Let's say you have a small SMPS transformer like this one......


Should it show at least some voltage fluctuations at the primary and secondary terminals if you place it in a field of magnetic flux or wave a powerful magnet past it?

I placed this one on top of a powered AC adapter (wall wort) with VOM leads set on AC to the various output pins.
I observed no changes in output voltage.
I then waved a very strong magnet very close to it back and forth and observed no changes in output.

What does this indicate?

 

What does this indicate?

That it is working(if it actually is a good one and not burned out) as it should. The core is there to keep the magnetic field where it belongs to work as a transformer, at the center of the coils/windings.

 

Try it again with the voltmeter set to DC (better is DC millivolts).

 

Really, a "perfect" transformer would show nothing at all because all of the magnnetic flux that produces output voltage would be unaffected by an external field. And neither that wall wart nor an external magnet will have much effect on the magnetic path through the coils. The megnetic materil core is there to keep the internal fields internal and it seems to be working.

 

…except for a small possible change in permeability of the core which may go unnoticed,

 

To really know what is happening you need an oscilloscope showing millivolts. The hum pickup in small, low level, transformers can be a problem sometimes, and so a small amount of pickup is possible, but not likely to be noticed in a power transformer. And in a transformer with a higher frequency core like the one shown, it is even less likely.

 

Another thing that some people don't seem to know about transformers is that when powered up the iron core is not an electromagnet. All but a very small amount of magnetic flux is contained to the center of the transformer. Even a large transformer won't hold a paper clip to its self if powered.