Control Strategy For PMSM Driving System At Low Switching Frequency

Permanent Magnet Synchronous Motors(PMSMs)are used in many industrial fields due to their characters of high-efficiency and high-power density.In a high-power motor drive system,the impact of inverter switching losses and its heat dissipation conditions on the overall system should be considered.Therefore,the maximum switching frequency of the inverter is commonly limited.The motor vector control system will occur the issue of d-q dynamic coupling at low switching frequency condition,and the performance of the control system is generally non-ideal.In addition,low switching frequency will also lead to poor harmonic characteristics of the inverter output waveform,and the hybrid modulation strategy optimized for this problem is mostly used in the open-loop control systems.Consequently,the research on combination of high-performance motor closed-loop control algorithms and hybrid modulation strategy has important engineering practical significance.This paper takes the motor control system at low switching frequency as the research object,for the purpose of achieving the collaborative optimization of the control performance of the high-power PMSM traction system from the respects of the inverter modulation strategy and the current controller.Firstly,according to the actual working conditions of the wide-speed electric traction system,the current single closed-loop vector control system is established.The current loop control strategy of this system adopts Maximum torque per ampere(MTPA)algorithm below the base speed,and combines with the field weakening control in order to extend its speed range.The two-level voltage source inverter(2L-VSI)commonly-used modulation strategies are also analyzed,and the weighted total harmonic distortion(WTHD)between different modulation strategies under the same pulse number is compared to illustrate the superiority of synchronous modulation strategy at low switching frequency.Secondly,for the sake of improving the harmonic spectrum of the inverter output signal at low switching frequency,the hybrid modulation strategy is introduced.Comparing the pros and cons between synchronous space vector PWM(SVPWM)and selected harmonic elimination PWM(SHEPWM),this paper designed a hybrid modulation strategy using synchronous SVPWM and SHEPWM in the middle and high frequency band respectively.From the point of view that the electromagnetic torque of PMSM is directly related to the d-q current,the appropriate switching points of hybrid modulation strategy are determined by the instantaneous value of the three-phase current and the equivalent circuit model of PMSM.Thirdly,aiming at the issue of poor current loop control performance in the vector control system at low switching frequency,this paper designs a precise decoupling current loop controller.Based on the complex vector theory,the dynamic coupling items in the d-q coordinate are analyzed.Considering the hybrid modulation strategy and the delay coupling of the algorithm digital implementation,the PMSM mathematical model is refined and transfer function is obtained based on complex vector theoretical analysis.According to classical control theory,it is proved that the controller designed in this paper can ensure the stability of the motor control system in the full speed range,and effectively realize the combination of decoupling control algorithm and hybrid modulation strategy.Finally,the above-mentioned theories are verified in MATLAB/Simulink and the hardware-in-loop system based on d SPACE.The experimental results show that the hybrid modulation strategy can effectively improve the total harmonic distortion of lode side current at low switching frequency.In the PMSM vector control system,the transient process of PMSM output current and torque at switching periods of the hybrid modulation strategy can keep stability,and the performance of current loop precise decoupling controller will not change with the variation of motor speed.To sum up,when the switching frequency is limited,the presented control strategy can effectively improve the performance of the high-power PMSM vector control system in full speed domain.