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Re: Racing Spark Prediction

Original poster: Vardan <vardan01@xxxxxxxxxxxxxxxxxxxxxxx>


We had a discussion way back as to the "best method" of measuring coupling. My new search thing popped it right up ;-)

From: terryfverinet.com[SMTP:terryfverinet.com] Sent: Thursday, November 27, 1997 9:14 AM To: Tesla List
Subject:   Best Method to Find Coupling Coefficient

        I have tried all the suggestion I have received (Thanks Malcolm, Fr.
Tom, John C., Mark Rzeszotarski). The best method I have found that does not require expensive equipment or great theoretical challenges consists of the following.

Apply a heavy 60 Hz AC current to the primary coil. This is best done by placing a space heater, hair dryer, etc. in series with the primary to limit the current to about 10 amps. Measure this current with a multimeter. Note that the space heater gives a fairly stable resistance. Light bulbs have a non-linear resistance through the AC cycle and distort the measurement (they must cool down substantially at the nodes of the AC cycle). Of course, use great caution with the live AC on the primary so as not to kill yourself. Only the isolated primary need be connected to the AC. The capacitors, transformers, and other wiring should be disconnected from the primary for this test. Be cautious of the AC finding its way on to the secondary!

Place a 10k ohm resistor and a 1uF capacitor across the secondary and measure the AC voltage. It will be on the order of say 100 mV AC. The resistor and capacitor will eliminate stray noise picked up by the secondary and swamp any resonance which is significant at these low levels.

The mutual inductance is found by:

        M = V / (w * I)


        M = Mutual inductance in Heneries.
w = the line frequency in radians per second (377 for 60Hz or 314 for 50 Hz).
        I = The measured current in the primary in amps AC.
        V = The measured secondary voltage in volts AC.

As an example:
If the current in the primary is 10 amps and the frequency is 60Hz and you measure 0.100 volts AC, you would get:

0.100 / (377 * 10 ) = 26.52 uH for the mutual inductance.

k can then be found by using the formula:

        k = M / sqrt(L1 * L2)

Where L1 and L2 are the inductances of the primary and secondary coils.

This method is rock solid in theory and easy to do. The accuracy is excellent. There is little that can go wrong compared to other methods and you don't need anything special other than a multimeter to do the test. The accuracy is dependant on the accuracy of your multimeter. My tests could easily get within 1%.

Thanks again for all the great suggestions and do be careful with the AC if you try this.




At 11:14 AM 5/6/2006, you wrote:
Hallo John.

> Original poster: FutureT@xxxxxxx

> Seems that when using the aiding and bucking inductances method
> for measuring k, along with the typical formula that's generally used
> (I can't remember it offhand), the higher couplings give wrong results.
> It makes the k seem tighter than it really is.

something like "measure the series inductance twice, once with one
coil reversed (a1 and a2), subtract one result from the other, and
solve the resulting expression for M.", then


yeah, i`ve read about this gotcha in archives, but i think this method
prone to errors with any value of k - coz Lpri is about 3 order of
magnitude smaller than Lsec.
Marco Denicolai had used this method while playing with his Thor, he
make it even simplier - used only one msmt instead of 2 - measured
only a1, then


: D

> Using the 60 Hz AC current method you mentioned, then all is
> accurate for k measurements.

so no problemo here : )
i prefer measuring instead of calculating k not because i don`t trust
JAVATC or INCA for example - no, just coz i know for sure, that it would
be impossible for me to wind a primary exactly flat, or exactly at 30
deg, etc, etc, so at the end i would not have rights to compare my real
transformer with simulated one, that`s all % )