Research On Direct Torque Control Strategy Of Dual Three-Phase Permanent Magnet Synchronous Motor

Compared with traditional three-phase motor drive system,multi-phase motor has the advantages of large power output with low voltage,high reliability.It is suitable for electric vehicle and aerospace applications.With the development of power electronics and control theories,phase number is no longer a restriction in the motor drive system,multi-phase motor has attracted more and more attention.Direct torque control(DTC)has the advantages of simple structure,fast torque response.However,it usually suffers from large torque ripple and harmonic current.In this article,the dual three-phase permanent magnet synchronous motor(PMSM)is researched,and the direct torque control is deeply studied.Firstly,the structure and the mathematical model under six-phase stationary reference frame of dual three-phase PMSM have been introduced.By the use of coordinate transformation theory,the mathematical model is then set up in the vector space decomposition reference frame.And simulation results verify the correctness of the motor model.Secondly,the basic principle of direct torque control of dual three-phase PMSM is described in detail.The functions of each component of the control system are introduced.The switching state of six-phase voltage source inverter and the effects of different voltage vectors on torque and flux are analyzed.The vectors switching table of traditional direct torque control is given for the purpose of the fastest torque response.To solve the problem of large torque ripple in traditional DTC system,different improved DTC strategies are proposed,including high order torque hysteresis controller and duty ratio modulation.The six-phase inverter provides abundant voltage vectors,it is easy to extend the conventional three-order torque hysteresis comparator to a seven-order hysteresis comparator,which realizes the hierarchical control of the torque error.For the problem that one active vector acts on the whole period in direct torque control,the duty ratio modulation is introduced to reduce the torque ripple by reasonably allocating the operating time of active vector and zero vector.And two different duty ratio calculation methods are given,including duty ratio DTC based on online calculation and offline switching table.Finally,in order to suppress the current harmonic in direct torque control of the dual three-phase permanent magnet synchronous motor,the spatial distribution of inverter switching vectors in the two sub-planes is deeply analyzed.Based on the constraint that the voltage amplitude in harmonic plane is zero,a control algorithm of synthetic virtual vector is presented.Two different types of virtual vector synthesis methods are compared in detail.The simulation results show that the direct torque control based on virtual vectors can effectively suppress harmonic current.