No abstract

A HTS induction motor, with a HTS squirrel-cage rotor, is analysed using an electrical equivalent circuit. The squirrel-cage winding in the rotor consists of rotor bars and end rings, and both are considered to be made of YBCO film conductors. A wide range of electric field versus current density in YBCO film is formulated based on the Weibull function, and analysed as a non-linear resistance in the equivalent circuit. It is shown that starting and accelerating torques of the HTS induction motor are improved drastically compared to those of a conventional induction motor. Furthermore, large synchronous torque can also be realized by trapping the magnetic flux in the rotor circuit because of the persistent current mode.

HTS squirrel-cage induction motor was fabricated and tested in this study. Both of rotor bars and end rings were made of Bi-2223/Ag multifilamentary tapes in order to realize superconducting current loops, and the conventional (normal conducting) stator, 3-phase and 4-pole, was utilized. Rotating characteristics of the fabricated motor were tested for different input voltages at 60 Hz. The performances were also analyzed by means of the theoretical method based on the electrical equivalent circuit. It was shown that the minimum starting voltage was experimentally confirmed and agreed semi-quantitatively with the analysis result. The rotation at synchronous speed was realized by applying the load, at least, 1.5 Nm.

The mechanical and superconducting properties of MgB 2 wires heat treated at various conditions have been investigated. Superconducting MgB 2 wires, fabricated by the powder-in-tube (PIT) method, show a transition temperature T c at 32.4 K for the as-rolled wire as shown by dc-resistivity measurement. It is found that the T c is improved (37.3-38.4 K) by subsequent Ar annealing treatment under various sintering temperatures and times. Remarkably, the annealed samples show significant improvement in the overall quality of the samples. On annealing at 1323 K for 0.6 ks, we obtained the best quality sample with a J c of 470 A mm −2 at 2.0 T. In order to find out the irreversible field (B irr ), the technique of the Kramer plot was applied. The B irr obtained was 7.6 and 9.6 T, for the as-rolled as well as the annealed samples at 1073 K for 3.6 ks, respectively. The mechanical properties were measured under tensile load. The yield stress was 450 MPa for the as-rolled sample and decreased by annealing at 1323 K for 0.6 ks to 110 MPa. After the wires were heat treated at temperatures between 1073 and 1323 K and tensile-stressed up to their yield stress at room temperature. The critical current measurements were carried on such samples at 4.2 K under applied magnetic fields of 2 and 5 T. The stress level for the 95% tolerance of critical current was determined to 97 MPa.

We study the angular dependence of E-J characteristics and dissipative properties due to thermal disturbance in the Bi-2223/Ag multifilamentary tape in the low-voltage region. In order to simulate the dissipative properties, we take into account the analytical expressions of the E-J characteristics as a function of temperature, external magnetic field and field direction, which agree well with the experimental data. It is shown that the simulation results of the dissipative properties agree with the experimental data semi-quantitatively. The relationship between the current sharing rate and its contribution to the temperature increase is also discussed, based on the simulation. These results are crucial for high-T c superconducting power application systems with a persistent current mode where even low-power dissipation is detrimental to the performance.