Research On Direct Torque Control Of Open-End Winding Permanent Magnet Synchronous Motor For Electric Vehicle

Three-phase open-end winding permanent magnet synchronous motor(OEW-PMSM)drive with common DC bus is considered as a competitive solution for electrical vehicle and it has been paid attention by researchers owing to its high driving power,good controllability and strong fault-tolerance ability.Direct torque control(DTC)for OEW-PMSM drive system is studied in depth in this thesis,where some corresponding control strategies have been proposed for electric vehicle occasion.Several DTC strategies,such as DTC with duty ratio modulation(DRM-DTC),DTC with minimum torque ripple(T-DTC),predictive direct torque control(PDTC),deadbeat direct torque and flux control(DB-DTFC)and fault tolerant DB-DTFC(F-DTFC),are proposed to lay a solid foundation for the practical application of OEW-PMSM.The specific research contents and main achievements of this thesis can be summarized as follows:1.On the basis of analyzing the mathematical model of OEW-PMSM and the voltage vectors output by dual inverter,the conventional DTC has been studied.The research shows that the conventional DTC has some disadvantages,such as large ripples of zero-sequence current,electromagnetic torque and stator flux.In order to address the above problems,a DRM-DTC is proposed.In DRM-DTC,a duty ratio adaptive adjustment mechanism of the active voltage vector and zero vectors is designed to make zero-sequence voltage error free tracking and stator voltage control error minimization.As a result,the ripples of zero-sequence current,electromagnetic torque and stator flux would be reduced at the same time.Furthermore,an optimized pulse width modulation(PWM)scheme is proposed according to the redundancy characteristics of zero vector.The appropriate zero vector is picked out based on the principle of minimizing the total number of switches in a control cycle.Thus,the average switching frequency of dual inverter can be greatly reduced.2.In order to further reduce the ripples of zero-sequence current and electromagnetic torque,a T-DTC is proposed.In the T-DTC,the zero-sequence current with sinusoidal AC component is decoupled into two DC components.Then,a zero-sequence current controller based on dual PI regulator is proposed to track these two DC components.As a result,the zero-sequence current can be suppressed to almost zero.Whereafter,by inserting the zero vector with non-null zero-sequence voltage,the duty ratio of active voltage vector can be modulated to ensure that the torque ripple is minimum in theory.The research and experimental results show that T-DTC has better zero-sequence current and electromagnetic torque ripple suppression effect than the conventional DTC and DRM-DTC under different operation conditions.3.PDTC has the advantages of intuitive concept,easy to deal with multivariable and nonlinear constraints.In this thesis,PDTC is applied to OEW-PMSM with common DC bus for the first time.However,the basic PDTC(B-PDTC)has some problems,such as tedious tuning of weighting factors,non-optimized elements in the finite control set.To overcome the above problems,a PDTC based on weighting factorless and finite control set optimization(WF-PDTC)is proposed.Firstly,a deadbeat control strategy is proposed to convert the control of zero-sequence current,electromagnetic torque,and stator flux into the control of stator voltage vector.Since the equivalent control objects have the same unit,the weighting factors are not required.Secondly,a two-step optimization scheme for finite control set is proposed,every new element are synthesized by three different voltage vectors.Compared with B-PDTC,WF-PDTC eliminates the tuning process of weighting factors.Meanwhile,it has smaller ripples of zero-sequence current,electromagnetic torque and stator flux.4.In order to further improve the performance of the motor drive system and make the actual values of zero-sequence current,electromagnetic torque and stator flux of OEW-PMSM with common dc bus track the reference values within a control cycle,a DB-DTFC strategy is proposed.The proposed DB-DTFC derives the direct torque and flux controller based on the concept of active flux.Then,the relationship between the stator voltage vector and the electromagnetic torque and stator flux is revealed.On this basis,a novel space vector pulse width modulation(SVPWM)based on hysteresis control of zero-sequence current is proposed.The voltage vectors,whose polarity of zero-sequence voltages is opposite with the polarity of zero-sequence current,are selected as the basic voltage vectors to synthesize the reference stator voltage vector.The proposed strategy has easy implementation and excellent zero-sequence current suppression effect.In addition,the zero-sequence voltage still has wide linear modulation range under the condition of high fundamental modulation ratio.Simulation and experimental results verify the effectiveness of the proposed DB-DTFC.5.The previous DTC strategies of OEW-PMSM with common dc bus are proposed for fault-free condition.In order to improve the reliability of the system and ensure that the motor drive system can still operate continuously after one phase open circuit fault occurs,a F-DTFC strategy is further proposed.The proposed strategy establishes the mathematical model of OEW-PMSM with one opened phase and reveals its operation law.Accordingly,a fault tolerant direct torque and flux controller is constructed.On the basis of analyzing the voltage vectors output by the remaining healthy phases,an improved SVPWM is proposed.The research shows that the proposed F-DTFC can ensure that the motor drive system still has smaller ripples of electromagnetic torque and stator flux under fault tolerant condition.In this thesis,DTC for OEW-PMSM with common dc bus is applied to electric vehicle occasion to improve the system performance,and it also can be extended to other drive occasions.In the meantime,the theory of DTC is enriched and developed.