Abstract: This paper introduces the technical transformation of the electrical control system of a 1250 centrifuge using a Mitsubishi PLC to achieve frequency converter speed regulation and details the system design. Practical operation results show that the system operates stably, is easy to operate, and can meet the requirements of production processes. Keywords : PLC, centrifuge, frequency converter speed regulation 1. Introduction The 1250 centrifuge is a vertical scraper discharge automatic filtration centrifuge, mainly used for solid-liquid phase separation of granular suspended materials, and can also be used for solid-liquid phase separation of fibrous materials. It is widely used in industries such as minerals, environmental protection, pharmaceuticals, and chemicals. Currently, most centrifuges are still controlled by relays, using stepped speed regulation. This results in limited centrifuge speed adjustment, high equipment failure rate, and low production efficiency. To overcome these problems, we technically transformed the electrical control system of a 1250 centrifuge in a pharmaceutical plant, adopting PLC control and frequency converter speed regulation. This system has a high degree of automation, good stability, and reliable operation, and has been successfully applied in several pharmaceutical plants. 2. System Principle The centrifuge works by feeding the material to be separated into the high-speed rotating centrifuge drum through the feed pipe. Under the force field of the centrifuge, the material is filtered through the filter cloth (screen). The liquid phase is discharged through the outlet pipe, while the solid phase is retained in the drum. When the filter cake in the drum reaches the machine's specified loading amount, feeding stops, the filter cake is washed, and the washing liquid is filtered out. After the separation requirements are met, the centrifuge runs at low speed, and the scraper device is activated to scrape off the filter cake, completing one working cycle. Figure 1 shows the structure of a 1250 centrifuge. Centrifuge centrifugation process: 1) Feeding: When the frequency converter speed reaches 20Hz, the feed valve and the material level detection valve are opened first. When the material level is detected to be full, the feed valve is closed and delayed for 10 seconds. After the material level full signal disappears, the feed valve is reopened, and the above actions are repeated twice. 2) Centrifugation: When the third full signal is generated, the feed valve is closed, and the frequency converter speeds up to 50Hz for high-speed separation. The centrifugation time can be set via the touchscreen. After the time is up, the frequency converter speeds down to 40Hz. 3) Cleaning and Drying: The cleaning valve is opened for cleaning. The cleaning time, pause time, number of cleaning cycles, and the types of drugs to be separated are set via the touchscreen. After the cleaning process is complete, the drying process begins. The frequency converter speeds up to 50Hz, and the drying time is set via the touchscreen. After the time is up, the unloading process begins. 4) Unloading: If the material layer is too thick after drying, the scraper uses segmented timed rotation for unloading. The scraper rotates (time can be set) → pauses for 2 seconds → the scraper descends (descent height can be set). This process is repeated until the scraper descends to the lower limit sensor, then rises to the upper limit and stops. [align=center]Figure 1[/align] 3 System Design 3.1 Hardware Design The system adopts Mitsubishi FX2N-40MR programmable logic controller (PLC) for control. After the program is set, it can perform unattended automated operation or select manual control, and monitor the entire process of feeding, primary filtration, washing, fine filtration, and unloading. The centrifuge speed is adjusted using a PLC+D/A module and a frequency converter. The voltage (0-10V) controls the frequency of the frequency converter. The frequency converter is a 22KW EVF series frequency converter from Lenze, Germany. The touch screen is an EASYVIEW 5.7-inch 4-grayscale touch screen. The PLC has 20 input points and 15 output points. Figure 2 shows the PLC external hardware configuration. The main circuit of the control system and the external wiring of the frequency converter are shown in Figure 3. [align=center]Figure 2 Figure 3[/align] To eliminate jitter and prevent signal interference, input points X11, X12, X13, X14, X15, X16, X17, and X20 are connected to the PLC input through four isolation barriers. 3.2 Software Design The programming software used is the MELSECMEDOC professional software package, which is based on a personal computer and suitable for Mitsubishi PLC user program compilation and monitoring. The SC-09RS232/RS422 interface is connected to the PLC programming port, and ladder diagram or instruction programming can be used. The PLC ladder diagram program control requirements of this system are to use STL and SET step instructions. The main components include initialization settings, feeding, separation, cleaning, spin-drying control programs, and automatic control programs. Safety protection measures implemented in the program design include: speed detection, over-vibration protection, cover opening protection, motor overload and overheat protection, scraper rotation, dual mechanical and electrical control of lifting, and linkage locking between the scraper and the drum. The program flowchart is shown in Figure 4. [align=center]Figure 4[/align] 4. Conclusion This system has been successfully tested at multiple pharmaceutical plants, with satisfactory results. All indicators meet the on-site technical requirements. The system starts smoothly, the separation factor is adjustable, and the operation is simple and convenient. Since its modification and commissioning, it has operated stably, with convenient speed adjustment and requires no maintenance. It has created a highly efficient working environment for on-site operators and achieved relatively advanced control technology for 1250 centrifuges.