Research On Control Strategy Of High Speed Permanent Magnet Synchronous Motor For Electric Vehicles

With the expansion of PMSM's application field,its performance requirements are also increasingly higher.Especially for electric vehicles with high-performance requirements,the requirements for high torque at low speed,accelerated overtaking,and other high-speed operations have to be satisfied.The back electromotive force will increase with the speed grows up,it prevents speed to continue increasing due to the output voltage saturation,so flux weakening control is required.In addition,the permanent magnet synchronous motor can realize a static current decoupling with the vector control,but the dynamic coupling relationship still exists.The coupling further degraded the system because of the increasing speed,at the same time,the influence of the time delay during high-speed operation should not be neglected either.For high-speed operation of the permanent magnet synchronous motor,the research on the key problems is of great importance.Under this background,this paper took the permanent magnet synchronous motor as the research object,and aimed to solve the voltage saturation of the permanent magnet synchronous motor for electric vehicles,and the problems related to the current cross-coupling and time delay were studied.The main content of this thesis are listed below:Firstly,on the basis of the mathematical model for the permanent magnet synchronous motor,a complex vector model for the permanent magnet synchronous motor was built according to the complex vector principle.According to the discrete control theory,an accurate discrete complex vector model for the permanent magnet synchronous motor driven by a three-phase inverter was derived,which provided a theoretical foundation for further research.Secondly,the boundary for the flux weakening operation of the permanent magnet synchronous motor was introduced,and the common flux weakening control methods of the permanent magnet synchronous motor were studied.With respect to the parameter sensitivity problems of the feedforward method and the defects in the dynamic response performance of feedback method,a hybrid flux weakening control method was studied.As for the core of this algorithm,an industrially practical method was provided to enable fast table creation,which significantly reduced the workload of the look-up table method.Through the above steps,the common inherent problems of the look-up table method which is heavy in workload and time-consuming have been solved.Thirdly,with respect to the current cross-coupling and time delay of the permanent magnet synchronous motor during high-speed operation,the factors and influences of the digital control time delay were introduced.A compensation method for time delay based on double current sampling and PWM duty cycle double update time sequence were studied,and the angle error was compensated in real time without any hardware equipment.By drawing the pole-zero diagram according to the transfer functions,the shortcomings of the traditional PI regulator were illustrated,and the discrete regulator based on complex vector was designed using pole-zero cancellation.After both the system delay and the current cross-coupling of the high-speed areas had been considered,the complex vector regulator has better current control capability.Finally,experiments for hybrid flux weakening algorithm were carried out,and its integrity and feasibility were verified.On this basis,the traditional PI regulator and the complex vector regular were compared.The experiment results showed that the discrete complex vector regulator based on the direct design have better current control performance than the traditional PI regulator.