1. Sequence of steps for the elevator to run:
We shall pick up the elevator control and drive designs which existed in India during the mid-1980s and proceed from thereafter.
A basic elevator control and drive system should have the following minimum intelligence:
A means to know the position of the car.
A means to upgrade the car's position while moving.
Should be able to receive calls from the car inside or the landing.
Close the car and landing doors and move.
Decide the direction of travel by comparing the position of lift car with called floor.
Stop at the required floor.
Open the doors.
2. How does the elevator sense its position in the hoist way?
The car is mounted with many mechanical cams and they operate switches mounted in the hoist way. The control system looks for operation of SET switch which is mounted at the bottom most floor and sets its own position as Zero. If the control system does not find the signal, it means the car is above that floor. In that case the control system makes the car to move down at a very slow speed till the SET switch is operated. This has been the logic followed right from 1980s till now. Later designs had a battery backup to keep its position with power failure. But operation of SET switch at the bottom most floor was the most reliable way of knowing its position even today. In the recent designs, the lift car retains its position even if the power is turned OFF and ON thus avoiding correction run to the bottom floor.
3. How does the elevator upgrade its position during movement?
There are many methods followed. One popular method is to fix a metallic vane at each floor and a photo electric switch is mounted on the car top. As the car moves, interceptions of vane with the photoelectric switches are counted by the control logic to update its position. During the UP travel the counts are increased and while traveling down the counts are decreased.
Each time the car reaches the ground floor, the count is reset to Zero by virtue of the SET switch to take care of any possible counting errors.
4. How does the elevator know that a call has been registered?
Elevator control system is generally installed in the machine room. This controller is microprocessor operated. The call buttons in the COP (Car operation panel) mounted inside the car are wired to the controller. Similarly, there are hall buttons in each landing which are also wired to the controller. Whenever a Car or Hall button is pressed it is sensed by the controller. In the newer designs discrete wiring of these buttons has been replaced by serial communication. Serial communication saves cost due to reduction in the number of wires, especially as the number of floors increase.
5. How does the car move to the required floor?
The controller compares the position of the lift with respect to its call. For example, if the lift is in position 3 and a COP button corresponding to floor 8 is pressed, the controller decides the direction of travel by comparing the position of lift with called floor.
This is achieved by the control logic by comparing the car's position with that of the called floor. First the controller commands to Close the door. Before actual movement of car, the doors must be closed. Where manual doors are used, it has to be physically closed by the passenger. In case of auto doors, the controller signals the door operator, which is mounted on the car to close. The car door is closed by means of door operator control logic. The landing doors are mechanically coupled to the car door and hence the landing door closes along with the car door. Both the car and landing door must be closed before a run is initiated. This is sensed by operation of independent switches for car and landing doors. When the car door closes, the car and landing switches operate and he car can now move.
As the lift moves, the photo electric cells are intercepted and the controller increments its count. When the count matches with the called number of the call button, the controller makes the motor to slow down and stop at door zone. At each floor level, another photo electric switch mounted on car top is intercepted by a vane mounted in the hoist way known as door zone switch. Once the door zone is reached and if the car is moving at slow speed, a stop command is given by the logic control to the motion control and the car stops at the door zone. After stopping, the controller gives a command to the door operator to open the door.
6. Classification of Elevator:
For easy understanding, we has classified the elevators based on their different attributes.
The Indian Standard classifies the type of elevators as Passenger lift, Goods lift, Hospital lift and Service lift. The classification is made for the purposes of outlining the speed and load.
Geared and Gearless Elevators.
Elevators with machine room and machine room less (MRL) elevators.
Elevators based o type of drive system namely Single Speed, Two Speed and Variable Speed.
Elevators based on the type of control system namely Down Collective, Simplex, Duplex, Triplex and Multiple Elevators.
Classification as per Indian Standard:
The fundamental specification of an elevator is the load it can carry in KG and its rated speed in meters per second m/s. The other equipment is built to fulfill these parameters and normal operation is then guaranteed by the manufacturer.
The Bureau of Indian Standards IS 14655 specifies the weight of one passenger for the purposes of any calculation as 68 Kg. The European Standard as per EN 81 specifies the load of one passenger as 75 Kg for the purposes of calculations.
Passenger Lift: A lift designed for the transport of passengers.
Goods Lift: A lift primarily designed for the transport of goods but which may carry a lift attendant or other person necessary for the unloading and loading of goods.
Service Lift (Dumb Waiter): A lift with a car which moves in guides in vertical direction, has net floor area of 1 sqm, total inside height of 1.25m whether or not provided with fixed or removable shelves and capacity not exceeding 250 Kg and is exclusively used for carrying materials and shall not carry any passenger.
Hospital Lift: A lift normally installed in a hospital/ dispensary/ clinic and designed to accommodate one number bed / stretcher along its depth with sufficient space around to carry a minimum of three attendants in addition to the lift operator.
