[ARC5] Mutual Inductance and coupling coefficient measurment

[Bruce Long]

Hello:
Some of you have expressed interest in seeing how to measure mutual coupling.  I present the procedure below.  My apologizes if you are not interested.




Procedure for Measurement of Mutual Coupling
 
Given two coupled inductors Lp and Ls
Remove any resonating or adjustment capacitances from the
circuit.
Measure Lp with Ls open circuited
Measure Ls with Lp open circuited
Connect Lp and Ls in series and measure
L+ and L-
L+ is the inductance of the two coils connected in series
aiding
L- is the inductance of the two coils connected in series opposition.
 
In practice this means connect the two coils in series and
measure the inductance
Then reverse the polarity of the connection of ONE coil and
measure inductance again.
The larger inductance reading is L+, the smaller inductance
reading is L-
 
With measured values for Lp Ls L+ and L- do the following
algerbra
 
Lm = (L+ - L-)/4                                                          ie    Lm = L+ minus L-   then divided by 4 


k = coupling coefficient = (Lm) /[(Lp)(Ls)]^(1/2)           ie   k = Lm divided by the square root of the product of Lm and
Ls 


Hope this is useful

Measurement accuracy is important because you need to subtract L- from L+ to get Lm and the subtraction of two numbers enlarges the measurement error  The less the coupling, the more error enlargement occurs.

We use this procedure at work on an almost daily basis.

bruce