Escalator is a stairway whose steps move continuously on a circulating belt or simply they are the moving stairs. An electrical services engineer should also know something about escalators.
These are moving staircases. They consist essentially of pivoted steps linked to each other and pulled along by an endless chain.
The steps have guide pins which move in tracks on either side of the tread arranged so that the steps come out of the concealed section horizontally, are then pulled or pivoted into the shape of a staircase and finally return to the horizontal before returning back into the concealed section. The steps complete their circuit within the concealed section, and on this part of their travel they are flat. This is shown in Figure.
The surface of each tread has grooves parallel to the direction of motion. The stationary platforms at the top and bottom of the escalator have fixed combs which mesh with the grooves in the treads in order to ensure a smooth run in and out of the treads under the fixed floor
Components of Escalators
The stationary platform is actually a floor plate over the recess under the moving staircase and covers the working mechanism at the top and bottom landings. It has an extension known as a comb plate and the comb-plate carries the projecting comb teeth.
An escalator also has a balustrade with a handrail, and the handrail moves on an endless chain in step with the stairs. There are separate chains for the handrail and the steps but they are both driven through a gearbox from the same motor. The usual speed of an escalator is 0.5 m/s and it ought never to exceed 0.75 m/s. The inclination varies between 27° and 35° to the horizontal (These are the standard values followed all over the world and may change according to the comfort level of its users).
The drive and transmission have to carry the total load on the escalator. Since people do not stand at even and regular intervals on the whole staircase the load averaged over the whole length of the escalator is less than the maximum load on individual treads. The peak load on each tread is of concern to the structural designer but the electrical engineer concerned with the power requirements can use the average passenger load taken over the total area of exposed treads. This average can be taken as 290 kgm2.
An escalator must have a brake which has to fail safe if there is an interruption to the electrical supply. The brake is therefore applied by a spring or a hydraulic force and is held off against the mechanical force by an electrically energized solenoid. As is the case with lifts, there is also provision for releasing the brake manually and hand winding the escalator. Since an escalator is in continuous operation there are no passenger controls, but there must be an on/off switch which can be worked by a responsible person in charge of the premises. It would be dangerous for the escalator to be started or stopped by someone who could not see the people on it and the switch must be in a place from which the escalator can be seen. Such a place is almost inevitably in reach of the public using the escalator, who ought not to be able to work the switch, and so the switch must be a key operated one.
Design of Escalators
According to the design standards a key operated starting switch to be provided at both ends of the escalator. Emergency stop switches are provided in the machinery spaces under the escalator and also in positions accessible to the public at the top and bottom of the escalator. The operation of any of these switches disconnects the electrical supply from both the driving machine and the brake. The removal of the supply from the brake allows the mechanical force to apply the brake and bring the escalator to a halt. Some escalators are fitted with a speed governor which similarly disconnects the electrical supply from the drive and brake.
There are further safety devices to disconnect the supply if one of the driving chains breaks. Most escalators are reversible. The driving motor is a squirrel cage induction motor and the drive is reversed by contractors which change the phase sequence of the supply to the motor.
A travellator differs from an escalator in being either horizontal or having a very small slope not exceeding 12°. This makes it unnecessary for it to form steps and passengers are conveyed on a continuous platform. The upper surface of the platform must have grooves parallel to the direction of motion which mesh with the comb plates. In other respects a travellator or moving walkway is designed in exactly the same way as an escalator.
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