Finite Set Predictive Torque Control For Three-Level Inverter-Permanent Magnet Synchronous Motor Systems

Permanent magnet synchronous motor(PMSM)has the advantages of simple structure,high degree of controllability and high power density.Neutral point clamped(NPC)three-level inverter has many advantages,such as small voltage stress of single power transistor when power is supplied at the same bus voltage level,high sinusoidal fitting of output current,and low switching losses at the same switching frequency.NPC three-level inverter fed PMSM system is widely used in high performance and high power applications such as electric vehicles,aerospace,and tram traction.The principle of model predictive control strategy is simple,and it stands out when dealing with multiple control targets.It has a good tracking effect and has lower torque ripple than direct torque control(DTC).Compared with vector control,the method is simple.However,there are certain problems in the adjustment of the weight coefficients of each control objectives and the steady-state performance of the control system.In this paper,the NPC three-level inverter fed PMSM system is used as the research object,and the research on Model Predictive Torque Control(MPTC)is carried out.Firstly,the sector is divided on the space vector plane,and the sector position of stator flux is determined.The mathematical model of PMSM in x-y two-phase rotating coordinate system is established,and the mathematical relationship between torque increment and flux increment and each axis component of stator voltage is deduced,and then transformed into the relationship between each axis component of stator flux and torque and flux.Then,the predicted value and reference value of electromagnetic torque and the predicted value and reference value of stator flux are input into the torque hysteresis loop and the flux hysteresis loop respectively,and the four situations of torque variation and flux variation are obtained.According to the four situations and the relationship between voltage and torque,flux,the candidate voltage vector set is selected,and the preliminary candidate vector set is obtained.Then,by analyzing the current loop formed by medium and small vectors and the direction relationship between DC side neutral point current and load current,the effect of different medium and small vectors on DC side neutral point potential offset is determined,and the vectors that balance the neutral point potential are retained to further screen the preliminary candidate vector set effectively,so as to establish a mechanism for quickly selecting candidate voltage vectors.This mechanism can remove the neutral point voltage term from the value function and keep the neutral point voltage balance.In this paper,on the experimental platform of NPC three-level inverter fed PMSM system.The steady-state and dynamic conditions of the control system are compared and verified.Compared with the traditional predictive torque control,the improved control method has obvious improvement in dynamic performance,and the neutral point voltage balance is realized while eliminating the weight term of neutral point voltage.The effectiveness and feasibility of the control method are verified.