The table below outlines the recommended Load and Speeds for a maximum travel of 30m. The code also provides the area required inside the lift for the passengers to stand comfortably in a fully loaded car.
| Passenger Lift | Goods Lift | Hospital Lift | Dumb Waiter |
Typical Speed (mps) | 0.7~2.5 | Up to 0.5 | Up to 1.5 | Up to 0.5 |
Load in Kg ('P' indicates No of Passengers) | 272 (4P) 408 (6P) 544 (8P) 680 (10P) 884 (13P) 1088 (16P) 1360 (20P) | 500 1000 1500 2000 2500 3000 3500 4000 4500 5000 | 1020 (15P) 1360 (20P) 1768 (26P) | 100 150 200 250 |
Table 1: Recommended Load and Speeds as mentioned in IS14665(part1):2000
Recommended car inside area: (Refer BIS)
No. of Passenger | Rated Load @68 Kg/ Pass | Max Net Inside Car Area | Proposed Allowable Minimum Area |
4 | 272 | 0.77 | 0.68 |
5 | 340 | 0.95 | 0.85 |
6 | 408 | 1.12 | 1 |
7 | 476 | 1.28 | 1.16 |
8 | 544 | 1.45 | 1.31 |
9 | 612 | 1.6 | 1.45 |
10 | 680 | 1.76 | 1.6 |
11 | 748 | 1.91 | 1.74 |
12 | 816 | 2.05 | 1.87 |
13 | 884 | 2.2 | 2.01 |
14 | 952 | 2.34 | 2.14 |
15 | 1020 | 2.47 | 2.27 |
16 | 1088 | 2.61 | 2.4 |
17 | 1156 | 2.74 | 2.52 |
18 | 1224 | 2.87 | 2.64 |
19 | 1292 | 3.00 | 2.76 |
20 | 1360 | 3.13 | 2.88 |
| 1500 | 3.38 | 3.11 |
| 2000 | 4.22 | 3.88 |
| 2500 | 4.99 | 4.59 |
Geared and Gearless Elevators:
In gearless elevators, the motor rotates the sheaves directly.
In geared elevators, the motor turns a gear that rotates the sheaves.
Let us first understand why the gears were required in an elevator. Elevators use Worm gears to reduce the speed of the motor to drive a large gear wheel to turn with more force (Enhanced torque). It is also useful for changing the direction of rotation in gear-driven machinery. A worm gear is chosen over other types of gearing possibilities because of its compactness and its ability to withstand higher shock loads. It is also easily attached to the motor shaft, sometimes through use of a coupling. The gear reduction ratios typically vary between 22:1 and 69:1.
By proper selection of gear ratio and the sheave diameter it is possible to attain speeds up to 2.5mps from a 1000 or 1500 RPM induction motor. For speeds above 2mps, it is preferable to use Gearless lifts. The major components of a geared machine are motor, brake, worm gear, Traction sheave and brake assembly. The control system, Governor, Interphone, Automatic Rescue systems etc. are all housed in a Machine room above the elevator shaft.
The motors used are normally 3 phase induction motor. When compared to other applications, in lifts, the motor is to start with load. Normal motors have 150% to 175% starting torque. Since the lift motors have to start with the load, anything between 250% to 300% torque is required. Motors for elevator applications are made with this specific requirement. With advancement in power electronic technologies, the high starting torque requirements are controlled by the electronic drive system, the ride quality and leveling accuracy have improved to a large extent along with the efficiency.
In order to obtain the benefits of gearless system, it was required to reduce the speed of the driving motor RPM to very low levels which was technologically difficult till recent times. The earlier types of gearless elevators used Ward Leonard speed control system, where in a Generator was driven by a prime mover and the generated power was used to control the speed of the DC motor by controlling its armature current.
The technological advances have made it possible to control the speed of the Permanent magnet synchronous motors by Pulse width modulation techniques. Permanent magnet motors are designed to have very low speed and with high torque. The machines come with wheel and brake all assembled as a package.
The geared system has the following disadvantages over the gearless system:
Less Efficient due to losses
More power consumption due to energy loss
Require oil for cooling the gears
Requires maintenance
Additional Space
Poor ride quality such as noise, vibration.
We consider a gearless traction machine superior to s Geared machine because it is nearly 40% more efficient and quieter in operation, requires less maintenance and has longer life, no bearing noise, less vibration, no oil for cooling the gears and hence no problem of oil leakage. The decision as to whether these advantages are worth the additional cost involved can be made only after careful analysis. Generally a gearless machine was preferred for high rise, high speed operation but with the advent of technology the gearless elevators are now used for even up to 1mps. For Speeds below 1mps, the geared systems are still popular, though fading.
