Research On Asymmetrical Six-phase Permanent Magnet Synchronous Motor Series-connected Decoupling Control Method

Multi-motor variable speed drive system is widely applied in more-electric aircraft and more-electric ship propulsion applications. Since multiphase machines have additional degree of freedom, which can be utilized to realize multi-phase multi-motor series-connected control. The asymmetrical six-phase machine can be connected in series with another asymmetrical six-phase machine or two-phase machine or one asymmetrical six-phase machine and one two-phase machine by an appropriate phase transposition, supplied from a single multi-phase inverter. This paper mainly studies the decoupling control method of three asymmetrical six-phase permanent magnet synchronous motor(PMSM) series-connected systems above.Firstly, the mathematical models of asymmetrical six-phase PMSM and two-phase PMSM in stationary reference frame are established. Then, the decoupling mathematical models in orthogonal rotating are obtained by using vector space decomposition(VSD). The direct torque control based on space vector pulse width modulation(SVM-DTC) technology of asymmetrical six-phase PMSM and two-phase PMSM are studied, on the basis of which, the SVM-DTC decoupling control method of three series-connected systems are proposed. Since the output voltage of space vector modulation can not be equal to the input reference value when using SVM-DTC technology to control asymmetrical six-phase PMSM, the improved scheme is presented to obtain the maximum achievable output voltage.Secondly, the multi-frequency output voltage generation method of two-motor series-connected systems using SVM-DTC is analyzed. Since the traditional method need to switch the space vector modualations, the developed method based on variable sub-period is presented, which makes every motors can utilize the full DC bus voltage and improve the DC bus voltage utilization. The further improved SVPWM modulation method can reduce the action frequency of switching devices, to get output voltage of less switching loss and harmonics components. These three methods are verified by simulation.Finally, due to the another main advantage of multiphase multi-motor drive system, fault-tolerance capability, the general model of asymmetrical six-phase PMSM under open phase fault is established. The behavior of flux and electromagnetic torque is analyzed when A phase is open. It can achieve a expected effect from the simulation of the asymmetrical six-phase two-motor series-connected system under open phase fault.