Re: Serious transformer problem


"Prune"

Hi Phil, thanks for the reply.

** That last one proves there is nothing basically wrong.

Off load primary current at rated voltage ( aka I mag) is a good
indicator of transformer's integrity.

If one *really* did have a lower primary inductance - Imag would be
much higher.

Thing is, even if the measurements are false readings, they're
consistent in that I get the same ratios for all windings. What could
be causing the factor-of-two difference to whatever it is the meter is
reading?


** The impedance difference you are seeing appears to be non existent at
normal operating voltage and frequency.

So it don't matter.



I also did another test, feeding to the primaries my scope's 1 kHz
square wave. One of the transformers loads the pulses twice as hard, as
seen by their distortion on the scope. I sketched the traces:
http://img258.imageshack.us/img258/6059/square2ml.png


** Gives the same info as the DMM test.


Someone suggested to me that one of the cores may be partially
magnetized.


** Nope - the Imags would be very different.

Anyhow, an AC power transformer will de-magnetise itself very quickly (in a
few seconds) at normal voltage and frequency.



Could it be the electrostatic shield? I can't really tell for sure if
the foil may be shorting its two ends together. I tried connecting the
drain wire to a pin and poking from the side to short it on purpose, but
didn't make any difference in either transformer.


** Not at all likely - any shorted turn would stick out like dog's BALLS
in the Imag figure.

You said they were both the same - but what is the figure?


I don't want to appear I'm trying to find something is indeed wrong, but
I would like to be sure. No one would be happier than me if I didn't
have to rebuild a transformer, as I already have three others waiting
for the winding tool anyway, one a 2 kVA...


** As I said before " Such transformer primaries have very large and NON
LINEAR impedances when
tested with low and medium frequencies."

The actual value of the impedance will in part depend on just how neatly
stanked the core is - ie if all potential air gaps are closed up very
tightly, the impedance will be higher.



........ Phil


.

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