How does the comparator set the motor speed at a fixed level?
The so-called "comparator" outputs a voltage proportional to the difference between the generator voltage (some kind of ideal generator that outputs a smooth voltage) and the reference voltage across the Zener. That difference signal is amplified and fed to the motor. 

This is a simple proportional control system and will have a steady state error inversely proportional to the gain, if it is stable. 

Calling it a "comparator" is just wrong, at least in English.