Converter and Application of PLC in Energy Saving Reform of Air Compressor

1 Introduction

An air compressor is a device that uses a motor to compress a gas in a compression chamber and apply a certain pressure to the compressed gas. As basic industrial equipment, air compressors have been widely used in almost all industrial industries such as metallurgy, machinery manufacturing, mining, power, textiles, petrochemicals, and textiles. Air compressors account for 15% of the power consumption of large industrial equipment (fans, pumps, boilers, air compressors, etc.). Due to structural reasons, most air compressors have obvious technical weaknesses. When the output pressure is greater than a certain value, the release valve is automatically opened, causing the asynchronous motor to run idle, which causes serious waste of energy. The asynchronous motor is frequently started and stopped, which affects the service life of the motor. The press-frequency startup current is large, and the impact on the power grid is great. , Motor bearings wear, equipment maintenance volume; poor working conditions, noise; low degree of automation, output pressure adjustment is to rely on artificial control valve opening to achieve, slow adjustment, fluctuations, instability, low precision .

In view of the above problems, the PLC and inverter were used to realize the energy-saving retrofit program for the screw air compressor. After analysis, the program has a high degree of automation, significant energy saving effect, and good practicality.

2 Principle of Frequency Conversion Reform of Air Compressor

2.1 The working principle of the air compressor

Figure 1 shows the working principle of the screw air compressor. Air is sucked through the air filter and the suction control valve. The control valve is mainly used to adjust the compression chamber formed by the air cylinder, rotor and sliding vane. Rotation operates eccentrically with respect to the cylinder. The sliding vane is installed in the slot of the rotor and pushes the sliding vane to the cylinder wall through centrifugal force. The efficient oil filling system can ensure good cooling of the compressor and the minimum comfort consumption of the lubricant. A thin layer is formed on the cylinder wall. The oil film prevents wear from direct contact between metal parts. The compressed air has a high temperature, which contains a certain amount of oil and gas, and is separated by an oil and gas separator. After that, the oil and gas are cooled by the oil cooler and flowed back to the oil storage tank through the oil filter. The air passes through the air aftercooler (air The cooling device) cools and enters the gas tank.

2.2 Air compressor frequency conversion energy-saving principle

The basic operating mode of the screw air compressor is loading and deloading. When the load is reduced, the motor idles and the energy is wasted. If the inverter is used to change the motor speed to adjust the speed, the inverter control is to change the speed of the motor to control the air output per unit time of the air compressor so as to achieve the pressure of the control pipeline. Has obvious energy-saving effect. The principle of the frequency conversion energy-saving system of the air compressor is as follows: The pressure value measured by the pressure transmitter is compared with the set value of the pressure, the deviation is obtained, and the frequency of the frequency converter acting on the asynchronous motor is calculated by the PID regulator. The corresponding frequency and amplitude of the alternating current output by the inverter are correspondingly obtained on the motor. Then the air compressor outputs the corresponding compressed air output to the air tank, so that the pressure changes until the pipe network pressure is the same as the given pressure value.

2.3 Frequency Conversion Considerations

(1) The air compressor is a large moment of inertia load. This starting characteristic can easily cause the occurrence of skip flow protection at startup of the inverter. It is recommended to use a speedless vector inverter with high starting torque to ensure that it can be realized. The continuity of the constant pressure air supply ensures reliable and stable operation of the equipment.

(2) Air compressors are not allowed to operate at low frequencies for long periods of time. The lower limit of work should not be lower than 20 Hz.

(3) It is recommended to use a frequency inverter one step higher than the compressor power to avoid frequent tripping of the air compressor.

(4) In order to effectively filter out the higher harmonic components in the output current of the inverter and reduce the electromagnetic interference caused by higher harmonics, it is recommended to select the output AC reactor and reduce the noise of the motor operation.

(5) The designed system shall have two sets of control loops for frequency conversion and power frequency, to ensure that the inverter will not affect production when abnormal jump protection occurs.




3 PLC-based air compressor frequency conversion control system

3.1 System Principle Design

The control system consists of the following components: frequency converters, programmable controllers, frequency conversion cabinets, reactors, pressure transmitters, and oscillating sensors.

PLC-based variable frequency control system schematic shown in Figure 2. The PLC consists of a touch screen, a power supply, a CPU, and an analog output module. Which use PLC to achieve the electrical part of the control. Including five parts: start, run, stop, switch, alarm and fault self-diagnosis.

(1) Start: Take two motors M1, M2 as an example, you can select variable frequency / power frequency start through the switch.

Operation: In normal condition, the motor M1 is in the state of frequency conversion and the motor M2 is in shutdown. The on-site pressure transmitter detects the outlet pressure of the pipe network and compares it with a given value. The PID instruction is used to calculate the frequency signal, and the dynamic speed is adjusted to reach the required pressure.

(2) Stop: Press the stop button, PLC controls all contactors to open, and the inverter stops working.

(3) Switching: Realize the mutual switching of M1, M2 power frequency and frequency conversion.

(4) Alarm and fault self-diagnosis: There are generally four quantities to be monitored inside the air compressor: cooling water pressure monitoring, lubricating oil monitoring, body temperature monitoring, and gas tank pressure monitoring.

3.2 Case Study

Take a factory air compressor as an example. The parameters before the transformation are as follows: motor power 110kW, outlet pressure 5.9~6.5MPa, running time 12 hours/day, 320 days of operation, load time 15s, load reduction time 15s; load current 190A, load shedding The current is 90A. After testing its energy-saving rate of more than 30%. The annual electricity (by 30%) is calculated as follows:

Power saving at W=12×320×110×30%=1.27×105(k•Wh)

It can be seen that the power-saving effect is obvious. In addition, there are other advantages of the system after the transformation. First of all, the machine noise is reduced. Second, two sets of control loops ensure normal and safe operation of the system. Finally, the degree of automation is high and overcomes the drawbacks of manual adjustment of the original system.

4 Conclusion

Using PLC and frequency converter to achieve the energy-saving retrofit program of screw air compressor, the experimental results show that the system has the advantages of energy conservation, high degree of automation, reduction of noise in the original system and reduction of equipment maintenance, and has practical value of in-depth discussion.