The relaxation link is the link that the formed fiber must be in the state of relaxation and set up in the dryer. The study of closed-loop control of general frequency converter shows that if U / F is constant, even if double closed-loop control of current and speed is adopted, the effect is not necessarily good. Due to the limited current protection of the General Inverter, the single closed-loop rectifier control is adopted, which not only has simple structure, but also can obtain good comprehensive performance index. Proportional Integral (P-I) controls the regulator of the proportional integral control system; JD is the absolute unit circuit to ensure that the control signal added to the BP is positive and limit the amplitude to 5V. M is 1.5 kw three-phase asynchronous motor, TG is DC tachogenerator, L is load. The mathematical model of the controlled object (inverter + asynchronous motor) , i. e. an integral connected in series with an inertia link, can be obtained by the method of rising curve. According to the amplitude approximation method, and adding the given filter and feedback filter, the regulating parameters which make the system have certain dynamic performance index can be determined. Under the conditions of r13k8, C16.8 LF and IA 1.5 a light load of generator,