Deadbeat Predictive Current Control For Permanent Magnet Synchronous Motors At High Speed And Low Carrier Ratio

With the advantages of simple structure,high power density and high reliability,permanent magnet synchronous motors are widely used in CNC machining,industrial robotics,aerospace and other fields.Since the differential-free predictive current control has the advantages of fast dynamic response,it is widely used in the current loop of permanent magnet synchronous motors.However,when the motor is running at high speed,the ratio of carrier frequency to motor operating frequency is low,and the discrete model error of the motor of traditional differential-free predictive current control is large and the control effect is poor.At this time,the three-phase current also fluctuates greatly,which affects the stability and efficiency of the motor.In response to the above problems,this paper carries out relevant research work.This paper first analyzes the discrete model of the motor obtained by the forward Euler method and determines that the linear approximation will lead to large errors in the traditional discrete model when the motor speed is high and the ratio of carrier frequency to motor operating frequency(carrier ratio)is low,which affects the control stability of the motor.After that,a discrete motor model suitable for high-speed and low-carrier ratio operating conditions was proposed.This model established a discrete model on the?-?-axis and took the back-EMF changes in a control period into account under specific working conditions,it still has good dynamic and steady-state performance.At the same time,other factors that affect the dynamic and steady-state performance of the current loop are analyzed,including : inverter switching frequency and A/D sampling time,PWM duty cycle update and other digital control delays.The deadbeat prediction current control algorithm needs a delay for a period of time to accurately track the reference current,so a one-shot delay compensation is generally required.In view of the above problems,the dual-sampling and dual-update strategy is combined with the proposed prediction model.Without changing the inverter switching frequency,the stability of the control system under low carrier ratio working conditions is further improved and it has a certain inhibitory effect to the current harmonics.When the motor runs at high speed and low carrier ratio working conditions,the harmonics in the motor phase current will increase greatly,and the current distortion will be serious.As a result,the loss increases and the control performance of the motor deteriorates,so it is necessary to suppress the current harmonics.The harmonic distortion in the current is mainly determined by the switching frequency and pulse width modulation strategy.By increasing the switching frequency,the ripple amplitude can be effectively reduced,but it will cause additional switching losses.In response to this problem,the traditional space vector modulation strategy is adjusted,combined with the double sampling and double update technology,a new modulation strategy is proposed.Without increasing the switching frequency,the current fluctuation is suppressed and the harmonics generated by PWM modulation strategy is reduced..Finally,the method proposed in this paper is experimentally verified on the experimental platform of the permanent magnet synchronous motor control system based on two-level voltage inverter.The experimental results show that the method proposed in this paper can still accurately tracking current without static error at high speed and relatively low carrier ratio working conditions,and current fluctuations and current harmonics can be effectively reduced.