AC Loss Reduction in HTS Coils and Armature Windings of Fully Superconducting Motors
High AC loss in armature windings is one of the greatest barriers to achieve practical fully-HTS (high-temperature superconducting) motors. At present, AC loss estimation for HTS armature windings is one of the core technical challenges. AC loss behaviours and AC loss reduction methodologies have not been fully investigated in REBCO (REBa2Cu3O7-x, RE: rare earth) HTS armature windings carrying AC current under rotating magnetic fields. Many open questions need to be studied, of which this thesis addresses the three most prominent: can significant AC loss reduction be achieved by using low-loss MFDs (magnetic flux diverters); is it feasible to apply MFD structure to the HTS armature windings; and what kind of REBCO conductor types for armature windings generate less AC loss.
The goal of this thesis is applying experimental and numerical methods to investigate AC loss behaviours and AC loss reduction methodologies in the HTS windings wound with REBCO wires. Specifically, it includes investigating AC loss reduction in HTS coil assembly by using low-loss MFDs; building numerical models for fully-HTS motors; and developing methodologies to achieve AC loss reduction in the HTS armature windings.
We firstly explored the effectiveness of AC loss reduction on a REBCO four double-pancake coil assembly by using low-loss MFDs, MPP (Molypermalloy powder) MFDs and HF (High flux) MFDs. These have not been studied with HTS coils before. The measured and simulated results show that both MFDs can significantly reduce the total loss (the summation of the losses in the HTS windings and MFDs) in the system. At 77 K and 40 A coil current, compared with the coil assembly without MFDs, the total loss with HF and MPP MFDs is reduced by 77% and 81%, respectively. Although MPP MFDs show slightly better performance in reducing total loss at low coil current, HF MFDs are the more obvious option for AC loss reduction in high magnetic field applications owing to their relatively high saturation field.
Next, we developed numerical models to investigate the AC loss reduction in the armature windings of a 50 kW fully HTS motor by applying pole shoes, which function as MFDs. For comparison, two motor designs with and without pole shoes have been considered in the finite-element method (FEM) models. REBCO coated conductors were considered for both the armature and field windings, and 2D FEM models of both designs were built using the T-A formulation (T: current vector potential, A: magnetic vector potential) and rotating meshes. The simulation results show that flux diverting pole shoes reduced the perpendicular magnetic field in the armature windings and provided a 51% AC loss reduction, without weight penalty or motor power rating reduction.
Finally, AC loss simulation in HTS armature windings of a 100 kW HTS motor wound with different types of REBCO conductor arrangements were carried out. Three types of conductors, 4 mm-wide REBCO conductors, 14/2 (14 strands, each strand is 2 mm wide) REBCO Roebel cables and striated REBCO conductors with four 1 mm-wide filaments, were considered in the armature windings. The simulation results show that both Roebel cables or striated conductors can significantly reduce the AC loss in the armature windings compared with 4 mm-wide conductors. Further, a 2% AC loss reduction can be achieved in the armature winding wound with REBCO conductors with asymmetric Ic(B, θ) characteristics by simply flipping the direction of the conductors.
This work provides a method to achieve large AC loss reduction in HTS coil windings by using low-loss MFDs. Also, it supplies practical implications for designing armature windings of fully-HTS superconducting rotating machines. The methodologies of AC loss reduction in this thesis also can be applied to other HTS applications, such as transformers, and fast ramping magnets.