Multi-star multi-phase winding for a high power naval propulsion machine with low ripple torques and high fault tolerant ability

In this paper, an original multi-phase Surface Mounted Permanent Magnet (SMPM) Machine designed for naval propulsion is proposed. The design objective of this high power low speed machine is twofold: to enhance the fault tolerance capability of the system and to optimize the quality of the torque by reducing the electromagnetic torque ripples which underlie the acoustic behaviour of the motor and of the global mechanical structure. A low level of ripple torques must also be ensured in faulty operations. To fullfill these constraints, the machine is equipped with a fractional-slot concentrated winding made up of four 3-phase windings each one being star-connected, each star being magnetically shifted by an angle of 15 degrees. This 4-star 3-phase configuration allows to reduce the cogging torque and to separate magnetically and physically the phase windings. The end-turns are also drastically reduced, which improves the compactness and the efficiency of the machine. This original multi-phase machine is supplied by four 3-phase PWM voltage source inverter with sinusoidal current law. The magnetic independences between the four star windings allow a very simple control of the four-star supply and a straightforward fault operating mode. Moreover, this 4-star winding configuration yields to very low torque ripples in nominal configuration (four stars connected) and in faulty operations if two magnetic non adjacent stars are disconnected. For all these reasons, this structure appears particularly suitable for naval propulsion application since it increases the machine performances in terms of compactness, reliability and quality of torque.