The air compressor is a kind of gas compression and conveying equipment, which is widely used in various fields of national production. In order to ensure the air pressure system security fund project: Hunan Provincial Science and Technology Plan Project 011FJ201S; Hunan Provincial Department of Education is fully efficient operation, it is necessary to control the pipe network pressure well. At present, there are two main methods for air compressor pressure control: (3) The normal addition and unloading mode is to adjust the air pressure by controlling the start and stop of the motor. When the pressure is lower than the loading pressure, the motor starts, when the pressure is higher than the unloading pressure. When the motor stops, the Q frequency conversion mode controls the air pressure by the frequency conversion speed control to adjust the air pressure. When the gas storage pressure is lower than the demand pressure, the motor accelerates, and when the gas storage pressure is higher than the demand pressure, the motor decelerates. Due to the large amount of air compressors used, it is a large energy consumer; improving the air compressor control performance and reducing its energy consumption has become a hot issue in the development of air compressors. The general energy-saving method is mainly to reduce the pressure loss and air leakage of the pipeline by rationally configuring the gas pipeline; reasonable lubrication, using lubricating oil with low viscosity and good lubricating performance to reduce the frictional power consumption; while ensuring the actual air volume It is possible to set the exhaust pressure of the air compressor low, because the lower the exhaust pressure setting, the less shaft power is consumed; the air compressor is regularly maintained to give the best performance of the machine; The motor; or seek to add and unload the optimal control scheme under operating conditions to achieve energy saving purposes. However, these measures can not fully tap the energy saving potential of the air compressor. At present, most of the air compressors are driven by ordinary asynchronous induction motors. The use of non-frequency control technology wastes energy. With the development of modern motor frequency conversion technology such as PWM technology vector control technology and direct torque control technology, inverter air pressure is adopted. The machine replaces the fixed speed air compressor as the main energy saving measure at present and in the future. This method not only avoids the frequent start and stop of the air compressor, but also has significant energy saving effect. At present, the main means of air compressor frequency conversion energy saving is to use air compressor to configure the inverter. Most of the operation and protection adopt PLC intelligent control. The whole system takes the output pressure as the control object, and the feedback signal is taken out by the pressure sensor. The operating frequency and the speed of the motor are determined according to the fluctuation amount of the pressure, and the automatic adjustment is realized. In this respect: He Fengyou et al. applied fuzzy PID control to the air compressor constant pressure air supply system consisting of PLC and frequency converter; Xu Zhihua used pressure transmitter, artificial intelligence regulator, frequency converter to screw type The air compressor is used for energy-saving transformation; Sun Libo and others use the modeless control technology to transform the vector control inverter of the air compressor, optimize the operating parameters of the air compressor, and improve the adaptability of the device. Direct-coupled air compressor driven by DC brushless motor, based on the idea of ​​direct torque control, according to the characteristics of direct-coupled air compressor load, a pulse width modulation based on single chip microcomputer (PWI variable duty cycle control) is proposed. Methods: These research work have played a positive role in promoting the optimization of air compressor operation control.
Based on the comprehensive analysis, this paper constructs a model of air compressor pressure control system including air compressor tank model, motor submodel and control submodel, and analyzes the dynamic characteristics of air compressor system pressure control. To provide a theoretical basis for the optimal control of the air compressor.
2 Compressed air supply system modeling 2.1 Air compressor system introduction Air compressor system is generally composed of main engine and motor system, lubrication and cooling system, oil and gas separation system, pneumatic system, control system, etc., where the main engine is the core of air compressor The component, the twin-screw air compressor main engine is a pair of intermeshing, oppositely rotating helical teeth. When the rotor rotates, the volume between the teeth gradually expands, and the gas enters the interdental volume through the suction port until the volume reaches the maximum, and is disconnected from the suction port. At this time, the volume between the teeth is closed to complete the inhalation process. After that, the rotor continues to rotate and the volume is gradually reduced. During the compression process, the oil forms a layer of oil film between the rotor slots, which not only avoids direct contact between the metal and the metal, but also seals the gap between the rotor parts and absorbs the heat of compression. The result of shrinking volume is that the compressed gas is sent to the exhaust pipe until the volume is at a minimum. The figure shows a system diagram of a screw air compressor (with oil) system.
For the air compressor system shown, in order to ensure its safe and effective operation, it is generally realized by online monitoring of key operating parameters in the air compressor system. Mainly include: exhaust pressure, exhaust temperature, motor temperature, fan motor temperature, full gas filter pressure difference, oil separator pressure difference and other parameters.
1. Oil separator 2. Pressure controller 3. Display 4. Minimum pressure valve 5. Temperature control valve 6. Oil filter 7. Cooler 8. Air filter 9. Control solenoid valve 10. Screw main unit 11. Motor 12 Oil tank 13. Safety valve For the convenience of analysis, the air compressor tank is assumed as follows: 0) When the air compressor tank is in and out, the temperature in the tank is basically the same. The temperature in the tank is 300K thermodynamic temperature; when the Q tank is in and out, the time for the gas to reach equilibrium is negligible, that is, instantaneous balance; the pressure gradient of the inlet and outlet of the Q tank is small, ignoring the influence of pressure gradient (4) Before the gas tank is not inflated, the pressure in the gas tank is balanced with the external pressure, which is about one standard atmospheric pressure; assuming that the volume of the gas tank is 1 m3, the shape of the gas tank has no effect on the gas state.
2.2.1 Van der Waals equation Since the actual gas can only approximate the ideal gas state equation when the temperature is high and the pressure is low, the air compressor adopts a three-phase induction motor, and the following idealized assumptions are made: 1 The stator and rotor three-phase windings are completely symmetrical; 2 the stator surface is smooth and has no cogging effect, and the magnetostatic force of each phase of the stator and rotor is sinusoidal in space; 3 magnetic saturation, eddy current and core loss are neglected.
A three-phase induction motor is essentially a high-order, nonlinear, strongly coupled multivariable system. The d-axis of the set rotation coordinate system and the rotor magnetic field direction 0=magic coincidence, the mathematical model of the asynchronous motor can be expressed by the formula ~0.
The actual gas state equation is used to describe the gas pFr relationship in the gas storage tank. Van der Waals made a fixed correction to the ideal gas state equation in the 1970s, and first proposed the actual gas state equation, such as.
2.2.3 Induction motor model Usi, Wa, stator voltage on the ¢ / axis component % - rotor flux R3, Rt - stator, rotor resistance 4d, 4q stator current on the mountain axis component Cs synchronous angular velocity called a slip The angular velocity flux linkage equation is less sd = Kong Dewen, Lin Weizhen, Cai Maolin, et al. Analysis of energy-saving operation under the condition of screw air compressor plus and under load conditions. Journal of Beihang University, 2012, 38 (FuzzyLogic He Fengyou, Bao Wei, Tang, et al. Design of air compressor constant pressure air supply system based on fuzzy PID controller. Industrial Automation, 2010, (1) Xu Zhihua. Screw Frequency conversion energy-saving transformation of air compressors. Energy and Environment, 2009, (6): 29-31. Sun Libo, Zhang Hongying. Air compressor no-mode control technology. Oil and gas field groundwork Zhong Meipeng, Zheng Shuiying, Pan Xiaohong. Direct-connected air Press PWM variable duty cycle Li Yongdong. AC motor digital control system. Beijing: Mechanical industry out of online detection and monitoring, precision measurement test, published more than 30 papers.
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