It's interesting, when I learned this stuff ( I won't tell you when, but my
then-new text was published in 1935!), albeit in the context of
utility/power engineering, about the LAST thing we learned was the tricks
and conventions about turns ratios, etc.

Just looking at a question in my book:

"Assuming a coil of thus & so dimensions surrounding a core of this &
that dimension & type of material, calculate: 1) The flux in the core, 2)
the flux in the air," etc.

Follow-up question:

"Assuming a second identical coil placed elsewhere on the core, calculate
induced voltage if only the flux in the iron passes through the second
coil," etc.

The whole text was written like that. The concept of a "perfect
transformer" was introduced much later, and only in certain contexts. For
utility purposes, perfect transformers are undesirable!

Kind of strange by today's standards; we were taught all the painful details
right up front, and later allowed to throw out the ones that didn't apply. I
think it's done the other way around today.

Oh, if anyone cares, book cited is "Alternating Current Machinery," Bryant &
Johnson, 1935.



"John Larkin" wrote in
message ...
On Sun, 6 Jun 2004 15:31:40 +1000, "Phil Allison"
wrote:


"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 ?


Not a bit. But the outer coil has - surprise! - a bigger diameter than
the inner, so more of the return flux is flowing *inside* the sense
coil, in the direction that reduces the induced voltage. Given a
typical drum/bobbin type inductor, I'd guess that the resulting error
might be in the 50% sort of turf; the actual error depends on the
geometry of the windings, and how close the sense winding can actually
get, given the insulation or epoxy or whatever cited.

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.

Up to saturation - and an drum core will usually vaporize before it
saturates - the voltage ratio, whatever it is, will be independent of
drive level; the coupling is linear. A high drive *frequency* will
limimize the effects of copper loss, although it can be approximately
accounted for.


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.

With a closed, high-permeability core, voltage ratios can track turns
ratios to a part per million, as in a precision AC ratio box. Since
virtually all the flux is concentrated in the steel, any loop of any
size pretty much slices the same amount of flux. A torroid is ideal
for close coupling. That's not the case with a system dominated by air
gap, because the flux is scattered all over in space.


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)


Transformar manufacturers routinely use DVM-looking gadgets that
indicate turns ratio, and can easily and accurately resolve whole or
half turns. But only when leakage inductance is low, as for a closed,
high-mu core with tightly-coupled windings.

John