1. Introduction AC speed control systems are a major means of saving energy, improving process structure, enhancing product quality, improving the environment, and promoting technological progress. An AC speed control system directly controls the speed of the gas blower motor. A closed-loop automatic braking control system for the suction force before the primary cooler is constructed using a PLC and pressure transmitter. This system effectively adjusts the motor speed according to load changes and automatically regulates the gas pressure. Using a variable frequency speed control system is the best and most effective method for regulating gas pressure and saving energy. It meets production needs while achieving energy saving, reducing consumption, extending equipment lifespan, and improving control levels. 2. Overview According to the process requirements of the reduction annealing furnace and curing furnace in the production line, the gas pressure delivered from the gas station cannot meet the production process requirements. Therefore, it is necessary to pressurize the low-pressure gas while ensuring a constant high-pressure gas pressure. The gas pressurization station has two three-phase asynchronous motors, one in use and one reserved. The gas blower, driven by an electric motor, pressurizes the gas. The corresponding pressure is set via a host computer interface program, and the PID program adjusts the output frequency of the frequency converter in real time to control the motor speed, ensuring a constant gas pressure required for production and stable operation of the color coating line. 3. System Design Scheme and Principle The blower frequency conversion speed control system adopts a highly reliable two-stage computer distributed control system, consisting of a frequency converter, PLC controller, monitoring station, and instrumentation system. The suction signal before the primary cooler is sent to the PLC as feedback value for the closed-loop control of the gas pressure before the primary cooler (the setpoint is set according to actual working conditions). The PLC transforms and processes the gas pressure signal before the primary cooler, providing a frequency setpoint to the frequency converter, achieving automatic frequency adjustment. This means controlling the speed of the blower motor to adjust the output power according to load changes, stabilizing the gas pressure before the primary cooler, and achieving energy saving and consumption reduction. 3.1 System Hardware Composition The system composition is shown in Figure 1. The main parts and their functions are described below. 1) Variable Frequency Speed ​​Control System: The variable frequency speed control system uses ABB's ACS600 fully digital AC inverter to convert 50Hz AC power to variable frequency AC power, and has automatic and manual, local and remote speed control functions. The fan room has two fans, one in operation and one on standby. One variable frequency speed controller is used to control one of the operating fans. Process calculations show that adjusting the speed of one blower can meet the process requirements for regulating the main gas pressure. Variable frequency speed controller speed control principle: Asynchronous motor speed n = 60f(1-s)/p (where f is the power supply frequency, p is the number of pole pairs, and s is the motor slip). From the above formula, it can be seen that changing the power supply frequency changes the speed. The rapid development of variable frequency speed control technology has enabled it to achieve a relatively perfect level in terms of frequency range, dynamic response, working efficiency, adjustment accuracy, and reliability. [align=center]Figure 1 System Composition[/align] 2) PLC Control System The PLC in the control system is a SIMATIC S7-300 series programmable controller. The PLC system communicates with the ABB 600 frequency converter via FROFIBUS bus. The S7-300 is the master station, and the frequency converter is the slave station. The S7-300 PLC completes the signal acquisition and control functions of the entire system and the automatic switching from frequency converter to power frequency when the frequency converter fails. The PLC and the host computer communicate via the Internet. 3) Monitoring Station The monitoring station uses a Siemens industrial control computer and high-performance industrial monitoring software WinCC (Windows Control Center) to realize the display and monitoring of the entire system's process flow and equipment operating status, realize curve display, historical data storage, access control, operation query, alarm, printing and other functions. 4) Instrumentation System The instrumentation system consists of transmitters, distributors, isolators, regulators and actuators, etc. It mainly completes the acquisition of pressure signals and the control and execution of valves. The regulator serves as a backup control means for the computer system. When the computer fails, it can switch to manual mode to control the gas pressure. 3.2 Software Basics and Implementation The software platform of this system adopts Windows 2000. The host computer uses WinCC V5.1 software configuration, and the lower-level computer programming application software uses SIMATIC STEP7 V5.2. The PLC system hardware configuration diagram is shown in Figure 2. [align=center] Figure 2 PLC System Hardware Configuration Diagram[/align] 4. System Functions When the pressure of the main gas pipe entering the pressurization station is lower than 700Pa, an audible and visual signal is emitted. When the pressure is lower than 500Pa, the pressurizer stops running. When the inlet and outlet pressures of the pressurizer change, the speed of the pressurizer can be automatically adjusted. That is, when the outlet pressure is set and the inlet pressure decreases, the speed of the pressurizer increases; when the inlet pressure increases, the speed of the pressurizer decreases to ensure a constant outlet pressure. Based on the user's requirement for stable gas pressure, and considering the unstable pressure of the low-pressure main gas pipe, a variable frequency speed control device needs to be installed for the pressurizer, with a speed adjustment range of 50-100%. Features of this control system: 1. The Siemens PLC system communicates with the ABB600 via a Profibus-DP network. 2. It automatically adjusts the outlet pressure of the gas system and the air-fuel ratio of the thermal oil furnace. 3. Online monitoring of all equipment is achieved through a supervisory control system (SCADA) screen, enabling sequential control of various process equipment, fault alarm handling, and display of high-pressure gas inlet pressure and historical trends. 4. The system can be configured, debugged, and simulated via the SCADA system. 5. Ethernet technology is used for network communication. 6. Manual and automatic operating modes are available, minimizing equipment downtime and reducing the maintenance workload for electrical workers. 5. Conclusion The gas controlled by this PLC system meets industrial requirements. It allows real-time monitoring of remote equipment operation, alarm information, and printed reports on the SCADA system, making operation more convenient and fault handling more timely, achieving the goal of automated production control. Contact Person: Li Xia Mailing Address: Automation Department, Jinan Iron and Steel Group, No. 21, Gongye North Road, Jinan, Shandong Province, 250101, China Telephone: (0531) 88866647 E-mail: [email protected]