Hiroyuki Ohsaki

Fully superconducting rotating machines, which have superconducting field and armature windings, attract attention because they achieve high efficiency and high-power density. In the design of a fully superconducting rotating machine, it is very important to estimate the AC losses in the armature windings to which AC magnetic fields and AC currents are applied. Therefore, we measure the AC losses of an armature winding using an AC loss measurement equipment, calculating the AC loss from the difference between the mechanical inputs obtained with and without the superconducting coil present. The rotation of the permanent magnet rotor causes transient phenomena due to the change in magnetic field distribution. In addition, temperature distributions occur along the coil. In this study, we conduct experiments and perform a numerical analysis to evaluate the AC loss characteristics of an MgB₂ coil under a rotating magnetic field, taking into account the magnetic field distributions. We evaluate the validity of the AC loss measurement using this equipment by comparing the measurement and the numerical analysis results.

There is a growing interest in researching the application of high-temperature superconductors to AC electrical equipment and systems. In particular, fully superconducting rotating machines with high power densities are attracting attention in aerospace and other industrial fields. However, AC losses occur when an AC current flows through and a rotating magnetic field is applied to the armature windings of a fully superconducting rotating machine. Therefore, it is important to evaluate AC losses in superconductors. On the other hand, there is an increasing demand for low-cost MgB₂ multi-filament wire with twisted, round, thin filaments, to reduce AC losses. Until now, three-dimensional (3D) numerical analysis for estimating AC losses in multi-filamentary round wires, such as MgB₂, has faced long calculation times and poor convergence. In this study, we constructed 3D numerical analysis models of superconducting multi-filament wires under rotating magnetic fields using the finite element method (FEM) based on the A-V formulation. AC loss calculations were performed for 2-filament and 36-filament wires, varying parameters such as the maximum magnetic flux density, the electric conductivity of the matrix, and the twist pitch. The validity of the model was confirmed as the numerical analysis results agreed with the theoretical values when the filament coupling was weak. Furthermore, the FEM analysis results showed that AC loss calculations could be performed for the cases with both weak and strong filament coupling. The proposed 3D numerical analysis model can be extended to the calculation of AC losses in superconducting multi-filament wires with complex shapes.

The application of superconducting technology to aircraft will develop the ability to solve the serious environmental problems caused by exhaust and noise from traditional aircraft and improve the performance of aircraft. With the merit of high-power density, the superconducting cable has raised interest from researchers and manufacturers around the world. To cooperate with aircraft electrification, its application on aircraft DC and AC power systems has been studied in recent years. To analyze the electromagnetic field and feasibility of the high-temperature superconducting (HTS) cable during the flight period, the finite element method (FEM) is used in this study to construct the model of the cable. To ensure the accuracy of the FEM model, the analytical AC loss model is applied for comparisons between T-A formulation and H formulation. The simplified electrical power system of the aircraft is simulated by MATLAB with a flight mission profile with specified propulsion architecture. Magnetic performance is analyzed from the aspects of cable types, the influence of harmonic current, and AC loss. Finally, wavelet transform is used to process the obtained magnetic field data to verify the effectiveness of the method and complete the HTS cable winding law analysis and magnetic optimization.

This paper reports numerical analysis results of AC losses when a rotating magnetic field is applied to a MgB₂ wire. The losses include coupling losses, hysteresis losses, and eddy current losses. The H-ϕ formulation is used for the finite element analysis for the loss analysis of the electromagnetic phenomena in the MgB2 wire. And the losses are calculated when the contact resistivity and contact length between MgB₂ wires are changed. The given rotational frequencies are 25 Hz and 50 Hz, and the flux density amplitude of the applied magnetic field is 0.01 T. As a result, it is shown that the coupling losses are inversely proportional to the contact resistance. And it is also found that the coupling losses are proportional to the square of the contact length. These tendencies can be explained by the theoretical formula for coupling losses.

We present the experimental characterization of energy loss in a rotation mechanism using a superconducting magnetic bearing (SMB), designed for the polarimeter of a cosmic microwave background polarization experiment. The design specification aims to achieve mW-level heat dissipation at 5 K with a rotation speed of 46 rpm. We estimated the heat dissipation from the SMB-based rotation mechanism, constructed a loss-mechanism model, and narrowed down the causes of the loss. The results indicate that the loss is predominantly caused by eddy currents in the drive mechanism.

