Don Foreman wrote:

Nick's idea of half-wave rectifiying is simple, small, and will work
well if the transformer doesn't saturate. (It probably won't).

Another possible gotcha is the current. While the diode responds to
average current, the copper responds to rms current. It would need a
test to see if the transformer overheats. BTW, don't forget a catch
diode across the motor.

Another way that would be very small would be to chop the rectified 24
volts at a high frequency, using a MOSFET driven by a 555 timer.

That's certainly the most elegant solution, IMO.

Ted