Elevators with machine room (MR) and machine room less (MRL) elevators:
At present the elevator industry is offering a relatively new elevator product termed Machine Room Less Elevators application in new construction or major renovation projects compared to standard elevator equipment. It is a fairly significant decision as it will affect the design of the elevator hoist-ways and equipment rooms.
The machine room les elevator is the result of technological advancements that often allow a significant reduction in the size of the electric motors used with traction equipment. These newly designed permanent magnet motors (PMM) allow the manufacturers to locate the machines in the hoist-way overhead, thus eliminating the need for a machine room over the hoist-way. This design is nowadays becoming the standard product for low to mid rise buildings. It was first introduced to Europe market by KONE though with geared arrangement Product acceptance was initially slow in the U.S. market because of its initial, limited applications, its inability to meet U.S. code requirements, and the limited number of manufacturers offering an equivalent product.
In the past few years the elevator manufacturers have overcome the obstacles to acceptance of the MRL product. All of the other elevator manufacturers are now marketing their versions of the MRL, and the product offering has been expanded to include most of the more popular elevator sizes and speeds. In addition, local code officials have become more receptive to the change in technology.
Recent developments by introduction of coated steel belts instead of ropes have made it possible to reduce the sheave diameter substantially to get 1mps speed at about 490 RPM with 2:1 roping. The coated steel belts are used only by a few manufactures and the steel ropes are still popular with most of the manufacturers.
While the hoisting motor is installed on the hoist way side wall, the main controller is installed on the top floor next to the landing doors. This controller is situated behind a locked cabinet which has to be unlocked using a key for maintenance, repair or emergency purposes. Inspection windows were provided to have visibility inside the shaft especially the main rope the governor and Governor ropes. Most elevators have their controller installed on the top floor but fewer elevators have their controller installed on the bottom-most floor. Some elevators (like OTIS and Schindler) have the controller cabinet installed within the door frame instead on the wall to save space. In some cases, Otis motors are installed above the counter weight rails. There is a continuous improvement happening in the positioning of the various equipment considering the maintainability and safety.
Like normal traction elevators, MRL elevators use the conventional steel cord ropes used as the hoisting cable. Some elevator brands (such as Otis, Schindler and ThyssenKrupp) are using flat steel rope belts instead of conventional ropes. Manufacturers using this technology claimed that with flat steel belt ropes, it saves much space on the hoist way and to allow a minimum size of the hoisting sheave.
Most MRL elevators are used for low to mid rise buildings. MRL elevators in mid-rise buildings typically serve up to 20 floors.
Benefits of the MRL Elevator:
Machine room is no longer needed, as all machineries successfully fit into the hoist way, except the control panel, which can be placed anywhere within a short distance from the traction machine.
A conventional machine room elevator applies load stress on a building's structure, whereas the guide rails support as much as 75% of the stress for added building friendliness in case of certain Otis and Mitsubishi elevator types.
In new building application, Architects naturally like to adopt the fact that no rooftop lift machine room is required allowing a clean roofline and maximizing available space.
Limitation of the MRL Elevator:
As previously stated, the machine room less lift depends on the relocation of machine room parts in order to negate the need for a separate room. This means that a new position for the control panels needs to be found. In most cases the panels are incorporated into extended architraves or special panels, usually at the top floor. Not only must the visual aspects of the panels be accepted by the Client, their position increases the maintenance necessity at this landing. In an office block this could cause some disturbance, but if children or animals are resident, it could cause an increased risk to the safety of all, including the maintenance engineer.
In certain cases, especially with five star hotels, We also attended complaints of elevator noise in the adjacent rooms due to machine noise and / or noise from the controller parts.
As far as the user is concerned, there is very little difference under normal conditions between a machine room less lift any other traction drive unit. However because the winding machine is located inside the shaft it is not directly accessible.
During a power failure, in case of elevator with machine room, it can be manually cranked up or down so that passengers can be released from the nearest floor. Under normal circumstances the car would be moved in the direction of least resistance determined by whether the car or counterweight is heavier. Should circumstances dictate that the car is moved in a particular direction, for example a fire on the floor below a fully loaded car, this can be achieved, if perhaps, it takes a little effort. Where the drive machine is not directly accessible, the full control in an emergency is not possible. On some machine room less applications a battery backup system can be provided, as an extra cost, but it often cannot move a fully loaded car.
When the elevator stops suddenly due to power outage, the MRL gearless elevators seem to introduce heavy jerk as compared to equivalent geared elevators. This causes discomfort especially to elderly and pregnant women.
Until the very recent past the MRL elevator products did not offer an economical advantage over traditional elevator products. The market is now experiencing certain MRL elevators priced more competitively, comparable to a traditional geared traction elevator. Certain manufacturers have tried to introduce a cost competitive product to compete with the low rise hydraulic elevator.
Thanks for reading this blog, for more topic comments the topic name.
コメント