Electric motor has been widely used as motion control, and many  type of motor controllers  have been designed to provide variable speed drives for the motor. The best controller is the servo controller, where a precise speed control is implemented using a speed sensor in the feedback path.  The circuit of the motor controller in this article use a novice method to control the motor. It uses the back electromotive force (back-emf)  sensing to keep the motor rpm at a fairly constant value. This motor speed controller is suitable for 3V battery operated devices like portable cassette player or portable audio cd player, keeping the motor rpm to be constant. This circuit is not a PWM motor control, so the motor power should be low power type to avoid excessive heating. This circuit only works for small  dc motor, it won’t work for ac motor. Here is the schematic diagram of the motor controller circuit.

small dc motor speed controller circuit schematic

The motor driver at the final output is integrated in the IC chip, ideal for low component count solution.  Unlike a servo controller, the motor speed is not kept at very tight reference because the feedback is not coming from a speed sensor, but only detect the dc motor’s electromotive force generation to maintain the motor speed at a fairly constant value. [Circuit's schematic diagram source:  STMicroelectronics Application Notes]