1 Air compressor working principle and frequency conversion energy-saving implementation feasibility First of all, to understand the basic working principle of the air compressor. The air compressor has a complex structure, a long operating time, and a large amount of power. Taking a piston air compressor as an example, during the operation of an air compressor, a piston reciprocates in a cylinder and cyclically changes the volume in the cylinder, thereby changing the volume of gas in the cylinder and responding to the air valve in the cylinder. The opening and closing movements are matched to increase the pressure of natural gas or lower pressure gas (primary cylinder gas) through inhalation, compression, and exhaust, and finally to the gas storage tank. In order to meet the gas demand of the equipment, the gas in the gas tank must maintain a certain pressure in order to act as a buffer. With the reason of the equipment itself, the air pressure must vary greatly. Usually, the adjustment of the intake air is used to change the row. Gas volume. The ideal condition is that the air supply pressure just meets the demand and the pressure remains unchanged. In fact, it is impossible to achieve this. Usually, the air compressor discharge volume is greater than the actual air consumption, and the air compressor operates at a constant speed. At this time, the air tank is in operation. The gas accumulates until the pressure rises to the set maximum pressure. Usually take the following two methods to solve the high pressure problem: First, the air compressor unloading operation, keep running but does not produce gas, this time the air compressor power consumption is generally about 50% of the rated power, all useless work; second is to stop The operation of the air compressor seems to save power consumption, but the air storage volume is limited. When the air pressure is lower than the lower limit pressure value, the air compressor pressurizes the air tank at the rated rotation speed again until the pressure reaches the upper limit pressure. And stop running, so loop, as shown in Figure 1. This frequent start and stop not only has great impact on the grid and equipment, but also shortens the life of the compressor. Actually, when the air compressor is driven, the shaft power is directly proportional to the exhaust pressure and its own speed. That is to say, in the actual operation, the air pressure of the equipment is changing at any time, so that the air compressor cannot be maintained at the rated value. Operating conditions, and the level of its exhaust pressure will directly affect the size of the actual shaft power.
For the operation of the air compressor, a dedicated frequency conversion energy-saving controller for the air compressor is designed, which can be easily and continuously adjusted to maintain the stability of pressure, flow, and other parameters (as shown in Figure 2). When the flow requirement is reduced, the rotation speed of the motor can be reduced, thereby greatly reducing the operating power of the motor and achieving the purpose of energy saving.
During the entire operation of the air compressor, the larger the proportion of time (duty cycle) = t2/(t1+t2)=t2/T (see Fig. 3) occupied by the no-load operation, the more serious the waste of electrical energy. The lower the efficiency of the air compressor, the smaller the wasted energy, and the higher the efficiency of the air compressor. In Figure 3, p0 is the no-load operating pressure, p0 = 0 (off / stop) to start the next loss, for a long period of time and more operating conditions. The use of variable frequency speed regulation technology can well solve the problem of no-load energy waste of air compressors. According to actual operation conditions and experience of air compressors, the power-saving effect after adopting variable frequency speed regulation is shown in Fig. 4, and the duty cycle is checked. Figure 4, to consider the oil pump selected separate pump splash valve.
2 Frequency Conversion Energy Saving Solution Since the speed of the asynchronous motor is obviously proportional to the speed n and f, as long as the f is changed, the speed of the motor can be changed. When the frequency f changes in the range of 0 to 50 Hz, the motor speed adjustment range is very wide. Frequency conversion speed regulation is achieved by changing the frequency of the motor power supply. Our implementation plan is to achieve effective control of the system through the sampling of the pressure at the outlet of the air compressor and the closed-loop automatic adjustment control technology. That is, the air pressure signal value is detected by the pressure sensor installed at the outlet of the air compressor, and then through the conversion of the analog digital signal, the pressure required by the work is ensured by the function of the PID regulator (according to the actual conditions of the air pressure for each device). Under the circumstances, guarantee the minimum power output of the motor, realize the soft start of the motor while controlling the pressure accurately, and will not produce the inrush current (usual direct starting current is 5~7 times of the rated current), make the air compressor Both service life and inspection cycle are extended.
In addition, the power factor is improved, the power quality of the power supply and distribution system is improved, the requirements for the power capacity are reduced, and protection against the failure of the power phase loss, under voltage, over current, overload, and overheating is provided. The corresponding benefits are: due to the stability of the air supply pressure, the air compressor can be operated under the set pressure value through the pressure regulator. The pressure is stable and reliable, and can be set steplessly. Tone. The displacement of the air compressor is controlled by the speed of the air compressor. The cylinder valves in the cylinder are no longer repeatedly opened and closed, and the working conditions such as valve seat and spring are also greatly improved, effectively avoiding the sharp rise of high temperature and high pressure gas. The flow and impact reduce the workload of maintenance. In the system design process, the free switching function of frequency conversion and power frequency is also considered. Once the frequency conversion control system fails, it can immediately switch to work under the power frequency condition to ensure the reliability of the gas supply. System control block diagram shown in Figure 5.
3 Conclusion Practice has proved that the application of frequency conversion speed control technology in air compressors is successful, and its energy saving rate consists of two parts.
However, to check whether there is excessive fluctuations in the outlet pressure when the speed drop is too large, the normal use is affected, and whether the supply of the lubricant oil is insufficient (splash oil supply, this problem is not present for the separate oil supply pump).
The intermittent operation of the air compressor has better power saving effect. The air compressor that adjusts the intake air volume has a smaller energy-saving effect. It is a constant torque load for the air compressor, and its output power
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