Research On Current Control Of Multiphase Induction Motor At Low Carrier Ratio

In recent years,as new energy electric vehicles have received more and more attention,motor drive control systems have been widely used in high-power applications such as electric vehicles.Multiphase induction motors are easy to achieve high power through low voltage power devices,and can inject harmonic currents in the harmonic planes to improve the magnetic field distribution,and improve the torque density.These advantages have made multiphase induction motors a viable solution for high-power applications.However,in the control of the harmonic planes,since the switching frequency of the high-power inverter is limited,the ratio of the switching frequency and the harmonic planar current frequency(the carrier ratio)is low,resulting in deterioration of the current control performance of the harmonic planes.In order to solve the problem of deterioration of the current control performance of the harmonic planes,this thesis has conducted in-depth research on non-sinusoidal power supply technology and model prediction control(MPC)technology.The main contents of the paper are as follows:Firstly,based on the motor winding function,the magnetic momentum of multiphase induction motors is analyzed,and the differences between multiphase motors and three-phase motors in terms of control freedom are elaborated.And this thesis establishes the model of multiphase induction motors in the rotating coordinate system.In addition,the basic principles of non-sinusoidal power supply technology are introduced.Based on the theoretical methods of related literature,the corresponding harmonic injection ratio is determined for the two power supply methods of only injecting the 3rd harmonic current and injecting the3 rd,5rd and 7rd harmonic current.Based on the experimental platform of the nine-phase induction motor drive control system,the motor control performance under sinusoidal power supply and two harmonic current injection methods are compared.The purpose of using non-sinusoidal power supply technology to optimize the magnetic field distribution of the motor and improve the utilization rate of the core and the torque density is achieved.At a switching frequency of 2k Hz,the limitations of non-sinusoidal power supply technology at low carrier ratio are analyzed through experiments.The results show that the current control performance of the PI controller of the harmonic planar current loop at low carrier ratio is poor.Secondly,aiming at the problem that the traditional PI controller has poor current control performance at low carrier ratio,the feasibility of using the new algorithm of MPC to replace the traditional PI controller is discussed.On the one hand,under the premise of injecting only the third harmonic current,the traditional MPC discrete models are established on the fundamental plane and the third harmonic plane of the nine-phase induction motor.On the other hand,the control performance of the traditional MPC algorithm at normal carrier ratio and low carrier ratio is analyzed separately through experiments.Experiments show that the traditional MPC algorithm has good current control performance and strong parameter robustness at normal carrier ratio.However,at low carrier ratio,the traditional MPC algorithm cannot accurately calculate the control voltage,which leads to the deterioration of the current loop control effect of the harmonic plane,and finally the system instability.Thirdly,aiming at the problem that traditional PI controller and traditional MPC algorithm cannot accurately control harmonic current at low carrier ratio,an MPC algorithm with prediction error compensation scheme is proposed.On the one hand,the MPC discrete model is established by means of incremental processing and state expansion,and the cost function is used to solve the control voltage.On the other hand,considering that it is difficult for the existing normal MPC methods to accurately calculate the optimal control voltage at a low carrier ratio,a scheme for correcting the control voltage using current prediction error is proposed.Experiments show that the improved MPC algorithm at low carrier ratio can overcome the shortcomings of conventional PI controller and traditional MPC algorithm,accurately track harmonic current commands,and still has strong robustness to motor parameters.