M. Konno

Development of a 7.5 kW YBCO Superconducting Synchronous Motor

We are developing a 7.5 kW-360 rpm synchronous motor with a superconducting rotor. The rotor is 6-pole type and has no iron core. The armature winding is wound with copper. The designed operating temperature is 40 K. It will be cooled by forced-flowed helium gas. We have finished the winding of the first superconducting field coil with YBCO superconducting tapes fabricated by IBAD-PLD method. It is a 62-layer solenoidal coil. The total length of YBCO tape is 400 m. The magnetic field in the field winding is about 0.4 T at maximum and that in the gap is 0.2 T. We cooled it down to 20 K by G-M type cryocooler and investigate the transport properties in advance of the winding of the other field coils. We verified the stable operation up to 100 A at 40 K against the rated current of 50 A. In addition we prepared the cryogenic rotor system and installed 6 pieces of dummy copper coils instead of superconducting coils. We cooled it down to 30 K by forced-flowing helium gas and tested it by rotating up to 360 rpm. We verified the satisfactory good mechanical and thermal operation.

Development of REBCO Superconducting Transformers With a Current Limiting Function

We are developing RE1Ba2Cu3O7-δ (REBCO, RE: Rare Earth, Y, Gd, etc.) superconducting transformers with a current limiting function. The target is a 3φ-66 kV/6.9 kV-20 MVA one for a distribution power grid. We designed and built two pieces of 1φ-6.9 kV/2.3 kV-400 kVA superconducting transformers with YBCO superconducting tapes. One was wound with a YBCO tape with a copper stabilizing layer with a thickness of 300 μm. The other was 50 μm in thickness of copper. Making sudden short-circuit tests with a 200 MVA short circuit generator, we investigated the response of the YBCO superconducting windings against the fault excess current over the critical current. Repeated sudden short-circuit tests revealed the quite different response between them. In the case of fully stabilized with copper 300 μm thick, short circuit current flowed without decay. On the other hand, in the case of copper 50 μm thick, short circuit current was reduced around three times as much as the rated current. Using the numerical simulation program, which we made up on the basis of the results of sudden short-circuit tests of a 10 kVA test transformer, we quantitatively investigated the transition phenomenon of REBCO superconducting windings to normal state through flux-flow state due to fault excess current. As a result, theoretical simulation explained experiment quantitatively and clarified how to control the short circuit current.

Relaxation of Shielding Current in Test Coils for MRI With REBCO Superconducting Scribed Tapes

Superconducting magnets for MRI need uniformity of produced magnetic field within ppm order in time and space. To develop the superconducting magnets for MRI operating in subcooled liquid nitrogen at 65 K, two pieces of small test coils were fabricated with Re1Ba2Cu3O7-δ (REBCO, Re: Rare Earth such as Y, Gd, and Eu) superconducting tapes. One was wound with a nonscribed tape and the other was with a laser-scribed tape into a four-filament structure. Each test coil was inserted inside a NbTi superconducting magnet. After bath cooling, the test coil with subcooled liquid nitrogen and applying a dc bias magnetic field of 0.5-2 T, transport current was applied step by step, and the variation of magnetic field at the coil center was observed by a Hall probe with high accuracy. Although magnetic field followed the variation of transport current with a delay in the respective coils, the drastic difference between them was revealed. It was demonstrated that the influence of the induced shielding current on the produced magnetic field was drastically reduced by scribing. Not only the magnitude of shielding current but also the decay time constant was reduced by scribing. The produced magnetic field of the test coil with the scribed tape reached to a target one in a short time as programmed. Scribing REBCO tapes brought about faster relaxation of magnetization.

New Method for Quick Decay of Shielding Current in REBCO Superconducting Coils

We are developing superconducting magnets for MRI with Re1Ba2Cu3O7-δ (Re: Rare Earth, Y, Gd, Eu, REBCO) superconducting tapes. We intend to operate them in subcooled liquid nitrogen at 65 K. Superconducting magnets for MRI need uniformity of magnetic field in time and space. The disturbance should be mainly caused by the shielding current induced in the wound tapes. To investigate this, we made small test coils with a laser-scribed four-filament EuBCO tape and a nonscribed GdBCO tape. After cooling down to 65 K with subcooled liquid nitrogen and applying a dc bias magnetic field, transport current was applied step by step to the test coil and the magnetic field variation at the coil center was observed. The actual magnetic field followed the variation of transport current with a delay. We derived the fitting expression for the observed field variations. The magnitude and the decay time constant of the shielding current increased with transport current. However, when further increasing transport current close to the critical current, the shielding current decayed quickly. Using this tremendous knowledge, we proposed a new operating method so that the magnetic field reached to the target one in a short time. In addition, the comparison between both coils demonstrated that not only the magnitude of shielding current but also the decay time constant was drastically reduced by scribing.

