Application of PLC in Air Compressor Unit Control
1 Introduction
As the air supply equipment for pneumatic control systems, the stable operation and reliability of air compressors directly affect production safety. Early electrical control systems mostly adopted relay circuits, which are prone to aging after long-term operation, resulting in reduced sensitivity and frequent shutdown failures during operation, disrupting normal production. Reconstructing air compressor control with programmable logic controller (PLC) technology overcomes the deficiencies of traditional pure relay control circuits. It can realize not only discrete variable control but also analog variable control, satisfying system requirements for control accuracy and safety.
2 System Operating Process
2.1 Operating Process of Air Compressor Unit
After equipment power-on, the system first checks the operating conditions of the air compressor. When cooling water pressure and crankcase oil pressure of the air compressor meet requirements, Unit 1 starts, and Unit 2 serves as standby. Both units adopt star-delta (Y-Δ) starting with a 6-second star-delta transition delay.
After startup, the air receiver begins charging. When the pressure of the air receiver reaches the set value of 0.7 MPa, the air compressor intake valve closes and the machine runs unloaded. When the air receiver pressure drops to 0.65 MPa, the intake valve opens and air charging resumes.
If the air receiver pressure falls to the set value of 0.55 MPa due to faults while Unit 1 is stopped, Unit 2 will start and operate normally to continuously charge the air receiver following the same operating logic as Unit 1. Similarly, if the pressure drops to 0.55 MPa with Unit 2 stopped, Unit 1 will start automatically.
In addition, the continuous operating time of each unit is limited to 12 hours. That is, once one unit runs for 12 hours, it will shut down and the other unit will start automatically, regardless of whether faults occur or whether the air receiver pressure is lower than 0.55 MPa.
2.2 Working Principle of Gas Drying Equipment
The two compressors share one set of gas drying equipment, the GWU series heatless regenerative gas dryer manufactured by Liuzhou Liu Erkong Machinery Co., Ltd.
After power-on, Tower A runs in adsorption mode and Tower B runs in regeneration mode. Under preset sequential control:
Solenoid valve A2 opens, while A1, B1 and B2 are closed. Compressed air flows through valve A2 and enters Tower A from the bottom. As the air flows upward, moisture inside the gas is adsorbed by the desiccant in the tower. Dried gas passes through shuttle valve C and enters the air receiver.
10 seconds after A2 opens, valve B1 opens. Residual gas inside Tower B flows from top to bottom to carry moisture desorbed from the adsorbent out through valve B1 and exhausts via the silencer. The 10-second duration is used for desorption of Tower B.
Ten minutes after A2 opens, solenoid valve B2 opens and valve A2 closes; Tower B starts charging. After another 10 seconds, valve A1 opens. Residual gas inside Tower A flows downward through valve A1, exhausting moisture via silencer D to realize desorption of Tower A. Valve A1 closes after a further 10 seconds.
At this moment, the pressure inside Tower A drops while pressure in Tower B rises. Shuttle valve C closes the exhaust port of Tower A and opens the exhaust port of Tower B.
Likewise, ten minutes after valve B2 opens, valve A2 opens and B2 closes. After a 10-second delay, valve B1 opens for desorption of Tower B. The two towers operate alternately to continuously dry compressed air.
3 System Control Requirements
3.1 Control Requirements for Air Compressors
(1) Switch on power before startup. All status indicator lights installed in the central control room and on-site are illuminated to display real-time operating status.
(2) Press the start button. The air compressor starts via star-delta mode, the intake solenoid valve opens, and charging of the air receiver commences. The two units do not need to run simultaneously during startup; either unit can be selected to start first.
(3) When the running unit operates for more than 12 hours or suffers a fault, the standby unit starts and takes over operation.
(4) Insufficient water pressure or oil pressure during operation triggers immediate shutdown and alarm signals.
(5) Press the stop button to shut down the unit.
3.2 Control Requirements for Gas Dryer
The operation of the gas dryer is synchronized with the air compressor. It is powered on together with the air compressor, and its startup is controlled by the main contactor of the air compressor.
4 System Hardware Design
4.1 System Configuration
This design adopts the standard CPU of S7-300 series. Digital input/output modules SM321 and SM322, as well as analog input module SM331, are expanded sequentially on the second SM interface of the No.3 expansion slot.
4.2 I/O Allocation and Wiring of Expansion Units
I/O addresses of Siemens S7-300 expansion modules are allocated as follows:
Discrete signals are collected by pressure switches and transmitted to digital input module SM321 with the starting address range I100.0 ~ I100.3.
A well-functioning cooling and lubrication system is essential for high-speed operation of air compressors to avoid thermal damage to equipment. Therefore, water pressure and oil pressure are priority monitoring parameters.
