. Couture by way of Terry Fritz
><twftesla-at-qwest-dot-net>" <couturejh-at-mgte-dot-com>
>>
>> Many coilers have tried to measure the efficiency of their coils in the
>past
>> with varying results. In my Tesla Coil Construction Guide book I show a
>test
>> method that I used for a small TC that appeared to work for TC
efficiency.
>> However, I am now having second thoughts about the test results and
wonder
>> what other coilers think about this type of test.
>
>> The input energy is found by the setting of the variac. The output energy
>is
>> found by the intensity of an incandescent lamp inductively coupled to the
>> secondary coil.
>
>There are several problems with this technique. First, the maximum
>voltage
>and maximum current from the transformer don't have a so direct relation
>to the actual power going into the system. Second, with a lamp coupled
>to
>the secondary, the Tesla transformer will be heavily loaded, and all the
>waveforms will be different from the normal. The lamp may be also taking
>energy directly from the primary, and not from the secondary, with
>inductive
>coupling.
>
>A correct method for efficiency measurement would be to first measure
>what is the peak primary voltage V1 where the primary gap fires,
>and then to measure what is the maximum voltage level V2 reached at the
>secondary terminal, without breakout. The efficiency would then be:
>E = C2*V2^2/(C1*V1^2)
>where C1 is the primary capacitance and C2 the secondary capacitance
>(combination of Ccoil with Cterminal, almost their sum).
>An oscilloscope can make all the measurements, with an adequate
>HV probe or attenuator fo the primary voltage and an antenna for the
>secondary voltage, with a calibration obtained by exciting the secondary
>terminal directly with a signal generator at the resonance frequency.
>Complicated, but I don't see shortcuts. This is a difficult measurement.
>Even with all care, heavy interference is expected.
>
>Antonio Carlos M. de Queiroz
>
>
>
>