Fundamental Limit (was Avalon - First Order Designs)

["Greg Leyh" <lod-at-pacbell-dot-net> (by way of Terry Fritz <twftesla-at-uswest-dot-net>)]

Original poster: "B2" <bensonbd-at-erols-dot-com> 

> What design feature constrains a Tesla coil to a 
> "fundamental limit of 5 MVA"?

I've touched briefly upon this in the past.  The limit isn't 
defined by the power level of 5MVA, that just happens to be 
about the input power where other factors start to adversely 
affect the performance of the coil.

Basically, in order to produce longer arcs the physical scale
of the secondary needs to be increased, otherwise the ground
strikes define the maximum length.  As the size of the secondary
is increased in size, its natural freq must necessarily decrease
owing to increasing values for Ls and Cs.  This can be fought,
but only so far.

There are two necessary constraints for efficient production
of long output arcs:  A) A short discharge/charge time ratio for
the primary capacitors, to get sufficient power multiplication
and B) A sufficient break rate, to prevent the ions in the 
arc channel from recombining.

Constraints A and B fight each other -- A) pushes the break
rate lower, and B) pushes it higher.  But as long as the dwell
time is short enough, the machine can be operated at a break
rate that reasonably satisfies both A) and B).

One of the principal factors that defines dwell time is the
coil's operating frequency.  If the frequency falls too far
then the dwell time lengthens to a point where both constraints
cannot be satisfied, and coil performance degrades dramatically.

This is what I define as the fundamental limit to coil size.

>From observations on Electrum and the 40kW coil I've projected
that a coil's performance will begin to markedly suffer below
a critical operating freq of about 9kHz.  This knee is not 
sharp, but performance degradations at that scale are costly,
and should always be minimized.

The design for the ALF arose out of this limit, and considerable
effort has gone into increasing the coilform size while keeping
the freq at 10kHz, minimum.  At present the secondaries are about
110 feet high and 22 feet in diameter.  The final trick of course 
is to operate *two* towers in anti-phase, which doubles the total
output voltage without changing the frequency. 
-- 


-GL
www.lod-dot-org