The single trim motor was introduced on the NG. Previously the 737 used separate motors for A/P trim and pilot operated trim.
About how exactly speed of the motor is controlled, earlier I guessed it may use different voltages, but I'm probably wrong. I don't have much experience with motors.
I did find a document describing the NG trim motor upgrade between the B version and the C version (the max uses the D version). From that document:
Eatons new Model 6355C Stabilizer Trim Motor features: Brushless three phase motor design Low loss power bridge with IGBT switches Processor based motor commutation and velocity control Dual current limit (torque) control circuits Power up built-in test Continuous fault monitoring Fault storage (non-volatile memory) RS-232 test/maintenance interface Investment cast housing Two stage spur gear train Modular, bottom up assembly two electronic sub-assemblies, motor, housing with gear train
The D version on the max is probably an evolution of the C version, so many of those features could apply to it as well.
http://www.eaton.com/ecm/idcplg?IdcS...ame=PCT_201019