On 20.02.2016 13:55, Cursitor Doom wrote:
On Sat, 20 Feb 2016 01:14:09 -0500, legg wrote:

Recheck pin function before jumping to conclusions.

Right; now re-checked. DC measurments proved (unsurprisingly) too
close together so I re-tested using 100khz instead. These are the
impedances WRT ground of the output taps of the long winding in the
order they actually come out of the transformer: GND, 0.17ohms,
0.17ohms, 0.26ohms, 0.28ohms, 3.7ohms, 3.8ohms. So this doesn't seem
to tally up with the schematic. Or does it? I need a pint of strong
coffee to kick-start my head on this one. :-/ Anyway, later...

Hi

As you say, this really doesn't tally up. The ratios look ok (see my
other post), but the absolute values are obviously junk.

After all, Ohm's law still holds, even for reactive impedances, and 60
volts divided by 3.7 Ohms is 16 amps, which would be WAY too much for a
small transformer winding's magnetization current. That would indicate
very heavy overload, most probably due to a short circuit inside one of
the windings.

But there's something that makes me distrust those impedance numbers -
and that is your use of 100 kHz as the testing frequency.

First of all, did you really use 100 kHz as written? Most LCR meters
have 100 Hz, 1 kHz, 10 kHz and 100 kHz signals. Did you perchance use
the 100 Hz one instead? 100 Hz would be so low that the inductive part
might not even register properly.

Second, 100 kHz is not a good choice. The reason for this is the
self-resonance frequency (SRF) of your transformer. All coils and
transformers in the real world are not just coils, but LC circuits. The
L part is (obviously) provided by the winding itself and the C part is
the stray capacitance between the wires in the winding. Being really an
LC circuit, a winding has a resonance frequency, like any "true" LC
circuit would have. This is called the SRF of the winding. To make
matters worse, a transformer that has multiple windings wound with
different geometries and wire diameters has multiple SRFs, one for each
winding.

Windings with few turns of loosely packed thick wire have high SRF
values, while windings with many turns of densely packed thin wire will
have much lower SRFs.

Your particular transformer has 2 high-voltage windings for the kV
outputs. They have lots of densely packed thin wire, so their SRFs will
be very low. I don't know exactly how low, but I'm sure that they will
be much lower than 100 kHz, and that's what makes 100 kHz an unsuitable
choice for testing.

If fact, if you try to operate a winding above its SRF (let's say the
winding has a 20 kHz SRF and you try to apply 100 kHz), then the winding
will no longer behave like an inductor, but it will behave like a
capacitor instead. I know, this seems crazy, but that's how a winding
behaves above its SRF.

In a transformer, where there are multiple windings, there are also
multiple SRFs, so at some test frequency, some windings may happen to be
below their respective SRFs, while some other windings may be above
their respective SRFs, depending on how you choose the test frequency.

If any winding happens to be above its SRF, then it will behave like a
capacitor. As you know, capacitors behave more or less like a short
circuit at high frequencies, and an above-SRF winding will behave like
that too. That is, it will look (from an impedance measurement) like if
it was heavily overloaded or even shorted out altogether.

So your 100 kHz measurements indicated very low impedances, like some
winding was shorted out. But then you also have 2 high voltage windings
in there, which would have been way above SRF at 100 kHz frequency, so
they will effectively behave like shorted even if they were perfectly
fine otherwise. At 100 kHz they're no longer inductors, they're likely
just capacitors instead.

Now, to test transformer winding impedances, you need to select a
reasonable test frequency. It must obviously be lower than any SRF of
any winding - otherwise the transformer will appear overloaded. If you
don't know the SRFs' values, you can measure them out with a signal
generator and an oscilloscope. But you don't need to. Normally no
transformer is operated above its SRF (it would not work very well if
one tried), so you can assume the normally intended frequency of
operation to be a "safe" choice that is unlikely to hit SRF limits.

Your transformer is probably supposed to run at something like 20 kHz in
normal resonant operation, so 20 kHz should be ok. But because it has
high voltage windings, it may be very close to the HV winding's SRF.
Indeed Philips engineers may even have chosen to run the transformer not
below, but essentially right at SRF. They may have selected the
resonance capacitors for the primary in such a way that the primary
(together with the resonance capacitors) would have a resonant frequency
which closely matches the self-resonance of one of the high voltage
windings, being just a little bit below to account for tolerances.

If that's the case, you should use a lower frequency for testing
impedances. Most LCR meters don't offer 20 kHz anyway, just 10 kHz and
100 kHz as "nearest neighbors". 100 kHz won't do, so use 10 kHz. That
should give you realistic impedances (which you can manually multiply by
2 to get to the in-circuit conditions).

Regards
Dimitrij