Re: Coupling Questions & coil measurements

• To: tesla-at-pupman-dot-com
• Subject: Re: Coupling Questions & coil measurements
• From: Terry Fritz <twftesla-at-uswest-dot-net>
• Date: Mon, 09 Aug 1999 12:16:00 -0600
• Approved: twftesla-at-uswest-dot-net
• Delivered-To: fixup-tesla-at-pupman-dot-com-at-fixme

```Hi Alex,

Below is a "blast from the past" on measuring K.  I took suggestions from
about 50 post on this and combined them in the "best method" post in late
November if 97...

Cheers

Terry

At 04:44 PM 8/10/99 +0100, you wrote:
>Ross, Terry, All
>
>I have always wondered what the k of my system is. However I have only very
>basic equipment to test it with - an analog meter, a home-made RF probe and
>a few bits with which I could make a GDO. Any hints on an easy way I could
>measure k with these parts?
>

=================================
From: terryf-at-verinet-dot-com[SMTP:terryf-at-verinet-dot-com]
Sent: Thursday, November 27, 1997 9:14 AM
To: Tesla List Subject: Best Method to Find Coupling Coefficient

All,
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)

Where:
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.

Terry

========================

```