Re: Inductance and K

["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: "sundog by way of Terry Fritz <teslalist-at-qwest-dot-net>" 
<sundog-at-timeship-dot-net>

 >     Suppose I put a 10 turn primary, tightly coupled to the
 > secondary.  Then I put a similar variable inductor in series with it (off
 > axis and away from the primary).  This will give me a lot of freedom to
 > tune the system as it's running, but how does it affect K?  Will I have to
 > move the primary physically closer to the secondary to get the same
 > coupling if I add the off-axis variable inductor?  Will I see a change in K
 > as I vary the inductance of the primary?

If you add Lx in series with the primary inductance, "off axis", the
mutual inductance remains the same:

M=k(L1 L2)^(1/2)

But as L1 changes to L1+Lx, the coupling coefficient changes to:

k'=M/((L1+Lx)L2)^(1/2) = k(L1/(L1+Lx))^(1/2)

So, the resonance frequency of the primary alone, and k, are divided by
the same factor as you change Lx.

Antonio Carlos M. de Queiroz