Abstract : This paper summarizes the capacity expansion and modification of a polymerization unit's discharge pump variable frequency speed control system, which, after adding a melt filter, improves product quality, increases noise efficiency by up to 120%, and enables stable operation under full load for extended periods. The paper focuses on the design of the main circuit and control circuit of the control system, the selection of the motor and frequency converter, the installation and commissioning process of the frequency converter, and discusses how to improve the frequency converter's resistance to power grid fluctuations. Keywords : Polymerization unit, frequency converter, variable frequency motor, gear pump, capacity expansion and modification. In a liquid-phase polymerization unit, the melt after the final shrinkage reaction is pumped by a discharge gear pump below the disc reactor to the melt filter to remove impurities. It is then extruded into strips by a casting head, cooled and solidified by demineralized water, and finally pelletized by a pelletizer. The main process is: disc reactor, discharge gear pump, filter, casting head to pelletizer. The gear pump relies on the periodic change in the working space volume caused by the meshing of gears to transport the melt. The pump speed determines the reactor's discharge rate and the unit's output, and is mainly controlled by the flow rate of terephthalic acid (PTA). The pelletizer speed is dynamically matched to the discharge speed to ensure normal production and that the slices meet the required appearance. When the 20,000-ton polybutylene terephthalate (PBT) plant was initially introduced, a filter was not designed and installed, which affected product quality. After the filter was installed and operated for a period of time, its filtration capacity gradually decreased. At 50% and 75% load, the melt pump motor power could still meet production needs. However, when the load reached 100%, the viscosity reached 1.00 dL/g, the pump outlet pressure was less than 7 MPa, and the filter pressure difference was approximately 5.5 MPa, the current significantly exceeded the rated value of 28.5 SA, causing the frequency converter protection to trip. The gear pump load also increased accordingly. The filter's service life was only about 5 days, and the pump outlet and filter pressure difference gradually increased. Once the specified pressure was reached, the filter had to be switched and cleaned promptly to ensure product quality. Frequent switching and waste cleaning severely impacted production and resulted in significant economic losses. To ensure stable operation at 100% load and to meet the need for a 20% capacity increase, increasing the power of the melt transfer pump motor is now imperative. [b][align=center]For details, please click: Capacity Expansion and Retrofitting of the Variable Frequency Speed ​​Control System for the Discharge Pump of the Polymerization Unit[/align][/b]