Table of Contents
ELEVATOR OPERATIONS AND CONTROLS
The various controls like governing, starting, stopping, direction of motion, acceleration, speed and retardation are specified in IS 1860 : 1968.
ELEVATOR CONTROLS: The generally used controls are given below:
- Single Speed AC Control: A control for a driving Induction Motor which is designed to run at single speed.
- Two Speed AC Control: A control for a two speed driving Induction Motor which is arranged to run at two different synchronous speeds by changing the connection of the windings to obtain different number of poles.
Variable Voltage Control: A system of control which is accomplished by the use of an individual generator for each elevator wherein the voltage applied to the driving machine motor is adjusted by varying the strength and direction of the generator fields.
In this system the elevator motor is a D.C. machine. Each elevator has a motor generator set, thus having in effect its own source of current, the voltage of which is varied at will to change the speed of the elevator.
The majority of the geared machines use rheostat control (mainly AC single / two speed), but a no. of geared machines and practically all gearless elevators use variable voltage control, especially at speed 1.5 to 2.5 m/s for passenger lifts and 0.75 m/s and higher for heavy duty freight elevators.
ELEVATOR OPERATIONS: The two basic modes of lift operation are:
MANUAL: Car switch control.
PUSH BUTTON OPERATION: In the push button operation, pressure of a button starts the car, which then continues to run after the button has been released and stops automatically at the floor corresponding to the button pushed. In the straight push button operation when a button has been pushed, the car is entirely under the control of that one button and cannot be stopped or intercepted by any other button than the emergency stop button. The push button operation is of two basic types:
- Selective Collective Operation: In this system, the car can be started by pressing a button in the car or at the landing and it can afterwards be stopped or intercepted by another button, if that button be at the floor in between the points from which the car started and the floor at which it was called or dispatched by the first button pushed. In this case up and down buttons would be provided at all the landings except in the terminal landing where there shall be only one button. All up landing calls are answered when the car is traveling in the up direction and all down landing calls are answered when the car is traveling in the down direction.
- Non-selective Collective Operation: In this system the car stops at all landings for which button have been actuated making the stops in the order in which the landings are reached after the buttons have been actuated but irrespective of its direction of travel. In this case there shall be only one button at each landing.
SAFETY AND PROTECTIVE DEVICES
Over speed governor and car safety gear: If for any reason the car should descend at a greater speed than that pre-determined and for which the governor is set, the elevator car will be brought to a gradual and smooth stop. The mechanism consists of a centrifugal governor connected by means of a wire rope to a clamping device located under the car platform.
The governor revolves at a speed corresponding to the movement of the car by means of a wire rope one end of which is connected to the clamping device located under the car platform, the rope is then led up the hoist way to the governor, over the governor driving sheave and then down the hoist way to tension sheave in the pit and back to the underside of the car where the other end is fixed. The clamping device is fastened securely to the lower member of the sling supporting the car platform and enclosure. It consists of a system of levers and wedges so arranged that when the governor brings the safety into action, the guide rails on each side of the car is gripped between a pair of jaws.
- Electromechanical Locks for car and landing: These are provided to prevent the car from operating unless all doors are closed and locked. No landing gate can be opened unless the lift is in that landing stop position and without motion.
- Emergency stop switch: This is provided in tha car for the purpose of stopping the car in case of emergency, by canceling all the calls including the one being answered at the time.
- Normal terminal switches: These are provided to disconnect the control circuit and automatically stop the lift car at the terminal landings.
- Final limit switches: This switch is provided to be operated by the movement of the lift car beyond the normal limits of travel at the terminal floors, cutting off all current from the motor and control circuit and causing the immediate application of the brake to bring the lift to rest, in case of failure of the regular terminal switches.
- Buffers: A buffer is always provided for resting of lift after coming to ground. For lower speeds, spring alone is used, but for higher speeds a combination of oil dashpot and spring is used to provide the retarding effect.
- Overload relay: Should for any reason the current drawn by the driving machine exceeds the rated current, the current to the motor is cutoff by this relay.
- Phase reversal relay: Frequently phase failure and the phase reversal relays are combined in one device. The reversal of phases immediately opens the controller circuit and prevents the elevator motor being connected to the lines until the relation of the phases is corrected.
- Retractable safety shoes: A retractable safety shoe will extend to the full height of the projection beyond the front edge of the door. When the car doors are in open position, the safety shoe will retract and thus assuring a substantially clear opening, should the above touches a person or object while the car doors are closing, the car and the hoist way doors will return to the open position. The doors will remain open until the expiry of a predetermined interval and then close automatically.
- Fireman switch: A switch with a glass enclosure shall be provided at the ground floor near the entrance of the lift. By the operation of this switch, the lift car comes to the ground floor without responding to any other call. The above switch will be provided in the lift for buildings whose height is 24 m or more. For the above lift, all the materials used shall be fire resistant. Collapsible doors should not be provided for the above lift to prevent the spread of fire from one floor to another.
Automatic rescue device: This device is used to automatically bring the lift to the nearest landing in the case of failure of supply or due to any fault. It operates by means of a battery which is kept in the charged condition by means of a charger.
METHOD OF MANUAL RESCUE: If the automatic rescue device is not provided, the persons trapped in the lift has to be rescued manually. Normally when the lift is stuck up the car gates can be opened manually after operating the Emergency stop irrespective of the fact whether the car is at the landing or not. The landing gate, if it is in the approachable region, can also be opened by manually releasing the gate lock and then the passengers can be released. All this thing can be done only if experienced lift operator is there in the lift. If however the lift is in the unattended automatic operation, the passenger has to press the alarm bell or use the intercom, if existing. The lift car would then be brought down or up to the nearest landing by an experienced person by manually releasing the brake and using a winding handle to turn the motor. The people can be taken out by opening the landing door and the car door. Use of key would be necessary only when the car is precisely not at the landing level.