Research On SVPWM Overmodulation Control Strategies For Permanent Magnet Synchronous Machines Used In Vehicles

With the increasing demand for convenient travel service,the automobile industry has achieved a rapid development and brilliant achievements in a hundred years,and vehicles have been popularized in modern families.At the same time,the energy crisis and environmental pollution caused by the increasing number of vehicles have become the focus of attention in human society.For this reason,traditional vehicle manufacturers are actively developing new energy vehicles and laying out in this new strategic market in future.The permanent magnet synchronous machine(PMSM)has been widely used in the new energy vehicle used motor thanks to its high-power density,wide speed range and high operation efficiency.In order to obtain a higher power density and output speed,permanent magnet synchronous motors are often required to have a wide speed range of operation.Therefore,the use of flux-weakening control and overmodulation technology to improve the maximum speed and voltage utilization of motors with limited DC-bus voltage has now become the mainstream trend of current research in both academia and industry.As one of the solutions,the flux-weakening control technology can effectively reduce the magnitude of back EMF by the injection of a continuous flux weakening current.The other solution,overmodulation technology,can significantly improve the utilization of DC bus voltage and increase the RMS input voltage to stator winding.The cooperation of the above two technologies is of great significance for the high-speed operation of the vehicle used motor.A deep investigation to the above two technologies has been conducted in this thesis,and main contents are introduced as follows:(1)A deep analysis and investigation on SVPWM overmodulation strategies are conducted.Two widely used overmodulation strategies,viz.two-mode and single-mode overmodulations,are studied in this paper.The nonlinear overmodulation region is usually divided into the overmodulation I(0.906 ≤MI<0.952)and the overmodulation II(0.952<MI ≤1)according to the value of modulation coefficient.In the overmodulation region I,only the amplitude of the voltage is modified to ensure less harmonics under low level of overmodulation.In the region of overmodulation II,both the magnitude and phase of the voltage are redistributed in order to maximize the voltage utilization.In each section,the Fourier series expansion is used to ensure that the amplitude of fundamental component of output voltage is equal to that of reference voltage.Since the implementation of two-mode overmodulation algorithm is too complex,a single-mode overmodulation algorithm,which does not need to divide the overmodulation region,has been investigated in this thesis.This single-mode strategy only controls the phase of reference voltage without dealing with its amplitude,which can achieve a continuous transition from the linear region to the sixth-step region.(2)The influence of harmonics generated by the overmodulation algorithm on the system performance is studied.The output voltage harmonics under the conventional single-mode overmodulation strategy are analyzed firstly.Considering that the fundamental amplitude of the inverter output voltage is usually inconsistent with the reference voltage,an overmodulation strategy considering this difference in modeling is proposed.Afterwards,the reason of existence of low order harmonics in output currents when the utilization rate of voltage is increased during the overmodulation control,is analyzed.Aiming to solve this issue,a current harmonic suppression algorithm is then proposed,which is a combination of coordinate transformation and low pass filter,and synthesizes a synchronous filter for the real-time estimation and suppression of current harmonics.Finally,it has been experimentally verified on a set of PMSM test platform.(3)The application of overmodulation technology in the flux-weakening control is studied.Three regions of vector control,viz.MTPA,conventional flux-weakening and MTPV,are analyzed in detail.A flux-weakening control scheme based on the feedback voltage difference is studied,which uses the comparison error between the amplitude of dq-axis voltage vectors and the maximum limit of DC bus voltage to create a closed-loop feedback control of flux weakening current.In order to further improve the performance,a voltage boost flux-weakening scheme based on overmodulation control is developed in this thesis.Both theoretical and experimental analyses depict that the proposed control scheme can increase the output voltage and expand the speed range of flux-weakening control while the DC bus voltage is kept constant and consequently.