Introduction：
An air compressor is a device that uses a motor to compress gas in a compression chamber and make the compressed gas have a certain pressure. It has a wide range of uses and can be used in various industries such as metallurgy, machinery manufacturing, mining, electric power, textile, petrochemical and so on. Air compressors account for 15% of the power consumption of large industrial equipment (fans, pumps, boilers, air compressors, etc.). After investigation, most air compressors have the following shortcomings: (1) When the output pressure is greater than a certain value, the load relief valve is automatically opened to make the asynchronous motor idle, which is a serious waste of energy. (2) The asynchronous motor is easy to start and stop frequently, which affects the service life of the motor. (3) Bad working conditions and loud noise. (4) The degree of automation is low, and the adjustment of the output pressure is realized by artificially adjusting the opening of the valve. The adjustment speed is slow, the fluctuation is large, the stability is unstable, and the precision is low. (5) The starting current of the power frequency of the air compressor is large, the impact on the power grid is large, the motor bearing wear is large, and the equipment maintenance is large. In view of the above problems, a frequency converter is designed to realize the energy-saving transformation scheme of the screw air compressor. After analysis, the scheme has a high degree of automation, significant energy-saving effect and good practicability.
Features：
D31 is a cabinet-mounted inverter, the power range is 0.75KW~500KW, the incoming voltage is 380V~480V, and the control methods are sensorless vector control and vector control. The speed sensorless vector control is a kind of vector control, but without a speed sensor, the speed feedback value of the motor is estimated through the calculation of the motor model. The control accuracy and dynamic performance of the speed sensorless vector control are not as good as the vector control with speed closed loop, but it also has the advantages of simple system, no need to maintain speed sensors, and low price, especially for fans and pumps. The traditional V/f control also does not need a speed sensor, but for high-power fan and pump motors, such as V/f control without current loop control, it is easy to cause current fluctuations. The speed sensorless vector control not only has a speed closed loop but also a current closed loop, and the steady-state current of the motor is more stable. D31 has a wealth of client terminals, including 8 digital inputs (LI1~LI8), 2 analog inputs (AI), 2 relay outputs (TA), 2 analog outputs (AO), 5V auxiliary power supply for analog power supply Fixed input, 24V auxiliary power supply is used for digital input, all LI/TA, AI/AO can be freely programmed to define functions.
MODBUS interface is standard configuration of D31, no option is required. The D31 can be connected to higher-level automation systems via MODBUS.
D31 series inverter has built-in PID function, which is more flexible in operation. There are many ways to control the interface, which can be hard-wired or communication.
Advantage：
System Introduction：
In this system, the system has two control loops of frequency conversion and power frequency. On the basis of the original power frequency starting system, the inverter control system is added to ensure that the air compressor can be started at the power frequency after the inverter fails, without affecting production. Through the built-in PID function of the frequency converter, the constant pressure control of the air compressor is realized. The frequency conversion operation of the motor can keep the outlet pressure of the air storage tank stable, and the pressure fluctuation range cannot exceed ±0.2bar.
Technical Parameter：
| Inverter technical parameters | Device configuration | |
 Model: D31 | Output current: 215A | D31 One inverter |
| Input voltage: 3-phase, 380-480V | Frequency range: 0-400Hz | One 110kW air compressor cabinet |
| Input frequency: 50/60Hz ±5% | Overload capacity: 150%, 60 seconds | Pressure Transmitter |
| Output power: 110KW | Ambient temperature: -10-40°C | |
| Output voltage: 0-480V (the maximum value is determined by the input voltage) | Protection level: IP20 | |
On Site Usage：
According to the on-site space, a D31 series inverter is installed outside the control cabinet, and the inverter installation and on-site motor are as shown in the figure
Running Result：
The air compressor frequency conversion transformation system using D31 inverter has been put into operation for more than two years, and has achieved good economic and social benefits:
The control device operates reliably and has a low failure rate;
The air pressure is stable, so that the system pressure change of the pipe network is kept within the range of ±0.2bar, which effectively improves the quality of the working conditions;
Enhance the reliability of the system, reduce the noise of the air compressor, and prolong the service life of the compressor;
The economic benefits are remarkable, the energy cost is reduced by 44.3%, and the maintenance cost is significantly reduced. The statistics of actual operation show that it can save 651,900 kWh of electricity a year. Calculated at 0.3 yuan per kilowatt-hour, about 195,600 yuan in electricity savings. The electricity bill is saved by more than 20%, and the invested funds can be recovered in about half a year.
Customer Benefits：
The pressure of the pipe network is stable and does not change with the change of gas consumption, which improves the quality of gas supply;
The impact on the equipment is reduced, the maintenance and repair volume is also reduced, and the maintenance cost is reduced;
The energy-saving effect is remarkable, and the increased investment of the inverter control system can be fully recovered in about half a year.