Re: Recycled: Measuring Coupling Coefficients
From: John H. Couture[SMTP:couturejh-at-worldnet.att-dot-net]
Sent: Monday, December 01, 1997 11:27 PM
To: Tesla List
Subject: Re: Recycled: Measuring Coupling Coefficients
At 10:16 PM 11/30/97 +0000, you wrote:
>From: Thomas McGahee[SMTP:tom_mcgahee-at-sigmais-dot-com]
>Sent: Sunday, November 30, 1997 8:59 AM
>To: Tesla List
>Subject: Re: Recycled: Measuring Coupling Coefficients
>Terry originally thought the same as you about the operating
>frequency, but found that in fact the method is not dependent
>on frequency. In fact, as the title of one of his posts pointed
>out, this method is perhaps the best for most coilers as it does
>not require very much in the way of test equipment, and yet yields
>answers at least 1% accurate if you use a heating element or
>high power resistor as the current limiting element (as opposed
>to a light bulb, which is non-linear and introduces a bit of
>error at 60 Hz).
>Greg Leyh uses a variation on this method, where he applies 120
>VAC directly to the SECONDARY and then measures the voltage at the
>primary and the current at the powered secondary. It is always
>useful to have information on several different ways to do things.
>Then use the method that works best based on the equipment you
>have at hand.
>Always exercise caution when dealing with the 120 VAC power
>Hope this helps.
>Fr. Tom McGahee
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?
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
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.