DC and AC Characterization of Pancake Coils Made from Roebel-Assembled Coated Conductor Cable

TL;DR

This study experimentally and numerically characterizes Roebel cable pancake coils, revealing how winding tightness affects magnetic field strength and AC losses, aiding optimized coil design for HTS applications.

Contribution

It provides the first combined experimental and FEM analysis of DC and AC behavior of Roebel coil geometries, highlighting the impact of winding tightness on performance.

Findings

Tighter winding produces higher magnetic fields despite lower critical current.

AC losses are similar across coils for the same magnetic field, except in loosely wound coils.

Finite-element simulations support experimental results and assist in coil optimization.

Abstract

Roebel cables made of HTS coated conductors can carry high currents with a compact design and reduced AC losses. They are therefore good candidates for manufacturing coils for HTS applications such as motors and generators. In this paper we present the experimental DC and AC characterization of several coils assembled from a 5 meter long Roebel cable built at KIT, which differ in the number of turns and turn-to-turn spacing. Our experiments, supported by finite-element method (FEM) calculations, show that a more tightly wound Roebel coil, despite having a lower critical (and therefore operating) current, can produce a higher magnetic field than a loosely wound one. For a given magnetic field produced at the coil's center, all the coils have similar AC losses, with the exception of the most loosely wound one, which has much higher losses due to the relatively large current needed to produce the desired field. The experiments presented in this paper are carried out on the geometry of pancake coils made of Roebel cables, but they are exemplary of a more general strategy that, coupling experiments and numerical simulations, can be used to optimize the coil design with respect to different parameters, such as tape quantity, size, or AC loss, the relative importance of which is dictated by the specific application.