Electronic control of D-C motors

IN recent years the need for an improved adjustable-voltage d-c motor drive operated from an a-c supply has been continually increasing. Although the conventional drive of that type using a motor-generator set, and sometimes referred to as the Ward-Leonard system, adapts a d-c motor to an a-c line and provides adjustable armature voltage for the motor, it does not solve all of the problems involved. Thus, good speed regulation at all speeds, ability to deliver full torque at speeds as low as 1/50 of the rated speed of the motor, and automatically controlled acceleration and reversal are but a few vital problems which have been awaiting solution.