Re: Recycled: Measuring Coupling Coefficients

From: 	terryf-at-verinet-dot-com[SMTP:terryf-at-verinet-dot-com]
Sent: 	Wednesday, December 03, 1997 7:10 AM
To: 	Tesla List
Subject: 	Re: Recycled: Measuring Coupling Coefficients

John wrote..........

>   Fr. Tom, All -
>  I agree that the K factor (coupling coefficient) appears to be independent
>of the frequency for Tesla coils. However, further research indicates the K
>factor is a number that depends on the type of operation. Refer to Terman's
>Radio Engineers' Handbook. For example, in radio circuits the K factor
>varies with the frequency because of the variable  tuning capacitor.
>Changing this capacitor changes the operating frequency and changes the
>coupling. This means that a Tesla coil with a fixed capacitor would have a
>fixed coupling  and be independent of frequency . However, if the TC
>capacitor is changed and the operating frequency changed the coupling would
>also be changed without changing the physical characteristics or geometry of
>the coil. Has anyone ever checked this?

For my work I am only interested in the K between the primary and secondary
coils.  When you add the effects of capacitances things become very
different indeed.  In order to study the more complex cases I need to know
the K of the coils alone as a starting point.

>  The mutual inductance test for K factor introduces two additional
>variables, Lp and Ls, to the voltage and current that is necessary to
>determine the Lm. This test would be less accurate compared to the other
>tests using only two variables. If all variables are  measured to the same
>accuracy the other tests would have twice the accuracy of the mutual
>inductance test.

In many cases the values of Ls and Lp or well known or easily measured.
Some formulas look good on paper but when you actualy try them, the
measurments become very diffucult.
        A good example of this is the test where one puts the coils in
series so that the inductances add and then reverse one coil so that the
inductances subtract to find Lm.  The difference between the two cases is
tiny and very difficult to measure accurately.  Yet, this test is very
commonly referred to in books and such.  I was looking for a good test that
works well and dosen't need a lab full of equipment. 

>  As for the light bulb shown in the Tesla Coil Design Manual for the mutual
>inductance test, the non linearity of the bulb is not involved. This test
>uses a fixed current that is independent of the non linearity of the bulb.
>The fixed current is obtained by varying the primary voltage with a variac.
>This has the advantage that the mv in the secondary is the uh of the mutual
>iinductance so calculations are not necessary. This test is also safer
>because it operates at low voltage.

A variac is certainly and advantage in this test but not everyone has one. 

>Comments welcomed.
>  John Couture