Research On Direct Torque Control Strategy Of Marine Six-Phase Propulsion Motor

Compared with the traditional mechanical propulsion system,the electric propulsion system has become the primary ship propulsion method based on its unique advantages.With the continuous increase of the propulsion power of the electric propulsion system,the traditional three-phase motor transmission system is difficult to meet the demand.Meanwhile,the six-phase permanent magnet synchronous motor is widely used because of its advantages of low-voltage high-power transmission performance.Compared with vector control,direct torque control has faster response speed and stronger robustness,which has broader application prospects in marine electric propulsion systems.This dissertation takes the six-phase permanent magnet synchronous motor as the research object,starting from the traditional direct torque control theory,and studies the methods of torque ripple suppression,harmonic current suppression and high-performance speed regulation.Finally,the ship electric propulsion system model is built on the Matlab/Simulink platform to verify the feasibility and effectiveness of the proposed strategy.Firstly,this dissertation establishes the mathematical model of the six-phase permanent magnet synchronous motor in the natural coordinate system and deduce the mathematical model of the motor in the rotating coordinate system based on the vector space decoupling theory.Then,the voltage vector of the six-phase inverter and its effect on the motor torque and flux linkage are analyzed.And the direct torque control system of the six-phase permanent magnet synchronous motor based on the switch table and the maximum voltage vector is built.The Matlab/Simulink platform verifies the control performance of the system.Secondly,in order to deal with the problems of excessive torque ripple and current harmonics in traditional direct torque control,the model predictive torque control strategy is proposed.A finite control set and cost function are designed.In order to suppress the current harmonics to a greater extent,a method of virtual voltage vector synthesis is proposed and verified by simulation.Then,in order to solve the overshoot problem caused by the PI controller in the model predictive torque control method,the model-free adaptive control theory is studied.The dynamic linearization model of the motor speed equation is established,then the speed control algorithm,pseudo partial derivative estimation algorithm and reset algorithm are given.Based on the algorithms,a model-free adaptive speed control algorithm is designed.The dynamic performance of the improved system is verified under three conditions of speed sudden change,torque sudden change and load disturbance.Finally,the mathematical model of the propeller and the ship is analyzed,and the model-free adaptive speed control law under the propeller load condition is given.Based on model predictive torque control and model-free adaptive control,a simulation model of marine electric propulsion system is built.The simulation study is carried out considering the three possible working conditions that could be encountered in actual sailing,such as graded speed regulation,propeller obstruction and propeller falling off.The simulation results show that the proposed control strategy performs surpassingly in speed regulation.Moreover,it can effectively limit the torque ripple and current harmonics to a low level,operate stably under various working conditions.The proposed method is worthy of application in the field of marine electric propulsion.