The Mechanism of Passenger Elevator I


Passenger Elevator can be rope dependent or rope-free. There are at least four categories of elevators, namely, traction elevator, hydraulic elevator, climbing elevator, and pneumatic elevator. This time, we will mainly discuss the traction elevator.

In practice, traction elevator can be subdivided into geared and gearless traction elevators.

Geared traction elevators are driven by AC or DC electric motors. Geared elevators use worm gears to control mechanical movement of elevator cars by "rolling" steel hoist ropes over a drive sheave which is attached to a gearbox driven by a high-speed motor. These elevators are generally the best option for basement or overhead traction use for speeds up to 3 m/s (500 ft/min).

Historically, AC motors were used for single or double speed elevator machines on the grounds of cost and lower usage applications where car speed and passenger comfort were less of an issue, but for higher speed, larger capacity elevators, the need for infinitely variable speed control over the traction machine becomes an issue. Therefore, DC machines powered by an AC/DC motor generator were the preferred solution.

The motor-generator set also typically powered the relay controller of the elevator, which has the added advantage of electrically isolating the elevators from the rest of a building's electrical system, thus eliminating the transient power spikes in the building's electrical supply caused by the motors starting and stopping, as well as interference to other electrical equipment caused by the arcing of the relay contactors in the control system.

The widespread availability of variable frequency AC drives has allowed AC motors to be used universally, bringing with it the advantages of the older motor-generator, DC-based systems, without the penalties in terms of efficiency and complexity. The older motor-generator-based installations are gradually being replaced in older buildings due to their poor energy efficiency.

Gearless traction machines are low-speed, high-torque electric motors powered either by AC or DC. In this case, the drive sheave is directly attached to the end of the motor. Gearless traction elevators can reach speeds of up to 20 m/s (4,000 ft/min), and a brake is mounted between the motor and gearbox or between the motor and drive sheave or at the end of the drive sheave to hold the elevator stationary at a floor. This brake is usually an external drum type and is actuated by spring force and held open electrically. A power failure will cause the brake to engage and prevent the elevator from falling. But it can also be some forms of disc type like 1 or more calipers over a disc in one end of the motor shaft or drive sheave which is used in high speed, high rise and large capacity elevators with machine rooms.

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