Traction elevators are the most common type of elevators. Elevator cars are pulled up by means of rolling steel ropes over a deeply grooved pulley (sheave). The weight of the car is balanced by a counterweight. Sometimes two elevators are built so that their cars always move synchronously in opposite directions, and are each other's counterweight.
Geared Traction Elevators are driven by AC or DC electric motors. As the name implies, the electric motor in this design drives a worm-and-gear-type reduction unit, which turns the hoisting sheave. While the lift rates are slower than in a typical gearless elevator, the gear reduction offers the advantage of requiring a less powerful motor to turn the sheave. These elevators typically operate at speeds from 38 to 152 meters (125-500 ft) per minute and carry loads of up to 13,600 kilograms (30,000 lb). An electrically controlled brake between the motor and the reduction unit stops the elevator, holding the car at the desired floor level.
Contemporary cheaper installations, such as those in residential buildings and low-traffic commercial applications generally used a single or two speed AC hoist machine. The widespread availability of cheap solid state AC drives has allowed infinitely variable speed AC motors to be used universally, bringing with it the advantages of the older motor-generator based systems, without the penalties in terms of efficiency and complexity. The older MG-based installations are gradually being replaced in older buildings due to their poor energy efficiency.
Gearless Traction Electric Elevator first invented in 1913, first installed in Woolworth Building in New York City, and the Majestic Building in Chicago. This type of drive system could be employed in buildings of any height and operated at much higher speeds than steam-powered elevators. This design has proven so durable that even now when a building is modernized—while the elevator control system is replaced with the most up-to-date electronics—it is rarely necessary to replace a well-maintained gearless machine. These elevators typically operate at speeds greater than 500 feet per minute.
In a gearless traction machine, five to eight lengths of wire cable, known as hoisting ropes (or wire ropes), are attached to the top of the elevator and wrapped around the drive sheave in special grooves. The other ends of the cables are attached to a counterweight that moves up and down in the hoistway on its own guide rails. The combined weight of the elevator car and the counterweight presses the cables into the grooves on the drive sheave, providing the necessary traction as the sheave turns.
To reduce the load on the motor, the counterweight is calculated to match the weight of the car and a half-load of passengers. As the car rises, the counterweight descends, balancing the load. This reduces energy consumption because the motor is required to lift no more than the weight of half a carload at any time. The grooved sheave in this traditional gearless system is quite large, from 0.6 to 1.2 meters (2–4 ft) in diameter. The electric motor that runs it must be powerful enough to turn this large drive sheave at 50–200 revolutions per minute in order to move the elevator at the proper rate.
Safety is provided by a governing device that engages the car’s brakes, should the elevator begin to fall. A powerful clamp clutches the steel governor cable, which activates two safety clamps located beneath the car. Moveable steel jaws wedge themselves against the guide rails until sufficient force is exerted to bring the car to a smooth stop.
Elevators with more than 100 ft (30 m) of travel have a system called compensation. This is a separate set of cables or a chain attached to the bottom of the counterweight and the bottom of the elevator cab. This makes it easier to control the elevator, as it compensates for the differing weight of cable between the hoist and the cab. If the elevator cab is at the top of the hoist-way, there is a short length of hoist cable above the car and a long length of compensating cable below the car and vice versa for the counterweight. If the compensation system uses cables, there will be an additional sheave in the pit below the elevator, to guide the cables. If the compensation system uses chains, the chain is guided by a bar mounted between the counterweight railway lines.
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