Analog signals are mainly used to detect air receiver pressure for air compressor operation control. Pressure transmitters convert pressure signals ranging from 0~1 MPa into 4~20 mA current signals and transmit them to analog input module SM331, with starting address range 672 ~ 687. Hardware wiring is omitted here.
For star-delta starting of air compressors, software interlocking has been configured for contactors KM2 & KM3, KM5 & KM6. For operational safety, hardware interlocking is added again to prevent short-circuit risks caused by contact ablation or other failures that prevent contactors from breaking off.
The gas dryer is equipped with four solenoid valves controlled by KM1 and KM4. Once either Unit 1 or Unit 2 starts, the gas dryer begins operation: valve A2 at the bottom of Tower A opens first, and the equipment runs following the working principle described above. Control via KM1 and KM4 ensures synchronous operation of the gas dryer and air compressors.
5 Software Design
5.1 Air Compressor Control
The operation program is designed according to the working principle of the air compressor. After power-on, water pressure and oil pressure are checked. When conditions are satisfied, Unit 1 starts and runs normally.
It should be noted that the startup of Unit 2 is controlled by both timers and air receiver pressure. When the air receiver pressure drops below 0.55 MPa, it indicates a potential fault of Unit 1, triggering startup of Unit 2. However, at initial startup, the air receiver pressure is zero, and both units are available for startup. Therefore, joint control by pressure transmitters and contactors KM1, KM4 is adopted. KM1 and KM4 are connected in series with pressure transmitters to realize interlocked operation of the two units.
The ladder logic for main unit and standby unit control: I672 & Q108.3 control startup of Unit 1; I672 & Q108.0 control startup of Unit 2. This configuration prevents simultaneous startup of both units when pressure falls below the set value of 0.55 MPa.
5.2 Gas Dryer System Control
The control logic of the air compressor gas dryer system depends on the startup status of the two air compressors. As shared equipment, the gas dryer must operate once any compressor starts. The ladder diagram for gas drying control is omitted.
6 Conclusion
After this technical transformation, operators do not need frequent on-site patrol inspections. The operating status of air compressors can be monitored directly in the central control room. Operators can respond rapidly to on-site abnormal alarm signals, instead of discovering faults in the air compressor system only after receiving low air pressure alarms from other pneumatic equipment as before.
After more than one year of operation, apart from mechanical faults of equipment, almost no control system failures have occurred, fully meeting design specifications.
The adoption of PLC for air compressor control simplifies operation, and further improves the safety and stability of unit operation.
As the air supply equipment for pneumatic control systems, the stable operation and reliability of air compressors directly affect production safety. Early electrical control systems mostly adopted relay circuits, which are prone to aging after long-term operation, resulting in reduced sensitivity and frequent shutdown failures during operation, disrupting normal production. Reconstructing air compressor control with programmable logic controller (PLC) technology overcomes the deficiencies of traditional pure relay control circuits. It can realize not only discrete variable control but also analog variable control, satisfying system requirements for control accuracy and safety.
2 System Operating Process
2.1 Operating Process of Air Compressor Unit
After equipment power-on, the system first checks the operating conditions of the air compressor. When cooling water pressure and crankcase oil pressure of the air compressor meet requirements, Unit 1 starts, and Unit 2 serves as standby. Both units adopt star-delta (Y-Δ) starting with a 6-second star-delta transition delay.
After startup, the air receiver begins charging. When the pressure of the air receiver reaches the set value of 0.7 MPa, the air compressor intake valve closes and the machine runs unloaded. When the air receiver pressure drops to 0.65 MPa, the intake valve opens and air charging resumes.
If the air receiver pressure falls to the set value of 0.55 MPa due to faults while Unit 1 is stopped, Unit 2 will start and operate normally to continuously charge the air receiver following the same operating logic as Unit 1. Similarly, if the pressure drops to 0.55 MPa with Unit 2 stopped, Unit 1 will start automatically.
In addition, the continuous operating time of each unit is limited to 12 hours. That is, once one unit runs for 12 hours, it will shut down and the other unit will start automatically, regardless of whether faults occur or whether the air receiver pressure is lower than 0.55 MPa.
2.2 Working Principle of Gas Drying Equipment
The two compressors share one set of gas drying equipment, the GWU series heatless regenerative gas dryer manufactured by Liuzhou Liu Erkong Machinery Co., Ltd.
After power-on, Tower A runs in adsorption mode and Tower B runs in regeneration mode. Under preset sequential control:
Solenoid valve A2 opens, while A1, B1 and B2 are closed. Compressed air flows through valve A2 and enters Tower A from the bottom. As the air flows upward, moisture inside the gas is adsorbed by the desiccant in the tower. Dried gas passes through shuttle valve C and enters the air receiver.
