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Re: K Quiz
Subject: Re: K Quiz
Date: Sun, 15 Jun 1997 17:58:24 -0700
From: Bert Hickman <bert.hickman-at-aquila-dot-com>
Organization: Stoneridge Engineering
To: Tesla List <tesla-at-pupman-dot-com>
References:
1
Tesla List wrote:
>
> Subject: Re: K Quiz
> Date: Thu, 12 Jun 1997 11:55:35 -0700
> From: Skip Greiner <sgreiner-at-wwnet-dot-com>
> Organization: Greiner, Ltd.
> To: Tesla List <tesla-at-pupman-dot-com>
> References: 1
>
> Tesla List wrote:
> >
> > Subject: Re: K Quiz
> > Date: Tue, 10 Jun 1997 11:41:13 +1200
> > From: "Malcolm Watts" <MALCOLM-at-directorate.wnp.ac.nz>
> > Organization: Wellington Polytechnic, NZ
> > To: tesla-at-pupman-dot-com
> >
> > Hello Mark, all,
> > Following on from Mark's little quiz, I have the
> > following query which might relate to something quite important
> > performance-wise.....
> >
> > > Hello coilers,
> > > Here is a little food for thought.
> > > "K" is the term applied to the degree of magnetic coupling between
> > > the
> > > primary and secondary coils of a conventional tesla coil (or the primary
> > > and
> > > driver
> > > coil in the magnifier configuration). Typically, we aim for values
> > > between
> > > 0.1 and
> > > 0.25 or so for a conventional tesla coil, and 0.4 or higher for
> > > magnifier
> > > primary/driver coil systems. "M" is the amount of mutual inductance
> > > between the
> > > primary and secondary coil. We define:
> > > K = M / SQUARE_ROOT [ Lp x Ls ]
> > > where Lp is the primary inductance, Ls is the secondary inductance, M
> > > is the
> > > mutual inductance, and K is the coefficient of coupling between the two
> > > coils.
> > <snip of quiz>
> >
> > k itself is relatively easy to measure and quantify. But as far
> > as I know, the figure says nothing about _how_ the primary is
> > coupling into the secondary. For example, I can score identical k's
> > with a flat primary with the same outer dimension as that of the
> > resonator when placed below the resonator as I can with a helical
> > primary with the resonator sitting inside it (easy). Has anybody
> > examined performance issues around the degree of coupling into say,
> > just the bottom few turns vs a primary that loosely couples a long
> > way up the secondary? I have a gut feeling that this may be a
> > significant determinant of system performance given the normal high
> > secondary Q's and the fact that all else being equal, performance
> > for a given power input varies so widely in different systems.
> >
> > Malcolm
>
> Hi Malcolm and All
>
> I formerly used flat spiral coils. I now exclusively use cylindrical or
> solenoid wound primaries. I have not taken the time and effort to
> quantify my results but my gut feeling is that I get much better overall
> performance with the cylindrical primaries. Usually the primaries are
> about 4" to 6" larger in diameter than the secondaries and the
> secondaries are set 2" to 4" above the top turn of the primary.
>
> Skip
Malcolm, Skip, and all
This is one of those questions that really makes you go hmmm....! At
first blush one would think that, if you've got equal coupling
coefficients, it shouldn't matter. The coupling coefficient should only
govern the relative amount of time that it takes to transfer the
primary's energy to the secondary. However, I suspect that, in practice,
it may indeed be more significant, since the secondary is not a simple
lumped parameter inductor. The more localized the primary's magnetic
field (as in a helical primary), the more concentrated the
electromagnetic energy coupling into the lower sections of the
secondary, and the more this would begin to look like a base-driven
resonator. In fact, if taken to an extreme, you'd almost be
approximating a magnifier - heavy magnetic coupling only to the lower
section, and the remainder of the secondary behaving more like a
base-driven resonator. Interesting speculations... Malcolm or Fr.
McGahee, do your collected coil parameters give any clues?
-- Bert H --