Here, Gavin Sevier, technical writer for Control Techniques, explains the inner workings of elevators and how drives & motors contribute to their operation.
Elevators (or lifts) are an integral part of modern buildings and considered one of the most important machines of all time. Without them our skyline would look very different as modern skyscrapers would not be practical without a reliable and quick way of transporting people.
Elevators are also the safest form of public transport, with many millions of journeys per day around the world, whilst being the only form of public transport to be driven by the public. Elevator companies’ state figures of around less than 1 chance of 12 million of something going wrong in an elevator system.
What is an elevator?
According to EU DIRECTIVE 95/16/EC (The Lifts Directive) it is a lifting appliance serving specific levels, having a carrier moving along rigid guides for the transport of persons, goods or both, with its controls being situated or reachable from inside the carrier with the appliance moving along a fixed course.
The A17.1/CSA B44 – Safety Code for Elevators and Escalators refers to them as a hoisting and lowering mechanism, equipped with a car, that moves within guides and serves two or more landings.
Overview of main elevator types
- Traction – This is the most common method used in modern buildings where the elevator car and a counterweight are suspended by wire ropes passing over a sheave driven by the elevator machine. Motion is transferred via the friction between the ropes and the sheave. More detail is given in the following section.
- Hydraulic – The elevator car is mounted on the direct-acting piston of a vertical hydraulic ram. The piston moves in a cylinder and the ram is raised using oil under pressure via an electrically driven pump.
- Roped hydraulic – The elevator car is suspended via wire ropes connected to the piston of a vertical hydraulic ram.
Typical traction elevator components
How traction elevators work?
The car is raised and lowered in a shaft by attached steel cables (known as ropes) that loop around a sheave (a pulley with grooves) which grips the ropes through friction or traction, which is in turn powered by a motor. The direction of the motor determines whether the car travels up or down the shaft.
A counterweight is attached to the other end of the rope to offset the weight of the car. This weighs around half the weight of a fully-filled car, so during an average journey the two are balanced. The motor then only needs to provide a nudge to tip the balance, causing the car to move. This saves energy and reduces wear on moving components, with the motor then controlling the falling object in the shaft.
Optimizing ride comfort and journey times
Electronic control systems, using variable speed drive and motors, are designed and rated to give optimum performance, regardless of traffic requirements or installation preference. High speed and smooth control provides a high quality, low noise and jerk-free ride for passengers. Cars are directed to the correct floors using “elevator algorithms” to ensure large numbers of people are moved up and down in the quickest, most efficient way. Intelligent systems can be self-learning or programmed to carry more people upward than downward at the beginning of the day and the reverse at the end of the day.
The role and typical advantages of using modern drives and motors in elevator systems
- The use of drives and motors in modern elevator systems provides many benefits:
- Highly accurate speed profiles and accurate positional control using encoders help to maximize passenger comfort. Drives can be programmed to precisely control motor/car movement with S-ramps providing gentle acceleration that increases until reaching top speed, with deceleration and creep speed occurring as it approaches the required floor.
Typical speed profiles
- Accurate and smooth electronic control of mechanical movements result in less wear of moving parts, requiring less maintenance and ultimately enhancing the active lifetime of elevators.
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- Emergency rescue procedures can be programmed into the drive in order to ensure the safety of passengers during emergency scenarios, along with back-up power supplies for rescue operation.
- Compact drives and motors mean that they can now be mounted in the shaft rather than buildings requiring the dedicated rooms for large machines.
- Silent and vibration-free operation of cars can be provided by:
- Elimination of motor contactors where possible
- Low noise motors
- Intelligent drive fan control
- High switching frequencies & ultra-fast current loop
- Drive and motor solutions are very energy efficient with standby sleep/wake mode allowing unused circuitry to be powered-down during prolonged periods of standby. Also many drives can be configured and connected to feed regenerated energy during braking back onto the supply.
- Modern drive and motor combinations are often able to autotune statically, ensuring optimum performance without the need to de-rope for encoder phasing tests or rotate the motor.
- Elevator builders are able to quickly set-up drives with full parameter sets and speed profiles using portable media such as SD cards to store and clone configurations. Furthermore, PC tools are often available to set-up and fine tune drive settings.
- Modern drives and motors are very robust with features such as active thermal management for trip-less operation under extreme conditions, conformal coating on PCBs for operation in harsh environments and drive phase loss detection.