Abstract : Traditional PI control is not ideal for controlling objects with linear, time-delay, time-varying, and strongly coupled characteristics. To address this issue, fuzzy PI control is introduced into the AC-AC frequency converter synchronous motor speed control system, replacing the traditional PI controller with a fuzzy PI controller. The principle and algorithm of the fuzzy PI controller are explained. Finally, it is applied to a 12kW AC-AC frequency converter three-phase synchronous motor system. Practical results show that the system's parameters are superior to those of traditional PI control. Keywords : frequency converter; speed control system; AC/AC frequency converter; fuzzy PI control 1 Introduction Traditional PI control algorithms are simple and parameter adjustments are convenient. It has a certain level of control accuracy, but it also has limitations. PI control is essentially a linear control. If the controlled object has nonlinear, large time-delay, time-varying, and strongly coupled characteristics, the linear parameter PI control cannot maintain the designed performance indicators, and its robustness is often unsatisfactory. Fuzzy control exhibits strong robustness, good dynamic response, fast rise time, and small overshoot, especially when the controlled object model is difficult to establish or the system possesses nonlinear, large time delay, time-varying, or strongly coupled characteristics, its superiority is even more pronounced. However, fuzzy control has relatively poor steady-state performance; therefore, fuzzy control is combined with PI control, employing fuzzy PI control to control an AC speed regulation system. 2. Algorithm of Fuzzy PI Control Figure 1 shows the basic structural block diagram of the fuzzy PI controller. (Read full text : Application of Fuzzy PI Control in AC Speed ​​Regulation Systems)