Application of Frequency Converter in Air Compressor Air Supply Control System

Application of Frequency Converter in Air Compressor Air Supply Control System

Air compressors are widely used in industrial production; for example, in our metal packaging industry, it is responsible for providing air source for all pneumatic components (including various pneumatic valves) in the production line; therefore, it runs well The damage directly affects the efficient operation of the production line. There are many types of air compressors, but their air supply control methods are almost always using loading and unloading control methods. For example, Nanjing Sanda piston air compressor, Sullair LS-10 screw compressor and atlascopcoga-110 screw air compressor used in our factory all adopt this control method. According to our many years of operating experience, although this air supply control method is simple in principle and easy to operate, it has many problems such as high energy consumption, easy damage to the intake valve, and unstable air supply pressure. With the development and progress of society, high-efficiency and low-consumption technologies have attracted more and more attention. Whether the frequency conversion speed regulation technology can be applied in the field of air compressor air supply, saving electric energy while improving air compressor performance and air supply quality has become a topic of our concern. Combined with the actual production, we chose an atlascopcoga-110 fixed screw air compressor for research.

　　 Introduction to air compressor loading and unloading air supply control methods

　　 I use the atlascopcoga-110 fixed screw air compressor electrical control schematic diagram to briefly introduce the air supply control method for loading and unloading. Press the start button sb2, the kt1 coil is energized, and its instantaneous closing and delayed opening dynamic contact contacts are closed, and the km4 and km6 coils are energized and the compressor motor starts Y-shaped start; at this time the intake control valve yv2 is energized , The control gas is released from the small gas storage tank into the piston cavity of the intake valve, the intake valve is closed, and the compressor starts at light load. When kt2 reaches the set time (usually after 5 seconds), the delayed open moving-break contact is opened, the delayed closed moving-close contact is closed, the km6 coil is de-energized and released, the km5 coil is energized, and air pressure The motor is automatically changed from y shape to delta shape to run. At this time, yv2 is powered off and closed, the control gas released from the gas storage tank is cut off, the intake valve is fully opened, and the unit runs at full load. (Note: The intake control valve yv2 only works during the starting process, while the unloading control valve yv1 works after starting).

If the required air volume is lower than the rated exhaust volume, the exhaust pressure rises. When the set minimum pressure value pmin (also known as the loading pressure) is exceeded, the pressure regulator acts to deliver the control air to the intake valve and pass The piston in the air valve partially closes the air intake valve to reduce the air intake volume and balance the air supply and consumption. When the pipeline pressure continues to rise and exceeds the maximum pressure value pmax (also called unloading pressure) set by the pressure adjustment switch (sp2), the pressure adjustment switch trips and the solenoid valve yv1 is powered off. In this way, the control air directly enters the air inlet valve to completely close the air inlet; at the same time, the vent valve is opened under the action of the control air to release the compressed air in the separation tank. When the pipeline pressure drops below pmin, the pressure regulating switch sp2 is reset (closed), and yv1 is turned on. At this time, the control gas to the intake valve and the vent valve is cut off. In this way, the intake valve is fully opened again, the vent valve is closed, and the unit runs at full load.

　　The problems with the air supply control method for loading and unloading

　　1 Energy consumption analysis

　　 We know that the loading and unloading control methods make the pressure of the compressed gas change back and forth between pmin and pmax. pmin is the lowest pressure value, that is, the lowest pressure that can guarantee the user's normal work. In general, the relationship between pmax and pmin can be expressed by the following formula:

　　Pmax=(1＋δ)pmin

　　Δ is a percentage, and its value is roughly between 10% and 25%.

　　 If the frequency conversion speed regulation technology can be used to continuously adjust the air supply, the pressure of the pipe network can always be maintained at the working pressure that can meet the air supply, that is, near pmin.

　　 It can be seen that compared with the air compressor under the control of the frequency conversion system, the energy wasted in the air compressor under the control mode of loading and unloading is mainly in two parts:

　　 (1) The energy consumed when the compressed air pressure exceeds pmin

　　 After the pressure reaches pmin, the original control method determines that the pressure will continue to rise (until pmax). This process will inevitably release more heat to the outside world, resulting in energy loss. On the other hand, the pressure of gas higher than pmin needs to be reduced to close to pmin through a pressure reducing valve before entering the pneumatic components. This process is also an energy-consuming process.

　　 (2) Energy consumed by unreasonable adjustment methods during unloading

　　 Under normal circumstances, when the pressure reaches pmax, the air compressor uses the following methods to depressurize and unload: close the intake valve to make the motor idling, and at the same time vent the excess compressed air in the separation tank through the vent valve. This adjustment method will cause a lot of energy waste. Closing the intake valve to make the motor idling can make the air compressor no longer need to compress gas for work, but the air compressor still needs to drive the screw to rotate during idling. According to our calculations, the energy consumption of the air compressor when unloading is about 10% to 15% when the air compressor is running at full load (this is still when the unloading time accounts for a small proportion). In other words, the air compressor is idle 10% of the time and is doing useless work. Obviously, in the loading and unloading air supply control mode, the air compressor motor has a lot of energy saving space.

　　 2 Other shortcomings

　　 (1) The intake valve is adjusted mechanically, so that the air supply cannot be continuously adjusted. When the air consumption is constantly changing, the air supply pressure will inevitably produce large fluctuations. The accuracy of gas consumption cannot meet the process requirements. Coupled with frequent adjustment of the intake valve, it will accelerate the wear of the intake valve and increase the maintenance volume and maintenance cost.

　　 (2) Frequent use of opening and closing of the vent valve, the durability of the vent valve cannot be guaranteed.

　　 Constant pressure air supply control scheme design

　　 In view of the many problems existing in the original air supply control method, after the above comparison and analysis, I believe that the variable frequency speed regulation technology can be used for constant pressure air supply control. When using this scheme, we can take the pressure of the pipe network as the control object. The pressure transmitter yb converts the pressure p of the gas storage tank into a standard electrical signal and sends it to the pid regulator, and compares it with the pressure set value p0. The magnitude of the difference is calculated according to the established pid control mode, and a control signal is generated and sent to the frequency converter vvvf, and the working frequency and speed of the motor are controlled by the frequency converter, so that the actual pressure p is always close to the set pressure p0. At the same time, this scheme can increase the power frequency and frequency conversion switching function, and retain the original control and protection system. In addition, after adopting this scheme, the air compressor motor can be started by the frequency converter from stationary to rotating, realizing a soft start. Avoid the start-up shock current and the mechanical shock caused by the start-up to the air compressor.