Research On Extremely Low Speed Control Strategy Of Three-phase Asynchronous Motor For Industrial Electric Vehicles

With the increasingly serious problems of fossil energy shortage and environmental pollution,the choice of low-carbon new energy electric vehicles has become the mainstream direction of the automobile market.Industrial electric vehicles are an important branch of electric vehicles,in which the drive motor and drive control system are its key core components.Asynchronous motors are widely used in the field of industrial electric vehicles because of their low manufacturing cost and high reliability.In the case of meeting the normal operation of industrial electric vehicles,in order to reduce production costs,low-cost and low-precision incremental photoelectric encoders are usually used to measure the speed.However,there is a lag in the speed measured by low-precision encoders at extremely low speeds,which degrades the performance of the asynchronous motor control system for industrial electric vehicles.Therefore,it is of great significance to study the control strategy of three-phase asynchronous motors for industrial electric vehicles at extremely low speeds in this paper.This paper firstly expounds the basic mathematical model and coordinate transformation principle of asynchronous motor,and then deduces the simplified asynchronous motor model under different coordinate systems.In addition,this paper briefly introduces the induction motor vector control strategy in the two-phase rotating coordinate system based on rotor flux linkage orientation,and analyzes the current loop,speed loop and flux linkage loop based on Proportional Integral Controller(PI Controller)in detail.Secondly,Based on the "M/T" speed measurement method of the incremental photoelectric encoder,this paper analyzes the influence of the delay generated by the method on the control performance in detail.By establishing the approximate small signal model of the encoder "M/T" method,the transfer function of the speed closed-loop control system is further improved,and its Bode diagram is drawn.It can be concluded that the measurement delay using the "M" method at high speed has little impact on the control performance;the measurement delay using the "T" method at low speed has a greater impact on the control performance.In addition,the lower the speed,the worse the control performance,and cause the system to enter an unstable state.Aiming at the above problems,this paper proposes a speed observation method fused with low-precision encoder and speed sensor,and then the stability analysis and parameter design of the method are carried out.Considering the influence of the low output pulse frequency of the low-precision encoder on the observation results,the speed error adaptive rate is derived according to the Lyapunov stability theory and used for speed compensation.Because the actual flux linkage error in the speed error adaptation rate cannot be obtained,this paper introduces the d-axis current error for improvement,so as to obtain more accurate error information for compensation.The method can obtain accurate speed information,thereby improving the control performance of the system.Finally,based on the TMS320F28335 chip as the main control chip,a0.67 k W asynchronous motor pair drag experimental platform is built.The experimental results demonstrate the correctness and feasibility of the proposed low-precision encoder and speed sensorless speed observation method.The method can obtain real-time and accurate speed information,thereby ensuring the excellent control performance of the asynchronous motor at extremely low speed,and realizing the smooth switching of speed at extremely low speed.