Analysis of AC losses and current distribution in parallel-wound HTS coils for superconducting motor armature windings

Abstract

Abstract

Abstract High temperature superconducting (HTS) coils composed of parallel conductors are being widely investigated to enhance the critical current in superconducting applications. Most studies on parallel-conductor wound coils focus on no-insulation coils under DC conditions, particularly their charging and discharging behaviour. In contrast, the parallel-wound coil considered in this work targets AC applications, such as armature windings in superconducting machines, for which the current distribution and AC loss characteristics remain insufficiently understood. Meanwhile, due to the electrical coupling at the HTS conductor terminals, the current distribution becomes a key issue, which is essential for evaluating the current-carrying capacity and quench limits of the coil. Due to the limited research on this type of coil for AC applications, three numerical methods were first employed to validate the accuracy of the model calculations, namely the weak form T-A formulation with Neumann boundary conditions, the T-A formulation with electrical circuit, and the H formulation, respectively. The three methods show strong agreement. Subsequently, the current distribution and AC losses of inductance-imbalanced and inductance-balanced coils were systematically compared under different excitation conditions. It is found that inductance imbalance induces circulating currents between parallel stacks, resulting in asymmetric current distribution, enhanced AC losses, and increased risk of local overcurrent, particularly under time-varying magnetic fields. In contrast, the inductance-balanced coil maintains more stable electromagnetic behaviour, where both the current distribution and AC losses exhibit weak sensitivity to variations in the externally applied parallel magnetic field. We also compared the current distribution and AC losses of coils with different numbers of parallel conductors under different excitation conditions and clarified the effects of transport current and applied magnetic field on the losses. These findings demonstrate the importance of inductance balancing for the reliable and safe operation of HTS AC power devices.