Design Study of 2-MW Fully Superconducting Synchronous Motors

This study designed various 2-MW fully superconducting motors with the recently developed REBa₂Cu₃O_7-δ (RE: Rare Earth, Y, Gd, Eu, REBCO) superconducting tapes to determine an optimum structure. The ac loss and critical current properties of the REBCO tapes were investigated and adopted in the design and analysis of the motor. The motors were designed by setting the operating temperature and the magnitude of magnetic field at the gap as parameters, and the properties of each motor were evaluated through numerical simulations with commercially available two-dimensional analysis software. Various factors, such as ac loss in the windings, efficiency, and the length of the required REBCO tapes, were evaluated, and the dependence of these factors on the operating temperature <italic>T</italic>, magnetic loading <italic>B</italic>_g, and diameter of motor <italic>D</italic> were analyzed. The ac loss induced in the armature windings could be reduced by our original technique, which involves scribing into a multifilamentary structure and special winding of the tapes. As a result, the motor could achieve an efficiency of 97.2% under optimum conditions, in which the REBCO tape is scribed into a 10-filament structure. Setting lower <italic>B</italic>_g and higher <italic>T</italic> is more favorable for enhancing efficiency, whereas the condition of lower <italic>B</italic>_g and <italic>T </italic> is more favorable for achieving compactness and lightness.

Numerical Analyses on the Influences of Armature Winding Shape and Yoke Arrangements on Total Losses in Fully Superconducting Synchronous Motors Using REBCO Tapes

In the fully superconducting synchronous motors using REBCO tapes, the magnetic fields in the direction of perpendicular to the face of the tapes in armature windings generate large ac losses in the windings. Therefore, it is effective to reduce magnetic fields in the direction of perpendicular to tape face for the reduction of ac losses. A one of the methods is to get closer a yoke to the armature windings as possible. However, installing the yoke in the cooling area in order to get closer the yoke to the armature windings substantially increase the iron loss in the yoke by the cooling penalty. Therefore, in general, the yoke is placed at room temperature. The purpose of this paper is to evaluate the effect of installing the yoke in low temperature. So, we analyzed the properties of 500-kW fully superconducting synchronous motor by finite element method. We estimated the influences of installing yoke into low temperature in the two cases of armature winding shapes, solenoid windings and pancake windings. In this calculation, we assumed that superconducting material is EuBCO, and the operating temperature is 65 K.

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

The development of fully superconducting motors using Re1Ba2Cu3O7-δ (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.

Development of cooling system for 66/6.9kV-20MVA REBCO superconducting transformers with Ne turbo-Brayton refrigerator and subcooled liquid nitrogen

We developed a turbo-Brayton refrigerator with Ne gas as a working fluid for a 3 - 66/6.9kV-2MVA superconducting transformer with coated conductors which was bath-cooled with subcooled LN2. The two-stage compressor and expansion turbine had non-contact magnetic bearings for a long maintenance interval. In the future, we intend to directly install a heat exchanger into the Glass-Fiber-Reinforced-Plastics cryostat of a transformer and make a heat exchange between the working fluid gas and subcooled LN2. In this paper we investigate the behaviour of subcooled LN2 in a test cryostat, in which heater coils were arranged side by side with a flat plate finned-tube heat exchanger. Here a He turbo-Brayton refrigerator was used as a substitute for a Ne turbo-Brayton one. The pressure at the surface of LN2 in the cryostat was one atmosphere. Just under the LN2 surface, a stationary layer of LN2 was created over the depth of 20 cm and temperature dropped from 77 K to 65 K with depth while, in the lower level than that, a natural convection flow of LN2 was formed and temperature was almost uniform over 1 m depth. The boundary plane between the stationary layer and the natural convection region was visible.