Bill Noble wrote:

simple electronic way. Have a magnet on the rotating part. Place a reed
switch near enough that it closes every time the magnet comes by. Have a
timer you set to two seconds. Reset the timer when you presss "start", and
every time the reed switch is closed by the magnet. You can make this timer
from a 555 chip, or just buy a timer. If the timer expires, drop out a
relay to de-energize the motor.

That'll work until the output shaft stalls with the magnet over the reed
switch, holding the 555 timer in a reset state permanently.

Google "watchdog timer", or go to maxim-ic.com and search on that term.
There are one chip solutions that handle this (and other) gotchas.

--
Paul Hovnanian
------------------------------------------------------------------
It's easier said than done.
.... and if you don't believe it, try proving that it's easier done than
said, and you'll see that it's easier said that `it's easier done than
said' than it is done, which really proves that it's easier said than
done.