Prune wrote:

I'm poor. So I rebuilt the core of a 16 lbs microwave oven transformer and
now it's very quiet -- outside the chassis. I mounted it in the 3u power
supply chassis, and the chassis resonates like crazy, at least 30 dB more
noise than the transformer alone. Rubber/springs/foam didn't help much. I
spent quite some time building a pneumatic vibration isolator with a pump
and everything. Imagine my surprise when it turned out most of the
vibration wasn't transmitted mechanically...the hum was being induced
electromagnetically into the chassis! I almost cried! Repositioning
didn't make much difference (more of the effect seems to come from the side
with the primary winding, but orienting that side away from the vertical
walls means having the core horizontal, which makes the hum worse; standing
the transformer on its narrowest side doesn't fit in the chassis). All my
testing was done with no load as the salt water resistor boiled too quickly
and I took it apart. Would it get better/worse/same when it's loaded? And
what can I do to deal with the issue, in terms of shielding or anything
else? I swear I'm going to go crazy trying to figure this out.

Microwave oven transformers operate with parts of the core very nearly
saturated during parts of the cycle, so they spray magnetic fields all
over the place. Any iron in their neighborhood will be bumped around
like a vibrator. Does your unit still have the magnetic shunt between
primary and secondary? Is the primary the same number of turns it
was, originally? Adding more primary turns (lowering the volts per
turn) will help. Wrapping a thick band of copper around the core in
the direction the turns wrap (an eddy current shield) may help contain
the stray fields a bit, also, at the cost of more waste heat and less
efficient heat removal.

--
John Popelish