Satsuki Okumura

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.

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.