RE: FW: Re: Tesla Coil Efficiency Test

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

Original poster: "Dave Larkin by way of Terry Fritz <twftesla-at-qwest-dot-net>" <teslaman15-at-hotmail-dot-com>

Hi John,

>It is obvious from your explanation below that we are talking about two
>different types of TC efficiency. You are referring to a transfer 
>efficiency
>based on  E = 0.5CV^2 and I am referring to an overall black box 
>efficiency.
>These two efficiencies will never be the same for any particular Tesla coil
>because the transfer efficiency leaves out many of the necessary TC system
>losses that the black box efficiency takes into consideration.
>
>Both methods, however, can be used  for finding the efficiency of a Tesla
>coil depending on what you are looking for. The transfer efficiency will
>always be greater than the black box efficiency because of losses that are

The transfer efficiency is of interest to coilers because it tells us how 
well the spark gap is performing.  The 'Black Box' efficiency would also be 
an interesting metric, how exactly does one go about measuring it?

>omitted. The efficiencies found by both methods will decrease as the TC
>becomes larger. This is contrary to most electrical apparatus where larger
>means greater efficiency. One reason is because larger with Tesla coils
>means larger voltages rather than larger currents. The higher voltages mean
>higher losses and they increase faster than the input wattage. This

I would have to disagree here, everything I have ever observed suggests 
Tesla coils get vastly more efficient as they get bigger. Coils in the 500VA 
class normally burn about 50% of their power in the spark gap, whereas a 
50kVA coil *probably* burns 20% or less of its power input as gap losses.  
The relationship between volume and area (and hence inductance and corona 
loss) would suggest that the power loss/unit inductance will in fact 
decrease as the scale of the coil increases.


means
>there is a secondary voltage limit for every size of Tesla coil. Above this
>voltage the insulation breaks down and the coil is damaged.

True, however given that even the largest coils ever built only just break 
the 1MV barrier I don't think any decently designed system will ever operate 
near the breakdown voltage of its resonator.

>The energies in the primary and secondary circuits represented by  0.5CV^2
>are easily determined if the primary and secondary voltages can be 
>measured.
>However, finding the secondary voltage usually means making only an 
>estimate
>of what the scope is indicating. The reason is because of the difficulty of
>calibrating the scope and secondary probe which usually gives poor 
>accuracy.
>These problems are eliminated when using the black box method for finding
>the TC efficiency.

Well..... A calibrated spark gap, a field mill or one of terry's fibre optic 
probes can all give you a secondary voltage to at least +/- 25%, if not 
better.


>I agree that finding the output energy of a TC "sparking machine" will
>always be difficult and may be only a guesstimate. The output energy for 
>the
>black box method, on the other hand, can be determined very accurately. The

But surely the only true 'black box' output measurement would be the power 
coupled to the arc?

>question arises, however, as to how accurately the black box efficiency
>represents the TC sparking machine efficiency.
>
>  For coilers who are bored with output spark tests the efficiency tests 
>are
>a good alternative. These tests would help to advance our knowledge of TC
>operation. It should be noted that for these tests you do not need a coil
>that produces the longest spark. You only need coils that are of good 
>design
>and properly tuned. It should also be noted that TC efficiencies can only 
>be
>obtained by tests and cannot be obtained by computer simulations. There are
>just too many unknowns in the TC system to do this job with computers.

Great advances in that area have been made in the last few years, and I 
think we can now computer model coils at least as accurately as I've ever 
been able to measure them, and probably better.

-Dave-

>
>John Couture