Borislav,
I know you've answered some of the following questions below, but I'm adding
all I can think of to help. Does both transformers have the same amount of iron
in them? Is the laminations the same size? Are the laminations the same
thickness? Are you sure both have the same number of turns in the secondary? Is
the iron type different between the two possibly (this really shouldn't make a
huge difference in the inductance though unless one has something added in the
iron which impedes the magnetic flux adding more reluctance).? Transformer iron
doesn't vary as much as does ferrite, or iron powder where you have different
AL factors unless it may be something like Permedure, etc. Could one core be
partially magnetized? If the shield were shorted of course you would have a low
voltage, high current short for what ever one turn equals out to in voltage by
using the turns per volt figure. When you applied current to the primary, you
would sure know it as it would be a big hum and sho
uld blow a fuse, etc. I would definately double check this! Any short
generally acts this way.
What controls the inductance is the number of turns and the amount of iron in
the core in square centimeters or square inches. A different core material
could possibly cause it if one type impeded the flux more than another (greater
reluctance). A gap will effect it some more because of flux impedance
(reluctance), but really it's used to keep a choke from saturating at high
current levels, or where you have DC mixed with AC, not for transformers. If
ones gaps a transformer, it needs to be a very thin gap like 0.002" or even
thinner as in a C-core type. These are actually ground and lapped so they fit
really close. Check all this over, and let me know what you find. I'll do my
best to help.
Best,
Will
*********** REPLY SEPARATOR ***********
On 6/30/06 at 9:10 PM Borislav Trifonov wrote:
>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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