Abstract : This paper describes the transformation of two liquefied petroleum gas compressors of Shandong Binhua Group Co., Ltd. using variable frequency speed control technology and PLC to realize automatic adjustment and various control functions. Practice has proved that the scheme is stable and reliable, and the economic effect is obvious. Keywords: liquefied petroleum gas compressor, PLC, frequency converter, PID, A/D Shandong Binhua Group Co., Ltd. has two liquefied petroleum gas compressors with a rated power of 75kW each. They are operated in a one-on-one standby mode, but only about 45kW of output power is needed in actual production. The compressors operate below the rated operating conditions, with a low load rate, and the wind pressure and flow rate need to be manually adjusted by valves, which is difficult to operate and wastes a lot of electrical energy. Therefore, variable frequency speed control technology is considered to be used for transformation, and PLC is used to realize automatic adjustment and various control functions. Operation practice has proven that the scheme is stable and reliable, and the economic effect is obvious. I. Process requirements (1) During normal production, at least one of the two compressors should be running. Even when switching between them, it is not allowed for both machines to stop completely. (2) Keep the compressor outlet pressure at the predetermined value. (3) It can analyze the operating status of the compressor to achieve predictive maintenance. II. System Control Principle (1) The process engineering department sets the normal outlet pressure of the compressor pipeline as P1, while the actual measured pressure on site is P2. According to the value of ΔP (=P2-P1), the PID function module in the PLC performs PID calculation to control the frequency converter to change the motor speed to achieve the required pressure. When ΔP>0, the on-site pressure is too high, so the output frequency of the frequency converter is increased to speed up the motor and increase the actual air pressure; when ΔP<0, the on-site pressure is too low, so the speed is reduced and ΔP decreases. In this way, the adjustment is continuously made so that ΔP tends to 0, and the actual on-site pressure fluctuates near the set pressure to ensure pressure stability. The system structure is shown in Figure 1. (2) The long-term operation of the compressor causes the gap between various components to become larger, which will cause the vibration to become larger and larger, which will easily cause damage to various components of the compressor. The PLC judges and analyzes the on-site vibration, and can carry out planned maintenance of the compressor in advance, which can greatly extend the service life of the equipment, improve the reliability of equipment operation, and reduce unplanned shutdowns caused by equipment failure. Three Design Schemes This scheme mainly consists of one Siemens ECO1-7500/3 frequency converter, one S7-200 PLC (CPU215/216, with matching EM235 expansion module), contactors, operation buttons, one field pressure transmitter, and two vibration measurement devices (vibration transmitters). The PLC is used to realize single-loop closed-loop PID control of the compressor outlet pressure and various electrical control functions such as compressor start-up, stop, switching, and fault handling. The vibration transmitter monitors and analyzes the compressor status to achieve predictive maintenance. The main circuit is shown in Figure 2. (1) Implementation of PID calculation function The CPU215/216 in the S7-200 series has special functions such as 32-bit floating-point operation instructions and built-in PID adjustment operation instructions. When using it, you only need to fill in one PID control parameter table (see the table below) in the PLC memory, and then execute the instruction: PID TABLE, LOOP to complete the PID calculation. The operand TABLE uses variable memory VBx to specify the starting address of the control loop; the operand LOOP is the control loop number (constant, 0~7). Parameters numbered 2, 4, 5, 6, and 7 are fixed and can be set in the PLC main program; parameters numbered 1, 3, 8, and 9 are real-time and must be filled in when calling the PID instruction. Since the input and output of S7-200 are digital quantities, while the signals of the frequency converter, pressure transmitter, and vibration transmitter are analog quantities, the EM235 module must realize D/A conversion. One EM235 module can simultaneously expand 3 analog input channels (connected to 1 pressure signal and 2 vibration signals) and 1 analog output channel (connected to the frequency converter). (2) There are two starting methods for M1 and M2, which can be selected by the conversion switch to start the frequency converter/power frequency. (3) Under normal operating conditions, motor M1 is in the frequency conversion speed regulation state, and motor M2 is in the standby state. The on-site pressure transmitter detects the outlet pressure of the pipeline network (4-20mA analog signal) and compares it with the predetermined value. The PLC performs calculations using a dedicated PID instruction to obtain the frequency signal required by the frequency converter, and automatically adjusts the motor speed to achieve the required pipeline network pressure. (4) Stop: Pressing the "stop button" will cause the PLC to control all contactors to disconnect, and the frequency converter will stop working. (5) Switching: When switching from motor M1 to M2, contactor KM2 will disconnect and KM1 will close. At this time, motor M1 will work at the power frequency. After the frequency converter stops completely, KM4 will close, and the frequency converter will restart. Motor M2 will start under the drive of the frequency converter. After it starts completely, KM1 will disconnect, motor M1 will stop, and the switching operation will end. The process of switching motor M2 to M1 is similar. (6) Alarm and fault self-diagnosis: Alarm and interlock protection are set through the PLC internal program. Once a fault occurs, the corresponding operation will stop immediately and an alarm will be triggered. For the fault self-diagnosis function, considering the cost, no host computer was designed. Only the corresponding fault codes were set and displayed through a 4-digit digital tube, so that maintenance personnel can easily find the fault point based on the fault information. For example: (a) Fault signals such as low oil pressure and low water pressure of the compressor can be measured by the pressure gauge of the explosion-proof electrical contact on site and sent directly to the PLC. The PLC controls the sound and light alarm and delayed shutdown; (b) Add a field vibration sensor and send the signal to the PLC to display and diagnose the compressor's operating status. Four points of experience and issues to be noted in the design (1) After adopting frequency conversion control, the compressor's soft start was realized, reducing the impact of the starting current on the power grid; the power saving effect is obvious, and the entire investment can be recovered within 1 year. (2) After adopting PLC, a closed-loop automatic control system is formed to realize automatic adjustment and more stable and reliable operation. (3) The frequency converter, PLC, contactor, etc. can be installed in a control cabinet and can be operated locally or remotely. The method is simple and flexible. (4) The system has good scalability. If multiple compressors are running under variable frequency/power frequency power supply, the system can be automatically controlled by simply sending the additional information or signal to the PLC. If production requires it, the system can also be easily connected to DCS or host computer to establish a human-machine interface monitoring system. (5) Predictive maintenance can greatly extend the service life of the compressor, improve reliability, reduce downtime losses, and reduce operating costs. (6) When the PLC controls the motor to switch between variable frequency and power frequency power supply, it must ensure the closing and opening sequence of each contactor and sufficient delay to prevent the induced electromotive force generated by the motor winding from being loaded onto the output inverter bridge of the inverter and causing damage. (7) The PLC must implement interlocking between KM2 and KM4 to prevent the two motors from starting at the same frequency and causing the inverter to be damaged due to overload. (8) Since the two motors will run at the same time under power frequency and variable frequency respectively for a short period of time, the main power switch of the variable frequency control cabinet must be considered according to the load of the two motors.