Subtitle: flies in the ointment.

Perhaps it's not possible to determine the turns ratio in the way I've
been suggesting, unless the coupling is very close to unity (in which
case there's no need to measure leakage inductances).

Consider the following coils/scenarios. Make three bobbins, all the
same dimensions. Make two of them from some nice dielectric like
Teflon or polystyrene. Make the third from a low-loss ferrite. Put
identical windings on all three. I'll assume here that the
permeability of the ferrite and the shape of the bobbin and winding
result in a coil with a self-inductance exactly four times that of the
other two coils. Neglect resistance and parasitic capacitance.

Now position the two "air-core" coils so the coefficient of coupling
is 0.5. Excite one with 1V. Measure the voltage induced in the
second without loading it: I believe it will be 0.5V.

Replace the second coil with the one wound on the ferrite bobbin, and
adjust for the same k=0.5. Now what voltage do you measure in that
coil? I believe it will be 1.0V, even though the turns ratio is the
same.

Sound right this time?

Cheers,
Tom




(Tom Bruhns) wrote in message om...
As has been pointed out in other postings to the thread, the
coefficient of coupling is important. Whatever flux from the primary
(driven winding) does not couple to the secondary will not induce
voltage in the secondary, and the measured turns ratio will be low as
a result. However, by measuring the inductance of the primary when
the secondary is open and again when it is shorted, and doing the same
with the secondary, you can find the leakage inductances and therefore
the coefficient of coupling, fairly accurately. (The second
measurement is really a check for consistency.) No need for xrays.
You could further improve the accuracy, I suppose, by including a
resistance value for each winding; ideally it would be the AC
resistance at the operating frequency. It will probably make for
easier calculations if you load the secondary very lightly for the
measurement.
....