Elevator energy-saving equipment suppliers remind you that with the continuous enhancement of environmental awareness, energy conservation and environmental protection have become a fundamental national policy with practical significance advocated by China. In today's increasingly competitive elevator industry, the adoption of new technologies, faster speeds, and heavier loads are the most prominent aspects that highlight product advantages. However, it cannot be denied that the economic and environmental benefits of elevators after they are put into use are also factors that must be considered when purchasing elevators.
1、 Basic Structure and Operating Status of Elevators
1. Basic structure of elevator
Nowadays, elevators are mainly composed of traction machine systems, guidance systems, car systems, and door systems. Composed of weight balance system, electric drive system, electrical control system, safety protection system, etc. These parts are installed in the shaft and machine room of the building respectively. Usually, steel wire rope transmission is used, with the steel wire rope winding around the traction wheel and connecting the car and counterweight at both ends. The traction machine drives the traction wheel to lift and lower the car.
2. Analysis of elevator operation status:
When the elevator runs upwards, it consumes energy, and when the elevator descends from a high place, it releases energy. The load dragged by the traction machine in the elevator is composed of the passenger car and the counterweight. In order to balance the drag load, the two are only balanced when the car load is added to 50% of the rated load of the car (for example, a passenger elevator with a load of 1050kg has about 7 passengers). Although this move changes the peak point of energy consumption, it cannot change the average energy consumption. In actual use, the frequency of occurrence of the weight of the counterweight is relatively low, as the weight of the car plus the weight of the passengers is exactly equal to the weight of the counterweight. So the operating state of elevators is basically in an unbalanced state, and it is also very likely that the car will descend when there are many passengers, and rise again when there are few or no passengers. If the first situation occurs when the gravitational potential energy of passengers is released, and the second situation occurs when the gravitational potential energy of the counterweight is released, due to the effect of the potential load, the speed is higher than the synchronous speed, that is, when n>no, the slip rate s=(no - n)/no<0, the rotor induced electromotive force is reversed, the stator winding feeds back electrical energy to the grid, and the T direction is opposite to the speed direction. The motor not only feeds back electrical energy, but also generates mechanical braking torque on the shaft. The sentence is:. However, due to the irreversibility of the AC/DC rectification circuit of the elevator's frequency converter, the generated electricity cannot be fed back to the grid, resulting in an increase in the voltage at both ends of the main circuit capacitor and the generation of "pump up voltage". Generally, variable frequency elevators use resistors to consume stored electrical energy in capacitors to prevent capacitor overvoltage. During elevator operation, these resistors emit a large amount of heat (with a surface temperature of over 100 ℃), and this wasted energy accounts for 25% to 45% of the total electricity consumption of the elevator. The energy consumption of resistors not only reduces the efficiency of the system, but also generates a large amount of heat that accelerates the flow of dust in the air of the machine room, adsorbs static electricity, and greatly affects the environment around the elevator control cabinet. At the same time, the increase in temperature will significantly shorten the service life of the original components of the elevator, and the aging and failure of the components will continue. In order to lower the temperature of the computer room to room temperature and prevent elevator malfunctions caused by high temperatures, users need to install air conditioners or fans with large exhaust volumes; In machine rooms with high elevator power, multiple air conditioners and fans often need to be started simultaneously. Make elevators and air conditioning the most energy consuming "electric tigers".
2、 Operating principle of elevator energy feedback device
To save energy in elevators, the key is to utilize the electrical energy generated by the traction machine during power generation. The energy generated by the braking resistor is then converted back into AC power through inversion, supplied to other electrical equipment, or fed back to the power grid. The general energy inversion efficiency is around 85%, and the energy consumption of the braking resistor mentioned above accounts for 25% to 45% of the total electricity consumption of the elevator. If the floor is higher or the elevator speed is faster, the feedback effect of electrical energy will be more obvious. The main circuit structure of the energy feedback system is mainly composed of filtering capacitors, three IGBT full bridges, series inductors, and peripheral circuits. The input end of the elevator energy feedback system is connected to the DC bus side of the elevator frequency converter, and the output end is connected to the grid side. When the elevator traction machine is operating in electric mode, all switches of the energy feedback system are in the off state. When the traction machine is operating in power generation mode, the pump voltage on the DC bus side of the frequency converter increases and meets other inversion conditions. After that, the energy feedback system starts to operate. As the current energy on the DC is fed back to the grid, the DC bus voltage decreases until it falls back to the set value, and the system stops working.
