I had rewound the secondaries on two 950 W power transformers a few
months back, both identically, each with four identical windings side by
side. I hadn't touched the primaries. There is a layer of aluminum foil
as an electrostatic shield between the primary and secondaries, not
making a complete turn, with drain wire. Yesterday I tried measuring the
inductances of their windings with my DMM, and to my surprise one of the
transformers measures more than twice as high as the other one, on all
its winding... WTF!
The meter uses 200 Hz in the range I was measuring with, so it gives me
a much smaller number than the actual inductance as the laminations are
designed for 60 Hz operation. Nonetheless, the huge difference between
the two transformers is consistent across all windings, same ratio.
Moreover, the transformer with the lower inductance buzzes the outer
magnetic shielding more when powered, indicating more leakage (I know
the cores do not saturate as I get fine sine waves on the scope for both
of them). Measurement of current through shorted secondary when
powering the primary through a ballast is the same for both
transformers, and both draw the same current from mains when secondaries
are open. All secondary windings produce the right voltages, and drop
the same under heavy load.
Yet, the measurement difference and the buzzing difference clearly
indicate something is wrong with one of the transformers. I was
thinking partially shorted winding, but then the voltage output would be
changed. It's possible the electrostatic shielding foil between the
primary and secondaries is shorted, though I'm pretty sure I had the
ends of the foil not touching each other (and poking with a needle from
the side and shorting the foil on the other transformer didn't seem to
create a difference anyway).
I considered gapping between the Es and Is of the transformers. However,
since I assembled both manually (interleaved, of course), I'd expect the
variations in gapping throughout the layers on each transformer to
average out to similar values for both transformers, I'd say up to 1/10
mm; putting them in the vice to squeeze Is towards Es does increase the
meter's measurement about 5%, but the same amount on both transformers,
so the ratio remains the same.
Well, I'm running out of ideas here. To start to take apart the more
buzzing/lower inductance (I guess more leaking) transformer, I'd have to
also take the other one apart simultaneously to do comparisons so I know
when I've reached the trouble spot. This especially sucks since
squeezing in the heavy gauge wire in the amount of space available was
very difficult work when I had put these together.
Help!
_______________________________________________
Amps mailing list
Amps@contesting.com
http://lists.contesting.com/mailman/listinfo/amps
|