On Figure 5 you can see the two printed circuit boards that exist inside the floppy disk drive. One is the logic board, which holds the drive’s controller. The other is the spindle motor board, which holds the spindle motor and its servo controller.
click to enlarge Figure 5: Logic boards.
The spindle motor uses a sensor for checking if it is rotating or not and also to check its speed. The servo controller will keep the motor spinning on its correct speed thanks to this sensor. So, this motor is a closed loop system, i.e. it has a sensor that feeds the controller in order to perform any speed adjustments that may be necessary to keep it rotating at its correct speed.
click to enlarge Figure 6: Spindle motor speed sensor.
