Description of traction elevators
The electric traction elevator use machines and cables to move the elevator up and down the hoist way typically with a counterweight system to counter balance the elevator.
Types of traction elevators
Geared traction machines are driven by AC or DC electric motors. Geared machines 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 machines are generally the best option for basement or overhead traction use for speeds up to 500 feet per minute (2.5 m/s).
In order to allow accurate speed control of the motor, to allow accurate leveling and for passenger comfort, a DC hoist motor powered by an AC/DC motor-generator (MG) set was the preferred solution in high-traffic elevator installations for many decades. The MG 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 (causing lighting to dim every time the elevators are used for example), as well as interference to other electrical equipment caused by the arcing of the relay contactors in the control system.
Contemporary 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 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 machines are low speed (low RPM), 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 2,000 feet per minute (10 m/s), or even higher. A brake is mounted between the motor and drive sheave (or gearbox) 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 (see inherent safety and safety engineering).
Machine room less[MRL] elevators are newer to the US commercial market. The technology gives the ability to utilize more of the building space that would be used for elevator machinery. The suspension means, cost, ride quality and reliability of this type of application is still a work in progress.
General information In each case, cables are attached to a hitch plate on top of the cab or may be “underslung” below a cab, and then looped over the drive sheave to a counterweight attached to the opposite end of the cables which reduces the amount of power needed to move the cab. The counterweight is located in the hoist-way and rides a separate railway system; as the car goes up, the counterweight goes down, and vice versa. This action is powered by the traction machine which is directed by the controller, typically a relay logic or computerized device that directs starting, acceleration, deceleration and stopping of the elevator cab.
The weight of the counterweight is typically equal to the weight of the elevator cab plus 40-50% of the capacity of the elevator. The grooves in the drive sheave are specially designed to prevent the cables from slipping. “Traction” is provided to the ropes by the grip of the grooves in the sheave, thereby the name. As the ropes age and the traction grooves wear, some traction is lost and the ropes must be replaced and the sheave repaired or replaced.
Sheave and rope wear may be significantly reduced by ensuring that all ropes have equal tension, thus sharing the load evenly. Rope tension equalisation may be achieved using a rope tension gauge, and is a simple way to extend the lifetime of the sheaves and ropes.
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