Synchronous motor variable frequency speed regulation system can be divided into two parts: Rectifier (power side converter) and inverter (motor side converter). The control system of rectifier is composed of speed regulator (ASR), current regulator (ACR) and rectifier trigger circuit (AT1), which is basically the same as the common DC motor double closed-loop control system. The difference is that there is an absolute value generator between ASR and ACR due to the unidirectionality of DC bus current. AT1 is responsible for the commutation control of inverter bridge.
The key to the reliable operation of synchronous motor frequency control system is the reliable commutation of inverter bridge. In this system, two commutation methods are adopted, i.e. back EMF commutation method is used at high speed, and intermittent commutation method is used at starting and low speed.
IGBT AC speed regulation is also widely used in continuous casting and steel processing line of iron and steel industry. The main drive of medium power rolling mill will be gradually occupied by AC motor variable frequency speed regulation with IGBT components.
Application of frequency converter in electric vehicle: with the continuous development and maturity of inverter technology, as well as the demand of energy saving and urban environmental protection, electric vehicles, especially the hybrid electric vehicles of urban public transport, have been developing rapidly, and the frequency converter for automobile drive has become the future trend of automobile power system.
The main driving modes of electric vehicles include DC (brush) motor drive system, asynchronous motor drive system, DC brushless motor drive system, permanent magnet synchronous motor drive system and switched reluctance motor drive system. The control methods of frequency converter in various drive systems are different due to different motors, and the characteristics of drive systems have their own advantages and disadvantages. Cage asynchronous motor used in electric vehicle drive has the advantages of simple structure, strong durability, low cost, reliable operation, low torque ripple, low noise, high speed limit and so on. In addition, its speed control technology is relatively mature, so it has obvious advantages over DC brush motor drive system. The permanent magnet brushless DC motor system has higher efficiency and power density, its vector variable frequency speed control technology is gradually mature, energy feedback is more simple, suitable for the drive control of electric vehicles. But there are also some problems, such as high cost, small power capacity, easy to produce noise, torque fluctuations, and the maximum speed is limited by electromotive force.
The special situation of electric vehicle must be considered in the design or selection of frequency converter for automobile drive. The power supply of electric vehicle is battery. The characteristics of battery power supply are voltage fluctuation and internal resistance. Taking a group of lithium-ion batteries as an example, the open circuit voltage can be reduced from 400V to 320v with its discharge rate; when the load output is 80kW, the terminal voltage will drop to 266v at room temperature. At low temperature, the capacity of the battery will be reduced, and the resistance at 0 ° C is more than twice that at 25 ° C. Therefore, when operating at low temperature, the voltage will be further reduced. Therefore, when designing the inverter, it is necessary to consider that the inverter can still output the maximum required inverter voltage and power under such DC power fluctuation. At the same time, it is necessary to study some PWM technology which can improve the output voltage of inverter. For example, the output voltage of the third harmonic superposition technology is about 15% higher than that of the conventional PWM technology. In addition, there are three problems that should be paid attention to in the design and corresponding measures should be taken: first, under low battery voltage, when all the energy of the battery is used, there should be measures to limit the output power; second, when the battery is fully charged, especially at low temperature, the regenerative braking torque should be limited to prevent large voltage rise; third, the charging and discharging of the battery should be considered The influence of life cycle on battery power and internal resistance. Even if lithium-ion batteries are used, the battery capacity will drop to 80% after 1000 charge discharge cycles.
At present, VVVF (constant voltage frequency ratio) and DTC (direct torque control) are widely used at home and abroad for variable frequency control scheme of AC motor for vehicle drive. The advantage of constant voltage frequency ratio control is that the control is simple and easy to realize. The disadvantage is that it belongs to the open-loop control mode, the dynamic performance is not good, and the utilization rate of power supply voltage is low. When it is used in synchronous machine, it is easy to cause out of step due to sudden load or sudden change of speed command. Direct torque control (DTC), through the detected stator voltage and current, calculates the torque and flux linkage of the motor by the instantaneous space vector theory, and realizes the direct control of the flux linkage and torque according to the difference between the measured value and the given value. The main application of DTC control is tractor control, which can also meet the performance requirements of the system, such as output torque and dynamic response, when the torque ripple and control accuracy are low. However, further research is needed for DTC control which requires high control performance.
As a way of AC asynchronous motor control, vector control technology has become the preferred scheme of high performance variable frequency speed regulation system.

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