To cope with various superconducting applications, the superconducting tapes in high-temperature superconducting (HTS) cables adopt different winding methods considering material properties and electromagnetic performance. AC or DC HTS power transmission cables used in large quantities are usually spirally wound, and the superconducting tape is stressed during the winding process. To evaluate the influence of electromagnetic factors and contact forces on HTS tapes in the cable generated by electromagnetic field and temperature variation, a theoretical analysis of the stress on the superconducting tape and HTS cable is performed. Based on the electromagnetic-thermal coupling finite element analysis model, MATLAB/COMSOL is used to jointly simulate the stress and strain conditions. The COMSOL electromagnetic-thermal model outputs electromagnetic and temperature parameters for stress and strain simulation. After mechanical simulation, the resulting changes in geometric parameters will be returned to the electromagnetic-thermal model. In this process, the critical current of tapes is affected by the magnetic field, temperature, and stress and is updated iteratively between models. The coupling model established in this study can be used to analyze mechanical performance of HTS cables under overcurrent and overtemperature conditions with different cases such as aircraft electrical power transmission, as well as to evaluate short-term safety and long-term reliability in the future.

In recent years, superconducting cables have been extensively studied for their characteristics of higher power transmission capacity, lower loss, and lighter weight compared with conventional power cables. With the development of aircraft electrification and environmental protection requirements, the advantages of superconducting cables and other superconducting devices are gradually manifested in all-electric aircraft applications. To analyze the feasibility and safety of the superconducting cable under fault conditions, this article takes the aircraft power transmission as the application background and conducts numerical analysis based on the aircraft power transmission structure, high-temperature superconducting (HTS) cable structure, etc. The COMSOL model for analyzing the transient electrothermal stress of the cable is constructed, and the simulation under the fault current condition is carried out by coupling with the MATLAB model. The electrothermal performance of the HTS tapes and cables under different fault conditions is evaluated. According to the analysis results of the HTS cable in the transient state and recovery state considering the fault conditions, the parameters of the HTS cable are optimized taking into account other factors such as weight and cost, which are important for aircraft applications.

We have proposed a rotor cooling structure using dilute gas for a fully superconducting motor for aircraft application. Numerical analysis was performed to investigate the feasibility of the proposed cooling method and structure, which can make a fully superconducting motor lighter and more compact than a partially superconducting motor. The helium gas pressure in the gap, the rotor rotational speed, and the rotor loss are key parameters of the cooling, and thermal analysis was performed to clarify the dependence of the field winding temperature on these parameters. In the analysis, the motor model has the maximum output of 5.5 MW and the maximum speed of 5000 rpm. The superconducting armature windings are cooled down to 20 K by conduction cooling with liquid hydrogen, and the superconducting field winding is cooled down to about 50 K indirectly using helium gas in the gap. Numerical analysis results indicate that the helium gas pressure should be increased when the rotor loss is larger, but if the pressure is increased too much, the temperature of the field winding becomes higher due to wind loss. When the rotor loss is several hundred watts, the helium gas pressure should be in the range of 10⁴ to 10⁵ Pa.

The International Energy Agency's High Temperature Superconductivity Technology Collaboration Program (IEA HTS TCP) analyzed energy delivery applications for HTS. The product was a Readiness Map that illustrates the Technology Readiness Levels (TRL) over time of HTS applications. Assessing these TRL levels is important since the electric power system that supplies our residential, commercial and industrial sectors is experiencing a transition to safer, more efficient and cleaner power supply. The international experts assessed the present degree of technological development of the main transmission, substation and distribution HTS applications in the energy delivery sector and estimated the pathway to commercialization phase. Instead of using a specific TRL number 1–9 for each of the applications, they are categorized into low, medium and high TRL levels. Superconducting fault current limiters and medium voltage AC cables to interconnect substations at the low side of the transformer are currently at a high TRL as they can be purchased in the market. High voltage AC and DC cables are at a medium TRL and could reach a high level TRL level in 2030–2035. Superconducting transformers are at a low TRL and they could reach a high TRL level by 2030–2035.