On Sun, 4 Mar 2013, Jim Mueller wrote:

On Sat, 02 Mar 2013 20:00:49 -0800, Frank wrote:

Clamshell-looking enclosure on small AC power transformers. Magnetic
shielding to prevent interference with nearby wires?

Magnetic fields are hard to shield. In a power transformer, it would be
more effective to use more iron in the core and more turns in the
windings to reduce the flux density. While a steel shell probably
provides a limited amount of shielding, it's more likely there to provide
mechanical protection. At the same time, it gives a better appearance.

Maybe protect the windings, too.

Definitely, nobody specifies between just a frame transformer and a metal
cased one. I'd honestly never given it any thought. Bigger transformers
often seem to be more likely to be cased in metal, so structural support
may apply. I don'thave it handy, but that one Hammond catalog I got
decades ago I think offered the same voltage/amps in open frame and metal
enclosed versions, which would add more to "looks" than anything else.

I have heard of the leads breaking off open frame transofrmers, right
where they come off the winding, which isn't a good thing. Any metal
cased transformer, I don't recall that happening, which may mean there is
some sort of terminal block inside, rather than just bringing out the
wire. Certainly I've seen open frame transformers where the wire from the
windings are brought to terminal points, which helps protect the winding
wire. it also exposes the line voltage going into that transformer, which
in some cases can't be a good thing.

Michael