Performance of Inverter Fed HTS Induction-Synchronous Motor Operated in Liquid Nitrogen

We studied the effects of changing the driving of a squirrel-cage type induction motor with high T_c superconducting (HTS) rotor windings. Bi-2223/Ag tape conductors were utilized for the secondary windings and the conventional (normal conducting) stator, 3-phase and 4-pole, was introduced. The fabricated motor was installed in the metal cryostat, and no-load tests were carried out in liquid nitrogen (77 K). A PWM (pulse width modulation) inverter was utilized in order to drive the motor. The test results are discussed based on the nonlinear electrical equivalent circuit. It is shown that the fabricated motor operates not only at slippage mode but also at synchronous mode for the frequency range from 10 Hz to 60 Hz. Furthermore, the rotating characteristics had frequency dependence, and higher input voltage was necessary for synchronism as the driving frequency increased. These characteristics were explained based on the conventional theoretical discussion and the nonlinear current transport property of HTS windings.