The active inverter that converts DC electrical energy into AC electrical energy is the essence of elevator energy feedback. The purpose is to feedback the electrical energy generated by the traction machine during power generation through the inverter, achieving energy conservation and avoiding pollution to the power grid caused by the inverter output. So in the process of energy feedback generated by traction machine power generation, four control conditions must be met in terms of phase, voltage, and current:
a) The system cannot be started casually. The inverter device will only start and provide energy feedback when the DC bus voltage exceeds the set value;
b) The inverter current must meet the demand for feedback power and cannot exceed the maximum current allowed by the inverter circuit;
c) The inverter process needs to be synchronized with the phase of the power grid, and the energy feedback to the power grid should be at the high voltage end of the power grid;
d) Minimize the pollution of the power grid caused by the inverter process as much as possible.
3、 Hardware Design of Elevator Energy Feedback System
1. Power inverter circuit
In the power inverter circuit, the direct current stored on the DC bus side of the elevator frequency converter during the operation of the elevator traction machine in the power generation state is converted into alternating current by controlling the on/off of the switch. It is the main circuit of the elevator energy feedback system, which has different structures according to different classifications of inverter circuits. By controlling the on/off of the switch, the DC power stored on the DC bus side of the elevator frequency converter during the operation of the traction machine in the power generation state is converted into AC power. In a circuit, the upper and lower switches on the same bridge arm cannot conduct simultaneously, and the conduction time and duration of each item are controlled according to the inverter control algorithm.
2. Grid synchronization circuit
The phase synchronization control plays a key role in whether the elevator can effectively feedback the energy on the DC bus to the power grid. The grid synchronization circuit adopts grid line voltage synchronization, and in order to avoid dead zone effects during commutation, switches are operated at 120 degrees on the same bridge arm. The logical relationship between the grid synchronization signal and the zero crossing signal of the power grid is obtained through a comparator, and the relationship between the grid synchronization signal of each switching device and the power grid voltage is obtained through Multisim simulation. Each switch has a working angle of 120 degrees and is spaced 60 degrees in sequence. At any time, only two switch tubes in the inverter bridge are conductive, ensuring safe and reliable operation. Additionally, each two switches operate in the highest voltage range of the power grid line, resulting in high inverter efficiency.
3. Voltage detection control circuit
Due to the high voltage on the DC bus side of the elevator frequency converter, it is necessary to first use resistors for voltage division, and then isolate and reduce the bus voltage through Hall voltage sensors, and convert it into a low voltage signal. In the voltage detection control circuit, hysteresis tracking comparison control method is adopted, which adds positive feedback on the basis of the comparator and provides two comparison values for the comparator, namely the upper and lower threshold values. Implemented by hardware circuits, control is both fast and accurate. The voltage detection control circuit can not only avoid the instantaneous superposition of interference signals on the voltage signal, causing the output state of the comparator to shake, but also prevent the energy feedback system from starting and closing too frequently.
4. Current detection control circuit
In the process of energy feedback, the current must meet its power requirements, and the power fed back to the grid must be greater than or equal to the maximum power when the traction machine is in the generating state, otherwise the voltage drop on the DC bus will continue to rise. When the voltage of the power grid is constant, the energy feedback power of the system is determined by the feedback current. In addition, the feedback current must be limited within the rated range of the inverter power switch device. Moreover, the reactance choke between the power grid and the inverter allows large currents to pass through while minimizing the volume of the reactor. Therefore, the inductance of the reactor must be a small value to ensure energy feedback. The speed of current change is very fast. Simultaneously using current hysteresis control can effectively control the feedback current and prevent overcurrent accidents.
5. Main control circuit
The central processing unit of the elevator energy feedback system is the main control circuit, which is used to control the operation of the entire system. The main control circuit consists of a microcontroller and peripheral circuits, which generate high-precision PWM waves based on control algorithms; On the other hand, based on the grid synchronization signal, IPM fault control ensures the safe and effective implementation of the entire energy feedback process.
6. Logic protection control circuit
The synchronization signal for grid connection, control signals for voltage and current, IPM fault signal, and drive signal output from the main control circuit all need to pass through the logic protection control circuit for logical operation, and finally be sent to the power inverter circuit to control the feedback process. In this way, it can ensure that the AC power output from the inverter is synchronized with the grid, and also block the drive signal in case of overcurrent, overvoltage, undervoltage, and IPM faults in the circuit, stopping the energy feedback process.
Due to the fact that the elevator energy feedback system only starts when the traction machine is in the generating state, its service life is longer than that of the elevator. From this, it can be seen that the application of elevator energy feedback systems, in terms of principles, energy-saving effects, and performance, is worth promoting vigorously in today's increasingly scarce energy environment. This not only creates a healthy and good green energy-saving environment, but also responds to the call of the country and the government for energy conservation and consumption reduction, and the construction of a conservation oriented society, contributing to the country's energy conservation and emission reduction efforts.