Study on the Optimum Arrangement of the Field Winding for a 20-kW Fully Superconducting Motor

The development of fully superconducting motors using $\text{Re}_{1}\text{Ba}_{2}\text{Cu}_{3}\text{O}_{7-\delta}$ (REBCO, Re: rare earth such as Y, Gd, or Eu) superconducting tapes has begun with the support of the Japan Science and Technology Agency. In this study, as the first step, the optimum arrangement of the rotating field winding for a 20-kW 600-r/min fully superconducting motor was evaluated in terms of torque. The ac loss and critical current properties of currently developed REBCO superconducting tapes were first investigated using a pickup coil method. Taking these properties into account, 20-kW fully superconducting motors with four types of ironless two-pole rotors were designed. The ac loss reduction technique for REBCO superconducting tapes, which was successfully adopted for a 66-kV 2-MVA transformer, was applied to the fixed armature winding. By numerically simulating the actual operation of the respective motors, the torque and ac loss properties were investigated. The torque, i.e., the output power of the motor, and the torque ripple were found to strongly depend on the arrangement of the field winding, despite the different arrangements having the same ampere-turn. The torque ripple becomes smaller as the rotating field winding approximates a 2-D dipole, which is indicated by the uniformity of the magnetic field inside the rotor.