10 seconds after A2 opens, valve B1 opens. Residual gas inside Tower B flows from top to bottom to carry moisture desorbed from the adsorbent out through valve B1 and exhausts via the silencer. The 10-second duration is used for desorption of Tower B.
Ten minutes after A2 opens, solenoid valve B2 opens and valve A2 closes; Tower B starts charging. After another 10 seconds, valve A1 opens. Residual gas inside Tower A flows downward through valve A1, exhausting moisture via silencer D to realize desorption of Tower A. Valve A1 closes after a further 10 seconds.
At this moment, the pressure inside Tower A drops while pressure in Tower B rises. Shuttle valve C closes the exhaust port of Tower A and opens the exhaust port of Tower B.
Likewise, ten minutes after valve B2 opens, valve A2 opens and B2 closes. After a 10-second delay, valve B1 opens for desorption of Tower B. The two towers operate alternately to continuously dry compressed air.
3 System Control Requirements
3.1 Control Requirements for Air Compressors
(1) Switch on power before startup. All status indicator lights installed in the central control room and on-site are illuminated to display real-time operating status.
(2) Press the start button. The air compressor starts via star-delta mode, the intake solenoid valve opens, and charging of the air receiver commences. The two units do not need to run simultaneously during startup; either unit can be selected to start first.
(3) When the running unit operates for more than 12 hours or suffers a fault, the standby unit starts and takes over operation.
(4) Insufficient water pressure or oil pressure during operation triggers immediate shutdown and alarm signals.
(5) Press the stop button to shut down the unit.
3.2 Control Requirements for Gas Dryer
The operation of the gas dryer is synchronized with the air compressor. It is powered on together with the air compressor, and its startup is controlled by the main contactor of the air compressor.
4 System Hardware Design
4.1 System Configuration
This design adopts the standard CPU of S7-300 series. Digital input/output modules SM321 and SM322, as well as analog input module SM331, are expanded sequentially on the second SM interface of the No.3 expansion slot.
4.2 I/O Allocation and Wiring of Expansion Units
I/O addresses of Siemens S7-300 expansion modules are allocated as follows:
Discrete signals are collected by pressure switches and transmitted to digital input module SM321 with the starting address range I100.0 ~ I100.3.
A well-functioning cooling and lubrication system is essential for high-speed operation of air compressors to avoid thermal damage to equipment. Therefore, water pressure and oil pressure are priority monitoring parameters.
Analog signals are mainly used to detect air receiver pressure for air compressor operation control. Pressure transmitters convert pressure signals ranging from 0~1 MPa into 4~20 mA current signals and transmit them to analog input module SM331, with starting address range 672 ~ 687. Hardware wiring is omitted here.
For star-delta starting of air compressors, software interlocking has been configured for contactors KM2 & KM3, KM5 & KM6. For operational safety, hardware interlocking is added again to prevent short-circuit risks caused by contact ablation or other failures that prevent contactors from breaking off.
The gas dryer is equipped with four solenoid valves controlled by KM1 and KM4. Once either Unit 1 or Unit 2 starts, the gas dryer begins operation: valve A2 at the bottom of Tower A opens first, and the equipment runs following the working principle described above. Control via KM1 and KM4 ensures synchronous operation of the gas dryer and air compressors.
5 Software Design
5.1 Air Compressor Control
The operation program is designed according to the working principle of the air compressor. After power-on, water pressure and oil pressure are checked. When conditions are satisfied, Unit 1 starts and runs normally.
It should be noted that the startup of Unit 2 is controlled by both timers and air receiver pressure. When the air receiver pressure drops below 0.55 MPa, it indicates a potential fault of Unit 1, triggering startup of Unit 2. However, at initial startup, the air receiver pressure is zero, and both units are available for startup. Therefore, joint control by pressure transmitters and contactors KM1, KM4 is adopted. KM1 and KM4 are connected in series with pressure transmitters to realize interlocked operation of the two units.
The ladder logic for main unit and standby unit control: I672 & Q108.3 control startup of Unit 1; I672 & Q108.0 control startup of Unit 2. This configuration prevents simultaneous startup of both units when pressure falls below the set value of 0.55 MPa.
5.2 Gas Dryer System Control
The control logic of the air compressor gas dryer system depends on the startup status of the two air compressors. As shared equipment, the gas dryer must operate once any compressor starts. The ladder diagram for gas drying control is omitted.
6 Conclusion
After this technical transformation, operators do not need frequent on-site patrol inspections. The operating status of air compressors can be monitored directly in the central control room. Operators can respond rapidly to on-site abnormal alarm signals, instead of discovering faults in the air compressor system only after receiving low air pressure alarms from other pneumatic equipment as before.
After more than one year of operation, apart from mechanical faults of equipment, almost no control system failures have occurred, fully meeting design specifications.
The adoption of PLC for air compressor control simplifies operation, and further improves the safety and stability of unit operation.
