Red writes:

Which brings up a question I've always wondered about: why do most
microwave oven turntables randomly reverse direction at turn-on?

It's a synchronous motor that runs equally well in both directions, and
the direction it goes is effectively random. (You could likely get it
to always start in a particular direction by picking the point during
the AC waveform to turn on the relay supplying power to the motor).

Did you ever wonder why, if you select 45 seconds of heating, the
magnetron runs for 45 seconds but the motor continues to run for another
5 seconds after the magnetron is off? Because the turntable motor
always runs for an integer multiple of the turntable rotation period,
usually 10 seconds. So if you put something at the front of the
turntable, near the door, it will be in that position again when the
oven shuts off.

Dave