Michael's last note reminded me of something else. The "AC motors" in
Lionel's trains are actually more properly called "universal AC-DC
motors". They are like permanent magnet field DC motors, but the field
magnets are replaced with a winding which is in series with the rotor.
Thus the proper polarity of the field is always present to propel the
motor in the proper direction with AC, since the current in the field
reverses at the same time as the current in the rotor. To reverse this
motor one need merely reverse the leads of the field.
An AC motor is one of several kinds of induction motor. These can be
readily distinguished from universal AC-DC motors because they have no
brushes and hence no arcing. Universal AC-DC motors produce noise that
can be heard on an AM radio or seen on the screen of a TV; induction
motors do not. Food mixers, weed eaters, and sewing machines are often
fitted with such motors. Higher power motors are almost always of the
induction type.
I always devote a full lecture in an introductory E&M course to eddy
currents, novel induction demonstrations, and induction motors. My
best demonstration (of many in this lecture) involves hanging a metal
ring from a long thread above a magnetic stirrer like the ones used in
chemistry laboratories. Inside the stirrer is a horizontal permanent
magnet which spins about a vertical axis. I open the cover of the
stirrer to show the students the magnet spinning and then replace the
cover and turn of the motor spinning the magnet. The ring is then hung
from above the stirrer, not touching the cover. The stirrer is turned
on and the ring begins to accelerate in the rotating magnetic field. I
then bravely go on to explain shaded pole, three phase, and single
phase induction motors.(I do a dandy standard shaded pole demo which
is standard equipment in the kit with our "jumping ring" apparatus, a
hollow copper ball. When floated in a beaker of water and placed above
the pole of the apparatus, shaded with a piece of aluminum plate, the
ball spins quite rapidly.
I think every student who obtains a physics degree from Simon Fraser
University (or any other respectable institution) should be able to
explain clearly how an induction motor works. I have to admit that
most of ours can't. I haven't changed the world in my 34 years of
trying to do so.