Torque Accuracy And Efficiency Optimization Measures Of Three-phase Motor PWM Driving System

Pure electric vehicle is a kind of green and environmentally friendly transportation tool,and it is also a travel tool vigorously promoted and developed by our country.The power source of pure electric vehicle is motor.In order to realize frequency conversion operation of motor,PWM strategy is needed.Vehicle operating conditions are complex.In order to cope with various emergencies,precise torque output and high performance algorithm control are needed.At the same time,in order to improve the range of the electric vehicle,the controller needs to ensure the power of the vehicle while having excellent system efficiency.These indicators are closely related to the PWM strategy.In this paper,two types of PWM,continuous PWM and discontinuous PWM,are discussed at first.Continuous PWM,such as SPWM,seven-segment SVPWM,discontinuous PWM,such as DPWMMAX,DPWMMIN,GDPWM,DPWM3,the main difference between the two is that in each carrier cycle of the former,threephase switches occur,while one phase of the latter does not need switches.Therefore,DPWM has less switching loss in theory and may achieve higher system efficiency.In addition,two implementations of these PWMs,zero sequence signal injection method and space vector method,are given,and the inherent consistency of the two methods is discussed.The output voltage of PWM is square wave,which contains abundant harmonics.These harmonic voltages can cause abundant harmonic currents.Taking R-L load as an example,the frequency spectrum and harmonics of output voltage and driving current waveforms of PWM method are analyzed,and the synchronous current sampling method commonly used in industry to avoid the influence of harmonic current on control performance is given.But when synchronous current sampling method is not synchronized due to delay of software,it will affect the accuracy of torque.A novel current oversampling filter scheme is proposed to improve the torque accuracy.In addition,the harmonic analysis of PWM current shows that the seven-segment SVPWM has the smallest harmonic component.When the modulation index increases,the harmonic components of PWM current tend to be the same.Therefore,when the modulation index is large,the variable PWM method may achieve higher system efficiency while guaranteeing the current control performance.At the end of this paper,the controller developed independently by our laboratory and test bench are used to verify the proposed scheme of improving torque accuracy and system efficiency.The results show that the proposed scheme is effective.