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[Amps] Serious transformer problem

To: amps@contesting.com
Subject: [Amps] Serious transformer problem
From: Borislav Trifonov <bdt@shaw.ca>
Date: Fri, 30 Jun 2006 21:10:01 -0700
List-post: <mailto:amps@contesting.com>
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!
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