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Re: Measuring Coupling Coefficients
From: John H. Couture[SMTP:couturejh-at-worldnet.att-dot-net]
Sent: Friday, November 28, 1997 2:02 PM
To: Tesla List
Subject: Re: FW: Measuring Coupling Coefficients
At 03:50 PM 11/27/97 +0000, you wrote:
>
>
>----------
>From: terryf-at-verinet-dot-com[SMTP:terryf-at-verinet-dot-com]
>Sent: Thursday, November 27, 1997 9:14 AM
>To: Tesla List
>Subject: Re: Measuring Coupling Coefficients
>
>SNIP............
>
>
>Although complex, the methode I used is accurate to a fraction of a percent.
>
>The method described on page 14-11 of the "Tesla Coil Construction Guide"
>requires inductance measurements with a precision of 1nH to get the accuracy
>I am looking for. This measurement is possible but more complex and error
>prone than my original method. Most typical LCR meters (the kind most
>people can afford) will not measure down to this level with any accuracy.
> The second test you mention is a repeat on the first test with the
>formula changed to give the k (coupling coeffiecent) instead of M (mutual
>inductance).
>Thanks for the info. I already have these texts.
>
> Terry
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Terry -
I was not referring to "the formula changed to give the K(coupling
coefficient) instead of M(mutual inductance)". This is the method you
described using a 60 HZ frequency. This is also shown in the Tesla Coil
Design Manual page 21-2.
I was referring to two other methods of finding the K factor of a Tesla
coil. One of these methods uses an induction meter and the other uses an RF
frequency generator. Both are open/short methods. The K factor equations
for these two methods are as follows:
K = sqrt(1-(Ls/Lo)) and K= sqrt(1-(Fo/Fs)^2)
Note that testing for the K factor is not the same as predicting the K
factor. The JHCTES TC computer program predicts an estimate of the K factor
at the design stage. This is a more difficult problem and best done by a
computer.
I agree that a test for the K factor can be more accurate than a
prediction for the K factor. I also agree that the measurements are
difficult to make. Several measurements should be made and averaged. More
accuracy can be found by using two or three different methods to compare the
K factors.
Note that finding the K factor using the mutual inductance test can be
less accurate because two additional variables Lp and Ls are introduced.
Adding these two variables will reduce the accuracy of the K factor.
John Couture