Abstract : This paper discusses the application of frequency converters and soft starters in AC motors through the modification practice of the control of a 90kW motor. The results show that soft starting and variable frequency speed regulation of AC motors not only reduce the impact of starting current on the power grid, but also achieve energy saving of the motor. Keywords : Electrical engineering; frequency converter; experimental research; soft starter; motor; The starting and energy saving of large-capacity motors in the field of motor control has always been a research hotspot. This is mainly because the starting current of induction motors is very large, generally 5 to 7 times the rated current. The large current during starting impacts the power grid, resulting in a large line voltage drop, causing voltage fluctuations in the power grid, and affecting the normal operation of other electrical equipment in the power grid. At the same time, with the change of motor load, the operating efficiency, power factor and other operating characteristics of the motor change significantly, and its power factor and operating efficiency are very low. The purpose of this study is to reduce the starting current and improve the operating characteristics of the motor. 1 Modification scheme of motor control A pesticide factory has two 90kW motors driving cooling water pumps. Originally, they were directly started. However, the pesticide factory's production is highly seasonal, and the changes in the production process cause large load changes. When the load is small (the flow requirement is small), one pump is started and runs at full speed, with the flow controlled by a mechanical valve. When the load is large, a second pump is started, and both pumps run at full speed, with the flow controlled by a valve. This control method has two major problems: first, the large current during motor startup has a significant impact on the power grid; second, adjusting the flow using a valve is too energy-intensive and places high pressure requirements on the valves, otherwise they are easily damaged. The reason is that, based on the pump's characteristics, especially when the load is small and the flow needs to be reduced, the valve is adjusted to a smaller size. Due to the increased frictional resistance of the valve, the head increases while the flow decreases. The pump's shaft power P is: P=KQH/n [b][align=center]For more details, please click: Soft Start and Variable Frequency Speed ​​Regulation of AC Motors[/align][/b]