Research On PMSM-DTC System Based On DC Bus Voltage Control Of Composite Power Supply In Electric Vehicles

For the demand of high specific energy and high specific power of the power supply in the actual driving process of electric vehicles,a composite power supply system composed of battery and supercapacitor is established,according to the characteristics of the battery and supercapacitor.Considering the large differences in the characteristics of the battery and supercapacitor,the output characteristics of the supercapacitor are so soft that they can't be matched directly and need to be connected to the DC bus through a bi-directional DC/DC converter.Due to the limitation of DC bus voltage and inverter output current,the stator voltage and current of permanent magnet synchronous motor must be limited in steady-state operation,thus affecting maximum speed and output torque range of the motor drive system in constant torque work area.However,the electric vehicles hope the motor work in a higher speed range,which requires the motor to work in the constant power area and run smoothly above the basic rate under the limitation of DC bus voltage.First of all,on the basis of introducing the principle of coordinate transformation and the mathematical models of permanent magnet synchronous motor in three coordinate systems,the basic principle of the traditional direct torque control theory are analyzed in detail.Aiming at the disadvantages of traditional direct torque control based on switching table,a new direct torque control system of permanent magnet synchronous motor based on space vector modulation is studied.Secondly,composite power supply system which composed of battery and supercapacitor through the bi-directional half bridge DC/DC converter in parallel is adopted,that can keep the DC bus voltage stable and reducing the effect of the DC bus voltage fluctuation on the performance characteristics of the motor.According to the characteristics of the battery with high specific energy and the supercapacitor with high specific power,this thesis studies a power allocation method based on the change rate of load power.The supercapacitor can provide or absorb the instantaneous power during the acceleration and deceleration period of the electric vehicles,and reduce the large current charging and discharging current of the battery.So it can effectively restrain the impact of the load change on the DC bus,and realize the stable control of DC bus voltage.Finally,in order to improve the speed of electric vehicles under the limitation of inverter capacity and DC bus voltage of composite power supply.According to the feedback of the DC bus voltage,it is judged whether or not to enter the weakening magnetic field.The maximum torque per ampere control and the field weakening control are adopted in the constant torque area and the constant power area respectively,thus realizing the full speed operation of direct torque control of permanent magnet synchronous motor.