The supplier of the inverter energy feedback device reminds you that the energy consumption of the electric motor driving the load accounts for more than 70% of the total power consumption. Therefore, the energy conservation of the electric motor and the load it drives has particularly important social significance and economic benefits.
There are two main ways for electric motors and their loads to save energy: one is to improve the operating efficiency of the motor or load, such as installing an elevator with a "memory brain" - in a building, multiple elevators often run in the same direction, which consumes a lot of electricity. How to make elevators smart and energy-efficient? Modern control technology can be said to have solved this problem. "Artificial neurons" are like information processing and memory banks, recording the operation of elevators for each week as a time period. According to the recorded information, the "artificial neuron" will generate the most energy-efficient operating mode, control multiple elevators in the building, make them have clear division of labor, arrive at the appropriate position at the appropriate time, facilitate passengers to get on and off, and reduce the number of elevator starts and runs. For group elevators, energy savings can reach over 30%. In addition, energy-saving measures aimed at improving the efficiency of electric motor operation include automatic shutdown of elevator lighting when no one is riding, automatic stopping or low-speed operation of escalators, etc; The second is to convert the mechanical energy converted by the motor to the load back into electrical energy and send it back to the power grid, so as to reduce the power consumption of the motor and load in a unit time, thereby achieving the goal of energy saving. Energy feedback is a typical device for saving electricity in the second category.
As is well known, electric motors have mechanical kinetic energy when they drive loads to rotate. If electric motors pull loads that move up and down (such as elevators, cranes, reservoir gates, etc.), they have potential energy. When the electric motor drives the load to decelerate, its mechanical kinetic energy will be released; When the potential energy load decreases in motion (potential energy decreases), its mechanical energy will also be released. If these two parts of mechanical energy can be effectively converted into electrical energy and sent back to the AC power grid, the goal of energy conservation can be achieved.
Energy saving analysis of elevators
The elevator using frequency conversion speed regulation has the maximum mechanical kinetic energy after reaching the maximum operating speed. Before reaching the target floor, the elevator needs to gradually slow down until it stops moving. This process is the period when the elevator load releases mechanical kinetic energy. The frequency converter can convert the mechanical energy during this period into electrical energy through the electric motor and store it in the large capacitor of the DC link of the frequency converter. At this time, the large capacitor is like a small reservoir with limited storage capacity. If the water injected into the small reservoir is not discharged in a timely manner, overflow accidents may occur in the reservoir. Similarly, if the power in the capacitor is not discharged in a timely manner, overvoltage may also occur. At present, the method of amplifying capacitors in frequency converters is to use braking units or external high-power resistors, which waste the electricity in the large capacitors to the external high-power resistors. Inverters can return the stored electricity in large capacitors to the power grid without consumption, thus achieving the goal of energy saving and eliminating the need for high-power resistors that consume electricity and generate heat, greatly improving the operating environment of the system.
The elevator is still a potential load, and in order to evenly drag the load, the elevator load is composed of passenger cars and counterweight balance blocks. Only when the load capacity of the elevator car is about 50% (such as a 1000kg passenger elevator with about 7 passengers), the counterweight balance block of the elevator car is in a basic balance state of mass between the two sides. Otherwise, there will be a mass difference between the elevator car and the counterweight balance block, which will generate mechanical potential energy during elevator operation. When the heavy components of the elevator move up, the mechanical potential energy absorbed by the electric motor and converted from the power grid increases. When the heavy components of the elevator move down, the mechanical potential energy decreases, and the reduced mechanical potential energy is released and converted into electrical energy stored in the large capacitor of the DC link of the frequency converter through the electric motor. The energy feedback device then sends this part of the electrical energy back to the power grid.
Analysis, calculation, and prototype testing show that the faster the elevator speed, the higher the floor, and the lower the mechanical rotation consumption, the more energy can be returned to the power grid. The amount of electricity returned can reach about 50% of the total consumption of the elevator, which means the energy-saving efficiency is as high as about 50%.
The above analysis indicates that the use of energy feedback devices has a significant energy-saving effect in fast up and down moving equipment such as elevators and cranes. In addition, there is also a significant energy-saving effect in equipment such as electric locomotives and gantry planers that frequently start and brake.
Structure and basic control principles of energy-saving devices
The main circuit structure of the energy feedback device is shown in Figure 1, mainly composed of a three-phase IGBT (Insulated Gate Bipolar Transistor) full bridge, series inductance, filtering capacitor, and some peripheral circuits.
Application of Energy Feedback Devices in Elevator Energy Conservation
Figure 1: PFE Energy Feedback Device Main Circuit Structure and Connection Method Diagram
Its output terminal is connected to the input terminals R, S, and T of the elevator frequency converter; There are two isolation diodes VD1 and VD2 connected in series at the input end, which are then connected to the PN line of the frequency converter. When the elevator generates electricity through regeneration, the bus voltage of the elevator frequency converter increases, and after passing through VD1 and VD2, the bus voltage of the feedback device also increases. When the bus voltage is higher than the set opening value, the feedback device starts working and feeds back electrical energy to the grid side.
The function of the energy feedback device can be described using Figure 2. The control circuit (within the dashed box) consists of a single-chip microcomputer programmable logic chip and a peripheral signal sampler, coupled with highly redundant software design, enabling the control circuit to automatically identify the phase sequence, phase, voltage, and current instantaneous values of the three-phase AC power grid, and orderly control the IPM (Intelligent Power Module) to operate in PWM state, ensuring that DC power can be promptly returned to the AC power grid.
Application of Energy Feedback Devices in Elevator Energy Conservation
Figure 2 Functional block diagram of energy feedback device
There are currently energy feedback device products available, which have the following characteristics:
① Replacing heating elements such as braking resistors, eliminating heat sources, improving the machine room environment, reducing the adverse effects of high temperatures on components such as motors and control systems, and extending the service life of elevators;
② It can instantly eliminate pump voltage, effectively improve elevator braking performance, and enhance elevator comfort performance;
③ By using phase control strategy, the harmonic interference of the frequency converter driving the elevator on the power grid can be effectively suppressed, purifying the power grid;
④ The output voltage waveform is good, the power factor is high, there is no pulsating circulation, and its voltage matches the grid voltage;
⑤ Disposer de mesures d’isolation électrique efficaces qui n’interfèrent pas avec d’autres équipements électriques et ne sont pas perturbées par des facteurs externes ;
⑥ Le produit possède un haut degré d'intelligence, un fonctionnement stable, une sécurité et une fiabilité élevées, et diverses fonctions de protection contre les pannes et d'alarme sont complètes ;
⑦ Tant que la sélection est correcte, le câblage est correct et qu'il n'y a pas besoin de débogage, il peut être mis en service ;
⑧ Le produit possède une structure simple, une taille compacte et une installation et un entretien faciles.