Re: Practical limit to number of turns on primary ? ? ?

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

Original poster: "Malcolm Watts by way of Terry Fritz <teslalist-at-qwest-dot-net>" <m.j.watts-at-massey.ac.nz>

Hi Jolyon,

On 27 Jan 2003, at 7:41, Tesla list wrote:

 > Original poster: "Jolyon Vater Cox by way of Terry Fritz 
<teslalist-at-qwest-dot-net>" <jolyon-at-vatercox.freeserve.co.uk>
 >
 > I too am intrigued by emphasis on turns rather than relative
 > impedance of primary and secondary. According to theory would it not
 > be possible to get a voltage step-up using a secondary with same
 > number or fewer turns than the primary if only the the secondary
 > inductance were greater; this of course would require a wider
 > diameter on the secondary with respect to the primary so perhaps
 > coupling would suffer.

Since the coupling is a function of geometry for uniformly wound
coils (i.e. flux linkages always connect to the same degree with the
same percentage of turns in such coils when they are positioned the
same relative to each other), coupling will suffer if the primary
ends up being a tiny coil sitting in the middle of a giant secondary
(the inverse applies also). The number of turns does not actually
affect the coupling.

       An example of this actually contributed to a drawback in
Tesla's CS machine. His idea of a "free resonator" was upset to some
extent by mounting it within the walls of the pri-sec system. I
measured this on a scale model (and checked the finding on both the
model and the real thing with Dr Reszotarski's MandK program) and
found there to be coupling to the extent of k = 0.06 between the
extra coil and pri/sec system (this despite the large difference
between coil diameters. The model actually performed better in spark
operation with the extra coil placed outside and away from the pri-
sec coils and there were clear differences in scope waveforms between
the two setups. Placing the extra coil outside the pri-sec system as
Robert Golka did was a real innovation.

       In fact 0.06 is really too low for a 2-coil so the effect
mentioned is seen to be deleterious for both cases for different
reasons. Sorry if the digression is considered irrelevant.

  Therefore, could it be the larger number of
 > turns normally used on TC secondaries is nothing more than a
 > "necessary evil" to ensure that there is adequate coupling between
 > primary and secondary windings -a coincidence which distracts from
 > the fact that it is the impedance ratio -not the turns ratio- that
 > is responsible for the voltage increase observed in Tesla coils?

I'd suggest that the large number of turns is an easy way to get the
secondary impedance high.

Regards,
Malcolm
<snip>