Electric-Powered Elevators

The electric motor was introduced in elevator construction in 1880 by the German inventor Werner von Siemens. His car, carrying the motor below, climbed its shaft by means of revolving pinion gears that engaged racks at the sides of the shaft. An electric elevator was constructed in Baltimore, Maryland, in 1887, operated by an electric motor turning a revolving drum on which the hoisting rope was wound. Within the next 12 years, electric elevators with worm gearing connecting the motor and drum came into general use except in tall buildings. In the drum elevator the length of the hoisting rope, and therefore the height to which the car can rise, are limited by the size of the drum; space limitations and manufacturing difficulties prevented the use of the drum mechanism in skyscrapers. The advantages of the electric elevator, however, including efficiency, relatively low installation costs, and virtually constant speed regardless of the load, spurred inventors to search for a way of using electric motive power in skyscrapers. Counterweights creating traction on electrically driven sheaves solved the problem.

Since the introduction of electric motive power for elevators, various improvements have been made in motors and methods of control. At first, single-speed motors only were used. Because a second, lower speed was desirable to facilitate leveling the car with landings, low-speed auxiliary motors were introduced, but later several systems were devised for varying speed by varying the voltage supplied to the hoisting motor. In recent years devices for automatic leveling of cars at landings are commonly used.

Originally the motor switch and the brakes were operated mechanically from the car by means of hand ropes. Soon electromagnets, controlled by operating switches in the car, were introduced to throw the motor switch and to release a spring brake. Push-button control was an early development, later supplemented by elaborate signal systems.

Safety devices have been highly developed. In 1878 Charles Otis, a son of the inventor of the original car safety, introduced a similar mechanism connected to a speed governor that applied the safety if the car was traveling at a dangerous speed, whether or not the rope broke. In later car safeties, clamps were used to grip the guide rails so as to bring the car to a stop gradually. Today so-called governors control a series of devices to slow down the car if it is speeding only slightly, to shut off the motor and apply an electromagnetic brake if the car continues to accelerate, and then to apply the mechanical safety if the speed becomes dangerous. Terminal switches independent of other controlling mechanisms stop the car at the upper and lower limits of travel. For low-speed cars, spring bumpers are provided at the bottom of the hoistway; high-speed cars are buffered by pistons fitting into oil-filled cylinders. Electric circuits, completed by contact points in hoistway doors on various floors and in car gates, permit operation only when gates and doors are closed.

The great advances in electronic systems during World War II resulted in many changes in elevator design and installation. Computing equipment was developed for compiling automatically information that vastly improved the operational efficiency of elevators in large buildings. The equipment, which became available in 1948, made possible the solution of such scheduling problems as morning and evening peak loads and traffic balance and the elimination of operators.

The use of automatic programming equipment eventually eliminated the need for starters at the ground level of large commercial buildings, and thus the operation of elevators became completely automatic. Automatic elevators are now generally employed in all types of buildings. The World Trade Center in New York City, with its two 110-story towers, has 244 elevators with carrying capacities of up to 4536 kg (10,000 lb) and speeds of up to 488 m (1600 ft) per min. The 110-story Sears-Roebuck Building in Chicago has 109 elevators with speeds of up to 549 m (1800 ft) per min. In 1995 Schindler Elevator Company developed a new elevator programming system to decrease the time of travel. The Miconic 10 replaces the up/down push buttons with a keypad that eliminates the need for floor buttons inside the elevator. The elevator user types the desired floor into the keypad, and the system computer chooses the most direct elevator.


National Elevator Manufacturing Industry, Inc. "Elevator," Microsoft (R) Encarta. Copyright (c) 1994 Microsoft Corporation. Copyright (c) 1994 Funk & Wagnall's Corporation.