Torque performance improvement on multi three-phase PMSM based on PWM drives for marine application

Multiphase drive systems have received a growing interest in recent decades for marine propulsion applications, due to their high power density, high reliability and good torque performance. Among the multiphase drives, the multi three-phase drive with independent neutral points is a popular option for this application, as it allows for the usage of standard control and standard power electronics for the individual three-phase systems. In high power systems the switching frequency of the power semiconductor is usually limited, which results in high frequency current ripple caused by the PWM of the DC/AC converter. The ripple affects the performance of the machine in terms of torque. This thesis presents a novel mathematical modelling of multi three-phase Permanent Magnet Synchronous Machines (PMSMs) fed by voltage source Pulse Width Modulation (PWM) converters. It is found that, based on the analytical models of the multi three-phase drive, the torque ripple introduced by PWM voltage excitation can be reduced by the shift of carrier phase angles among different three-phase inverters. For the torque ripple analyzed in this thesis, only the interaction between the armature field, resulting from the PWM voltage excitation, and the fundamental component of the permanent magnet field is considered. The proposed carrier phase shift angles are obtained for the case studies of a sectored triple three-phase PMSM and two dual three-phase PMSMs. Numerical and finite element analysis (FEA) and experimental results are presented to validate the analytical models of the multi three-phase drives. Additionally, the torque performance improvement and the effect on current ripple introduced by the proposed carrier phase shifts are presented and validated by means of both simulation and experimental results