"John Larkin"
"Phil Allison"

The turns ratio and the (unloaded) voltage ratio you measure are in
exact
proportion.

As long as the same flux traverses all the turns.

** That is not a very helpful remark.

But it's true.


** It is *unhelpful* because it is so damn ambiguous.


There's not a lot of sense pretending you can measure
something if you can't.


** It makes less sense to scorn a perfectly practical test method.


The suggestion was that the overwind be around the existing coil of the
inductor * PLUS * there is no load on the overwind so leakage
inductance
is irrelevant.


Leakage inductance means exactly that the same flux does *not* thread
all turns. So the unloaded voltage induced into the sense winding will
be less volts/turn than the main coil. This is the likely situation
for a drum core with a large air return path; some of the return flux
will sneak back *inside* the sense coil.



** The overwind is to be around the existing coil, wound in parallel and on
top of it, touching it - is that hard to comprehend ?

A further ( rather obvious) condition is that the inductor coil current for
the test be low enough to not generate a significant voltage drop across the
coil's resistance - or you calculate that drop and take it into account.


As the sense winding gets bigger in diameter, its signal level tends
to zero, loaded or not.



** I just took a small mains toroidal ( 30VA) and with the primary
energised at 230 volts passed a one turn loop through the core and measured
0.102 volts rms across the ends. The loop could be made as open as you
liked or tight wrapped as you liked with NO change in the measured voltage.

The primary magnetising current was only 1.5 mA and the primary resistance
was 94 ohms - so a negligible primary drop of 140 mV.

So I make the primary turns to be 2255 ( +/- the AC voltmeter's 0.3 %
error, or about 7 turns)



............. Phil