Variable Voltage Variable Frequency (VVVF) controller
Technology outline & How it can save energy
Operating characteristics of different motor drives
A variable speed drive system employs frequency inverter technology which rectifies AC voltages from the mains supply into DC, and then converts this into AC voltage with variable amplitude and frequency (i.e. VVVF control). The motor is thus supplied with variable voltage and variable frequency which enable infinitely variable speed regulation.
In lift applications, VVVF control regulates input voltage and frequency to the motor throughout the journey. The figures below illustrate the variation of electrical current drawn by different lift motor drives during the whole journey of a lift car. The speed of the lift increases (accelerates) and decreases (decelerates) gradually at the beginning and ending respectively of the journey as shown in the top graph. When compared with AC 2 speed drive (middle graph) and ACVV drive (bottom graph), VVVF drive draw much less current during acceleration and deceleration.
In escalator applications, VVVF control can be incorporated with automatic start/stop control or automatic two-speed control to vary the escalator speed according to the passenger flow. The operation of these kinds of escalator is determined by the presence or absence of passengers, hence energy can be saved when the escalator is idle.
How much energy can be saved
In lift applications, variable speed drives can reduce peak motor starting currents by as much as 80% compared with conventional motor drives. Further, wear and tear of the equipment can also be reduced during start/stop of the motor by using VVVF motor drive.
In escalator applications, the measured energy saving of automatic start/stop and two-speed escalator can be up to 52% and 14% respectively in an office building. Please click here for details.
Technology outline & how it can save energy:
The energy optimizer (also known as performance controller, energy saver or power factor controller) is a solid-state controller that reduces losses in AC induction motors in the form of energy efficiency and soft starting capability. It is not a variable speed drive and does not change the frequency of the motor. During the low load condition, induction motors usually operating at full supply voltage have very low power factor and are less efficient. The energy optimizer could provide the required motor operating voltage to suit various loading conditions, resulting in power factor improvement and reduction of motor losses. The other benefit of the energy optimizer is its soft starting property that reduce motor starting current and excessive wear of mechanical gears, chains, belts etc. associate with the mechanical transmission system. The energy optimizer could be use for any AC motor application with constant speed and variable load. Best applications are those motors with substantial variation in loading such as escalator and passenger conveyers.
An energy optimizer
How much energy can be saved
The amount of energy saving due to the energy optimizer in lift and escalator applications depends on the actual load of the lift or escalator. Based on the measurement of a retrofit project in a government office carried out by Electrical and Mechanical Services Department, the average energy saving can be up to 10% by using the energy optimizer.
This flash illustrates service-on-demand escalator. The paragraph below describes service-on-demand escalator.
Modern escalator systems can be designed to detect passenger presence and stop when no activity has been detected for a period of time. The system will only be re-started automatically once a passenger has been detected. Since the operation of the escalator is determined by the presence or absence of passengers, it is known as "service-on-demand" (SOD) escalator. There are basically two types of SOD escalator.
(a) Automatic start/stop control
When an approaching passenger is detected, the escalator will start running and complete the traveling cycle. The escalator will stop after a period of time when no further passenger is detected.
(b) Automatic two-speed control (crawl mode)
Similar to the arrangement of auto on-off controlled escalator, the auto two-speed control SOD escalator will be actuated by the presence of passenger to run at rated speed. The auto two-speed controlled SOD escalator will run at a lower speed (crawling speed) when it detects no passenger for a set period of time. The crawling speed is usually set at about 0.2m/s, while the rated operating speed is 0.5m/s to 0.75m/s.
Various kinds of detection methodologies can be employed for sensing the presence of passenger, such as optical detectors, step sensors, light barriers etc. The detectors for monitoring the approaching passengers can be integrated into a pair of sensing post installed at the entry of the escalator, or they can be incorporated into the handrail entry of the escalator.
How it can save energy
Service-on-demand escalators are designed to detect the presence of passengers and either stop or slow down when no activity has been detected for a period of time. Hence, energy can be saved when the escalator is idle or operate at lower speed.
How much energy can be saved
The amount of saving depends on the type of buildings and passenger flow pattern. Based on an measurement carried out by the Electrical and Mechanical Services Department, the energy saving of service-on-demand escalators can be up to 52% and 14% for automatic start/stop and two-speed escalator respectively in an office building.
Code of Practice on the Design and Construction of Lifts and Escalator published by EMSD.
Other energy saving measures in lift and Escalator systems
Reduce escalator step load
To replace the existing heavy escalator steps by glass fibre material. Whole step is 30% lighter in weight thus reduce step motor loading and result in lower energy consumption.
Reduce escalator motor load
Replacing power transmission chains with non-metallic materials or plastic-based materials (e.g. fibre glass reinforced plastic) can reduce the motor loading and result in lower energy consumption.
Standby mode of lift equipment
Some lift equipment can be shut down when the lift is being idled during off peak hours, while keeping the demand during off peak to be handled by the remaining equipment (e.g. shut down one of the lift in a lift bank).
The control of early lifts was achieved by electro-mechanical relays. Modern systems use software to provide flexible control. Some more expensive systems can "learn" where to position cabs at specific times, such as having all lifts return to the lobby in the early morning. Some will automatically save energy by matching the number of active elevators to the load in that interval.