An adaption of rare‐earth magnet and flexible power control of power electronics to traditional synchronous motors has made them more attractive electric drives in terms of high efficiency, flexible controllability and high densities of power and torque 1-6. With the spread of applications, there is a need for the electric drives to satisfy the diverse demands of users, and the motor should ensure high‐efficiency operation at multiple operating points on the speed–torque plane. Furthermore, a wider constant power speed range (CPSR) is also expected. In order to satisfy the requirements at various operating points under the electrical and mechanical restrictions, new control strategies typified by, for example, maximum torque per amperes and flux weakening (FW) control using d‐axis current adjustment have been developed.

Meanwhile, in order to respond to the diversified requirements of a wide range of motor application fields, such as electrification of automobiles, railways, aircrafts, ships, home appliances, and robotics including surgical robots, revolutionary motor technology is necessary, where it is essential to fundamentally change the magnetic circuit of a traditional synchronous motor to provide a variable field function like a direct current (DC) shunt motor on the premise of matured power electronics technology.