Speaker
Description
Fully superconducting synchronous motors have great potential to achieve a high-power density (kW/kg), which is very attractive technology for future electric propulsion systems for aircraft applications. This system is expected to be operated with low voltage (several kV) and large current (several hundred A or more). Hence, the armature coil is required to have a large current capacity. In order to realize this, our research group focused on the transposed parallel conductors to enhance the current capacity on the armature coils. In this method, the armature coil is composed of the parallel conductors stacked with multiple superconducting wires. However, differences in the interlinkage magnetic flux between the stacked wires disrupt the inductance balance of this coil, leading to non-uniform current distribution between each strand. To address this problem, the transposing technique into the armature coil has been developed to achieve uniform current flow. While we have studied the transposition methods which have been applied to the armature coil under the single-phase coil condition in previous study, an actual motor consists of the three-phase armature coils and the rotating field coil or magnet.
In this research, in order to evaluate the effectiveness of the transposition method that is developed in an actual motor situation, the effect of the magnetic field of the field magnet on the current distribution rate of the armature coil will be investigated. For this evaluation, we prepared racetrack-shaped double pancake coils consisting of the three-strand parallel conductors applying the optimal transposition. The sample coils with 18 turns were made of REBCO tapes provided by Fujikura Ltd. The stational armature coils consists of three phases and each phase is composed of two armature coils which are facing each coil with a distance of 150 mm. The rotating field magnet consists of a cylindrical iron core and permanent neodymium magnets. The neodymium magnets with a surface magnetic flux density of approximately 0.6 T were arranged to cover the iron core. In Addition, the magnetic field generated by the permanent magnet at the coil surface, located 75 mm from the magnet, was approximately 0.14 T. In this test system, an AC current was supplied to the three-phase armature coils in liquid nitrogen. The current distributions in the parallel conductors were measured using a Rogowski coil when the rotating magnet was rotating synchronously with the rotating magnetic field of the three-phase armature coils. When a current of 40 Hz and approximately 50 A applied to the armature coil and the field magnet rotated at 2400 rpm, the current distribution rates among the three-strand parallel conductors were 34.5%, 31.3%, and 34.2%, which is almost perfect uniform. The measurements also conducted with only the single-phase armature coil without the field magnets, showed distribution rates of 35.1%, 31.8%, and 33.1%. In addition, only the three-phase armature coils resulted in rates of 36.7%, 30.5%, and 32.8%. Here, when a current of 50 A was applied to the coil, the magnetic field at the coil surface was calculated by analysis to be approximately 0.013 T. These results indicate that the transposition method developed using a single-phase coil in nearly uniform current distribution even when the rotating magnetic field due to the field is larger than the magnetic field of the armature coil.
The effect of current distribution due to the magnetic field of the magnet will be clarified through analysis. The experimental results and details will be discussed in MT29.
