Re: can coupling be too high

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

Original poster: "Antonio Carlos M. de Queiroz by way of Terry Fritz <teslalist-at-qwest-dot-net>" <acmq-at-compuland-dot-com.br>

Tesla list wrote:

 > Original poster: "colin.heath4 by way of Terry Fritz 
<teslalist-at-qwest-dot-net>" <colin.heath4-at-ntlworld-dot-com>

 >          i have a question regarding coupling.here goes.
 > when we couple a coil tightly problems such as racing sparks appear so we
 > drop off the coupling. but why?

Probably just because the primary is too close to the secondary, and
the electric field becomes too distorted close to the base of the
secondary.

 > does coupling too tight really affect the
 > performance badly?

Quenching of the primary gap may be more difficult if the coupling is
high, and insulation between primary and secondary becomes problematic.

 > it is often stated that a coil too tightly coupled will
 > take on transformer action. i dont know if this is an old school of thought
 > or still applies.

Transformer action really tends to appear, in the sense that the pulse
applied to the primary coil when the gap fires tends to appear sooner
at the secondary coil. The picture:
http://www.coe.ufrj.br/~acmq/tesla/himodes.gif
shows the ideal secondary waveforms for the modes 1:2, 1:4, 1:6, 1:8,
and 1:10. The coupling coefficients are 0.6, 0.882, 0.946, 0.969, and
0.980. In all these cases, perfect energy transfer occurs at the 100 kV
negative peak, but as the coupling is increased, a very high positive
peak appears more and more quickly, with almost perfect energy transfer.
In practice, even professional systems don't go beyond k=0.6
(1:2 mode), that is still far from "transformer action", and results
in the fastest possible perfect energy transfer.
Usual Tesla coils work well with k=0.1 to 0.2.

 > i have experianced it with my small 2" coil but im not sure wether this was
 > due too the coil not being optimised for tight coupling perhaps ruining the
 > quenching of the gap.
 > should we be working on better insulation on our secondarys rather than
 > working around the problem ?

Insulation from a nearby primary would increase the capacitance of
the secondary coil, and would trap energy at it, instead of letting
it go to the terminal.

Antonio Carlos M. de Queiroz