Research On Multiphase Motor Drive System For Electric Vehicles

Electric Vehicles (EV) attracts widespread attention, due to its less pollution, low noise, energy saving and many other advantages. Meanwhile, the motor drive system for EV requires higher performance, because of its special requirement and harsh operating environment. Multiphase motor drive system has several outstanding advantages, compared to three-phase motor drive system. Thus, it can improve the performance effectively and provide a new power actuator solution for EV. This paper focuses on the key technologies for multiphase drive system in the application of EV, and the multiphase induction motor drive system is constructed. The organization of this article is listed as follow:Firstly, non-sinusoidal power supply technique is adopted in the multiphase motor, in order to improve the torque output and power density for EV motor drive system. The multiphase induction motor with non-sinusoidal supply can improve the output torque and efficiency, and the reasons are given in detail. According to winding functions method, spatial and temporal distribution of MMF and mathematical model for biplane rotor field orientation control are analyzed. The way of harmonics injection is discussed for the different phase number multiphase induction motors, including how to select the harmonics, how to determine the harmonics content and phase relationship as well as the effects on the teeth and yoke flux density. Maintaining the amplitudes of the air-gap flux density and yoke flux density unchanged, the motor performance with sinusoidal supply and non-sinusoidal supply are compared under various load. The results verify that the non-sinusoidal supply techniques can improve performance under heavy load. The rotor leakage flux impact on air gap flux is analyzed, because the air-gap flux density is affected by the motor load. Then, the fundamental and harmonic rotor magnetic field directions are required to make real-time adjustments, to ensure the flat-topped waves for flux density under various loads.Secondly, the fault-tolerant control strategies for different optimized targets are proposed to ensure its continuous operation, when the open-fault occurs in the EV drive system. The MMF keeps the same as normal condition, through reconstruction of the remaining stator currents under fault condition. The same current amplitude method, minimum stator copper loss method and minimum torque ripple method are analyzed in detail, according to three different optimized targets. The fundamental and harmonic planes are coupled under fault condition, and the magnetic coupling phenomena results in negative effects on the motor operation. Thus, the MMF theory is applied to solve the problem. In addition, the stator currents are asymmetrical after fault occurrence. In order to avoid the negative impact of hysteresis current control, proportional-resonant (PR) current regulator is adopted to achieve accurate tracking in the phase frame.Thirdly, field weakening control techniques are analyzed, in order to broaden the speed range for EV motor drive system. The optimized stator flux vector control in the field weaken region is proposed to improve steady-state and dynamic performance. Meanwhile, it is aimed to enlarge the torque output. The impact of the harmonic plane on weakening range operation is analyzed in detail, and non-sinusoidal power supply technique is adopted to improve DC bus voltage utilization. Motor efficiency is one of key technologies, because of limited batter energy storage problem. The modified method focuses on the overall motor efficiency by combining copper with iron loss calculation method, and the numerical values in the constant torque and field weakening region are calculated.Finally, the speed sensorless control method with rotor parameters identification is proposed, because the speed agency is fragile in the EV drive system. The multiple parameters are identified simultaneously by utilizing multi-plane properties of multiphase motors. Fundamental plane is used to estimate rotor parameters online, and harmonic plane is utilized to stator resistance and speed identifications. Futhermore, the sliding speed observer is modified under fault condition, and the speed sensorless control is realized even after fault occurrence. The sensorless algorithm under fault condition can improve the performance for EV in the extremely harsh environments.