Abstract :
Low-speed electric elevators, using d-c. motors, came into use about 1890. Later, a-c. motors were employed, but on account of the difficulty of speed control could not be used for the high speeds necessary in tall buildings. Since the height of buildings is dependent upon the elevator system, and in many districts only a-c. is available, the need for high-speed equipments that can be operated from a-c. is evident. A solution to the problem is found in the variable voltage system of control.
