In the mid-1960s, A.schomlng and others in Germany proposed the idea of ​​PWM frequency conversion, and applied the modulation technology used in the communication system to the frequency conversion speed regulation, which opened up a new development for the development and engineering development of modern AC speed regulation technology. path of. The AC front speed control technology has been widely used in pumps, fans, elevators, etc. Many coal machinery industries, fiberglass and petrochemical industries in China have air compressor stations to provide compressed air for production, which is the core power source. Take an example of an air compressor station consisting of five 132kW piston air compressors. When working 20 hours a day, the daily electricity consumption is 13,200 kWh. In practice, it is found that its power consumption is large, and the invalid loss is also relatively large. Therefore, the original system is modified by closed-loop control, which greatly improves the accuracy of gas supply and reduces the failure rate. This transformation has significantly improved the system's functions, achieving energy savings and reducing production costs.
Operating characteristics of a piston air compressor The piston air compressor is a positive displacement compressor. The piston reciprocates back and forth in the cylinder, and the air valve in the cylinder will cooperate with the corresponding opening and closing action.
In this way, the volume of the gas in the cylinder will change, and the gas pressure will change, and the high pressure gas will be sent to the pressure storage tank.
1.1 Single-stage compression working cycle Single-stage compression working cycle In the ideal working cycle, the leakage in the working cycle is not considered, the pressure loss when the airflow flows through the valve and the pipeline, and the inertia of the airflow in the unequal flow, and It is assumed that there is no heat exchange between the air and the cylinder wall, the temperature of the air is constant during the compression process, and the compressed air is completely discharged from the cylinder, for example.
Working cycle: 4:1~2*3 single-stage compression cycle theory working diagram The actual engineering application found that there is clearance volume, resistance loss, airflow inertia effect, air and cylinder wall heat exchange in the working cycle of the compressor, The leakage of the compressor and other factors, the compressor can not be carried out according to the theoretical working cycle, the actual working cycle diagram.
1.2 Pressure Control of Air Compressor The pressure control of the air compressor is controlled by the upper and lower pressure limits. Initially, the upper limit frequency of the inverter is HH, and the lower limit frequency is set to HL. When the actual working pressure of the single-stage compression cycle in the cylinder is greater than or equal to HH, the air compressor closes the intake valve by its own oil pressure, and no compressed air is performed. The high-pressure gas is generated, and the motor is in an idle state; when the pressure is less than or equal to HL, the intake valve is opened to generate compressed air, and the motor is in a loading operation state. Because this type of control has fluctuations in extra work and pressure at no-load operation, there is a large energy loss and waste in this mode of operation.
There are two main problems in the field: 1 In terms of pressure fluctuations, in order to make the system operation unaffected, the lower pressure limit must be set to the lowest pressure lower limit that can be used normally by the gas equipment. The power consumed by the lower full load state is PL. According to the relationship between the input power P and the exhaust pressure H: P = PLX (H / HL), the power P is proportional to the exhaust pressure H. :P is the power required on the compressor drive shaft. In actual operation, with the change of the gas consumption, the exhaust pressure H is directly greater than or equal to the lower limit air pressure HL, so that the air compressor is in an overload operation state, and the power consumed is greater than or equal to the full load power consumption PL. 2 due to the upper limit, The lower limit two-point control results in frequent unloading and overloading of the air compressor when the gas consumption is significantly different from the rated gas production, which causes a great impact on the motor, air compressor and power grid.
From the comprehensive analysis of the field in the application, the main problems solved by the system optimization and transformation are: 1 to minimize the displacement loss while satisfying the system gas consumption; 2 to minimize the pressure loss while satisfying the pressure. The scheme adopted in this paper is: when the difference between the gas consumption and the rated gas production is large, it is necessary to slightly adjust the rotation speed of the driving motor, increase or decrease the pressure of the exhaust gas, and then change the pressure of the gas in the cylinder to make the production of the air compressor. The amount of gas varies with the amount of gas used. In this way, the stability of the exhaust pressure is maintained, and on the other hand, the no-load operation of the motor is avoided, the system is stabilized, energy is saved, and waste is saved.
