Research On Optimized Design Of Field Orient Control For Low-voltage Electric Vehicle Induction Motor

The safety and economy of the low-voltage induction motor(IM)drive system make it a new choice for electric vehicle drives.In order to improve the output performance of the low-voltage IM drive system based on field orient control(FOC),ensure its reliability,reduce system cost,and improve power density,this thesis aims at the optimized FOC technology design of 48 V IM drive system for electric vehicles.Generally,this thesis studied the current distribution strategy to ensure the maximum torque output in the control,the speed estimation based on the discrete Luenberger observer,and the nonlinear factor compensation of the inverter.The implementation and verification conclusions of the corresponding control algorithm were obtained,which are summarized as follows:In order to meet the performance requirements of high torque output during electric vehicle operation,the current distribution strategy to ensure the maximum torque output in the FOC of 48 V IM and the field weakening controller was studied.Firstly,based on the voltage and current constraints of the IM drive system,the theoretical optimal current vector trajectory of the maximum torque output of the IM is derived.In view of the possibility that the 48 V IM may not have a constant power region,a suitable theoretical optimal current vector trajectory of the maximum torque output for 48 V IM is proposed.Secondly,it is the fact that the actual current vector trajectory is difficult to completely track the theoretical optimal current vector trajectory.Therefore,an optimized field weakening controller is proposed,which can improve the line voltage utilization rate of the field weakening region by introducing a voltage closed loop.In order to improve the reliability of the drive system and reduce the system cost,a speed estimation strategy based on discrete Luenberger observer for 48 V IM was designed.Firstly,comparing the effects of different discrete methods on observation performance,the forward Euler method is selected based on the compromise between estimation accuracy and calculation load.Secondly,the effect of observer poles placement on the observation is discussed.The traditional feedback gain matrix setting method is to enlarge the open-loop model poles by a certain multiple and use it as the observer poles.After discretization,there will be the problem of instability in the high-speed region.Therefore,the placement of shifting the open-loop model poles to the left as the observer poles is considered.To solve this problem,an optimal design method of the pole-shift distance of the open-loop model poles based on the speed range and switching frequency is proposed to ensure the stability of the observer.The experiment proves that the designed discrete Luenberger observer suitable for 48 V IM can accurately observe the rotation speed and maintain stability in the full speed range.Non-linear factors such as the dead time of the inverter,the voltage drop when the switching device is conducting,the voltage drop when the anti-parallel freewheeling diode is conducting,and the turn-on/off delay of the switching device will cause the error between the actual control output and the reference of the inverter,which affects the observation effect of the observer,and also causes the output torque ripple of the motor which affects the performance of the motor.In order to solve this problem,an inverter output voltage model considering the MOSFET conduction characteristics was established,and a method of inverter nonlinear factor compensation based on the inverter output voltage model was discussed.Secondly,for the problem that the compensation needs to know the current polarity accurately,and the current polarity is difficult to accurately detect near the zero-crossing point,a compensation method is proposed to promote the current to cross the zero in the vicinity of the current zero-crossing point.Finally,combining the compensation based on the inverter output voltage model and the compensation for prompting crossing zero in advance,a comprehensive method suitable for the compensation of the nonlinear factors affecting the inverter of 48 V electric vehicles is proposed.This paper builds a 48 V IM drive system control model based on the Simulink simulation platform and conducts simulation research.The verification of the field weakening control algorithm,speed observation algorithm and inverter nonlinear factor compensation algorithm proposed in this paper is completed.And based on the existing48 V IM drive experimental platform,the designed optimized field weakening controller,discrete Luenberger observer and inverter nonlinear factors affect the comprehensive compensation method of experimental verification,proved the effectiveness and feasibility of above 48 V IM high-performance FOC key technology.