Boris,
I can understand your frustration. It sounds like everything
is identical and nothing obvious is wrong.
It is not likely to be residual magnetisim since the soft
iron (magnetically soft) used in transformer laminations is
not able to be magentized. If it was able to be noticably
magnetized, it would have to be a bad or defective
lamination and would need replaced. Degaussing it would not
help a thing, because every time it carried flux it would
remagenteize to the same level. If the core is having a
major problem with residual magnetisim, it will make a poor
transformer.
My guess would be you either are on the right track, that
something is shorted. Another possibility is you are
measuring at much too low of power levels and things that
do not matter in operation, like a small hysteresis curve
difference, are showing up and the buzzing is just a
coincidence.
Here is what I would look at:
1.) There should not be a conductive path between
laminations. That means the laminations must be insulated
from each other, and the fasteners securing the laminations
must be insulated from the laminations. Make sure you don't
have the fasteners shorting the core from side to side. I've
seen people actually drill into laminations to mount
components!
2.) Run the transformers full voltage ( be SURE they are in
a grounded enclosure when powered up) with no load. See if
one gets significantly warmer or starts to smell. NEVER
touch the core while it is powered on!! A single shorted
turn will behave much like you descibe, and evetually make
the transformer fail. Generally you can smell that turn
getting hot if you run the transfomer for a while without
load.
Get rid of the electrostatic shields if you can the next
time. They really don't do anything in our applications,
they take up room, they reduce voltage breakdown, and they
are a potential souce of problems if they become shorted.
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