2 The basic principle of AC frequency conversion speed regulation technology The frequency conversion speed regulation technology is to rectify and invert the power frequency AC power of the power grid through high-power electronic switching devices and intelligent control technology, and change the fixed frequency AC power into frequency adjustable. AC power is used to drag the load of the motor, so that the stepless speed regulation of the motor can be realized.
2.1 Mathematical theory and the composition of the inverter The speed N of the asynchronous motor can be expressed as: number; s is the slip rate; f is the frequency of the power source. It can be seen from the formula that changing the power frequency can achieve the goal of changing the synchronous speed and the motor speed.
The realization of AC frequency conversion withering speed can rectify and filter single-phase AC power into DC power, then invert the DC power into a high-power drive module and convert it into AC power, and then change the conduction time and phase of the power module according to the operating law of the AC motor winding. A three-phase variable frequency can be generated while PWM modulation is completed.
The frequency converter is composed of a rectifier circuit, a DC intermediate circuit, an inverter circuit and a control circuit, for example.
The physical structure of the inverter 2.2 AC speed control technology has the following significant advantages (1) The output power can be continuously adjusted, and the motor can be steplessly regulated.
(2) It has great adaptability to grid voltage fluctuations, which can greatly reduce the pollution of the system to the grid.
(3) The power factor is high, the utilization efficiency is high, and the energy saving effect is remarkable.
(4) It can realize large-scale speed regulation (0~400Hz), and can ensure good output characteristics and dynamic response characteristics, and high speed regulation precision. Suitable for all kinds of pressure, temperature, speed, flow control.
(5) It has the electrical protection required for the start-up operation of the motor. It can be intelligently controlled by PLC, safe and reliable to use, and easy to operate.
(6) Start and stop stationary. The system controls the motor to accelerate or decelerate smoothly, thus avoiding the damage caused by sudden start or sudden stop, greatly extending the service life of the motor and the complete set.
3 control scheme of frequency converter 3.1 system block diagram of system optimization (such as) frequency converter system block diagram control scheme adjusts the initial value of set pressure through potentiometer, uses pressure sensor to collect the output pressure signal of air compressor, and The signal is fed back to the input of the regulator, compared with the set value of the pressure, and then adjusted by the regulator, and the signal of 4~20mA is output as the input end of the inverter. The air compressor output pressure signal is used as a control parameter to change the frequency of the frequency converter and adjust the operating frequency and speed of the air compressor. When the pressure of the air supply pipe network is lower than the initial pressure value, the output frequency of the inverter rises, the motor accelerates, and the air pressure of the pipe network rises. When the pressure of the air supply pipe network is higher than the initial pressure value, the output frequency of the frequency converter decreases. Slow down to reduce the air pressure in the pipe network. In this way, the balance between the pressure of the gas supply pipe and the initial value of the pressure is achieved, and the automatic constant pressure control is realized to ensure the stability of the system and the quality of the gas supply.
In combination with actual production requirements, the initial value of the system pressure is set to 600 kPa for testing.
3.2 Selection of Inverter The piston air compressor is a type of volumetric compressor with constant torque characteristics. Therefore, in the system transformation, constant torque inverters must be used.
Before the transformation, the original air compressor was matched with a 132kW asynchronous motor. The single-compressed displacement was 7200m3/h, and the pressure was <150kPa. It was found in the system debugging that when the motor speed is too low and the load is large, the motor is prone to death. The phenomenon. Therefore, through multiple tests, the upper limit frequency of the inverter is set at 50 Hz, and the lower limit frequency is set at 20 Hz. According to the site requirements, in the system transformation, the system uses the MASTER inverter of Siemens.
4 Conclusion After the system transformation, the effect of the system operation: before the air compressor is loaded and operated for 85 minutes, the unloading starts for 35 minutes, the current during the loading operation is 115A, and the current during the unloading operation is 5A; after the transformation, the operating frequency of the air compressor is 3040Hz. Between, the operating current is around 110A, and there is basically no unloading time. This saves a lot of power, improves the quality of the product, and extends the life of the device. According to the production situation, the gas supply is 20 hours per day, and the five air compressors can save about 250,000 baht per year.
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