Help! Transformer induces hum to chassis!
 

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.

On Sun, 07 Nov 2004 06:55:00 GMT, 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.

Learn to love hum. Let's face it: it's not as if your oven's going to
be running 24/7!

--

"What is now proved was once only imagin'd." - William Blake, 1793.

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

It's not mounted in a microwave; it's being bridge rectified into a CLCRC
filter for plasma supply. I get 3 kV DC unloaded. I need to have it in
a chassis as otherwise it makes too much stray interference.
I was thinking of mu-metal foil but those are too thin and would probably
saturate and be ineffective. Using a thick shielding on the other hand
means the shielding will be rigid and buzz instead of the chassis, plus
it presents a cooling problem. I've seen transformers where only the
exposed windings are shielded, with the outer part of the core exposed.
Maybe that will provide enough cooling. I belive they fill those with
epoxy for improved thermal exchange. I need to pull 350-400 mA DC and
the chassis has a fan.

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.

On Sun, 07 Nov 2004 11:43:44 -0500 John Popelish
wrote:

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.

These are your only 2 reasonable solutions.

-
-----------------------------------------------
Jim Adney
Madison, WI 53711 USA
-----------------------------------------------

These are your only 2 reasonable solutions.

Adding more turns to the primary -- will just a few % more turns make a
difference? I don't want to loose too much voltage. Also, I hope it's not
too important to match the gauge of the added turns to the rest of the
primary, as I don't have that exact gauge solid wire.

My original shield/epoxy question remains.

On Tue, 09 Nov 2004 06:24:21 GMT Prune wrote:

These are your only 2 reasonable solutions.

Adding more turns to the primary -- will just a few % more turns make a
difference? I don't want to loose too much voltage. Also, I hope it's not
too important to match the gauge of the added turns to the rest of the
primary, as I don't have that exact gauge solid wire.

The stray field will probably go down faster than the proportion of
windings added. Any wire as heavy or heavier can be used.

If you don't want to lose secondary voltage, you can just add the same
proportional number of secondary turns, too.

-
-----------------------------------------------
Jim Adney
Madison, WI 53711 USA
-----------------------------------------------

John Popelish wrote
Microwave oven transformers operate with parts of the core very nearly
saturated during parts of the cycle [...]

why are they designed that way?

Alan Horowitz wrote:

John Popelish wrote

Microwave oven transformers operate with parts of the core very nearly
saturated during parts of the cycle [...]


why are they designed that way?

Probably to save money on the core -- the oven will cost a few cents
less to make, a few dollars less to buy, then you pay that all back to
the power company as your kitchen gets hot. It would also add a modicum
of voltage regulation to the thing, but I doubt that's the reason.

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

Tim Wescott wrote:
Alan Horowitz wrote:

John Popelish wrote

Microwave oven transformers operate with parts of the core very
nearly
saturated during parts of the cycle [...]
why are they designed that way?

Probably to save money on the core -- the oven will cost a few cents
less to make, a few dollars less to buy, then you pay that all back to
the power company as your kitchen gets hot. It would also add a
modicum of voltage regulation to the thing, but I doubt that's the reason.

It is mostly done for current limiting, which a magnetron needs because
(in DC terms) it looks like a diode connected across the power supply.
Saving size, cost and weight is also important, so the transformers are
pared right down to the bone.


--
73 from Ian G3SEK 'In Practice' columnist for RadCom (RSGB)
http://www.ifwtech.co.uk/g3sek