RE: Mutual inductance and K factor

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "by way of Terry Fritz <twftesla-at-qwest-dot-net>" <couturejh-at-telocity-dot-com>


Weazle -

I have been working with your tests and find they give excellent results
when I use them correctly. The problem has been that I have left out a 2
multiplier in the equation I show in the Tesla Coil Design Manual. If your
test voltages are multiplied by 2 they give the correct mutual inductance. I
compared these test inductances with the ones produced by the JHCTES Ver 3.3
and they agree very closely. This change also made your test K factor agree
closely with the Ver 3.3 K factor. This same correction should be applied to
the equation you used.

The calculations -

A.  (4 Spacers) Sec bottom wire on same level as the primary wiring.
Your test   V = 42.97mv x 2 = 85.94 uh mutual inductance.
JHCTES Ver 3.3  83.57 uh mutual inductance
Difference  2.7%

Your test K factor = .199
JHCTES Ver 3.3     = .191
Difference  4.0%

B.  (no spacers) Sec bottom wire 1 inch above the primary wiring.
Your test   V =32.56mv x 2 = 64.74 uh mutual inductance
JHCTES Ver 3.3  66.15 uh mutual inductance
Difference  2.1%

Your test K factor = .150
JHCTES Ver 3.3     = .152
Difference  1.3%

I would be interested in hearing from other coilers who have used this test
to find the mutual inductance and K factor of their coils and may have had
trouble making the results agree with the JHCTES Ver 3.3 program.

John Couture

----------------------

-----Original Message-----
From: Tesla list [mailto:tesla-at-pupman-dot-com]
Sent: Thursday, October 25, 2001 10:06 AM
To: tesla-at-pupman-dot-com
Subject: RE: Mutual inductance and K factor


Original poster: "J. B. Weazle McCreath by way of Terry Fritz
<twftesla-at-qwest-dot-net>" <weazle-at-hurontel.on.ca>


At 10:14 PM 24/10/01 -0600, you wrote:
>
>Original poster: "by way of Terry Fritz <twftesla-at-qwest-dot-net>"
<couturejh-at-telocity-dot-com>
>
>
>Weazle -
>
>The equation you used apparently is not in the programs Terry has listed
>below.  Where did you find the equation?  Your calcs are correct.  It is
>the equation that is incorrect.
>

<snip>

Hello again John, coilers,

The formulas below are the ones I got from a posting Terry made to
the list some time ago.  He indicated then, that it was easy to do
the measurements and it gave reliable results, which I agree with.

The mutual inductance is found by:  M = V / (w * I)

Where:  M = Mutual inductance in Henries.
        w = Line freq. in radians per second (377 for 60 Hz.)
        I = The measured current in the primary in amps A.C.
        V = The measured secondary voltage in volts AC.

K can then be found by using the formula:  K = M / sqrt(L1 * L2)

If you look on page 21-2, Fig. 33 in your Tesla Coil Design Manual
you'll see that you have used essentially the same formula.

The only difference is that you use a specific current of 2.65 amps,
while I used 10 amps.  In my case, that meant the measured secondary
voltage (in volts) was divided by 3770 to give the mutual inductance
in Henries.  Having that value, I applied it to the other formula
to get the K factor, just as you show in your manual.