Finite difference modelling of bulk high temperature superconducting cylindrical hysteresis machines

A mathematical model of the critical state based on averaged fluxon motion has been implemented to solve for the current and field distributions insid e a high temperature superconducting hysteresis machine. The machine consis ts of a rotor made from a solid cylindrical single domain HTS placed in a p erpendicular rotating field. The solution technique uses the finite differe nce approximation for a two-dimensional domain, discretized in cylindrical polar co-ordinates. The torque generated or equivalently the hysteresis los s in such a machine has been investigated using the model. It was found tha t to maximize the efficiency, the field needs to penetrate the rotor such t hat B-0/mu(0)J(c)R = 0.56, where B-0 is the applied field amplitude, J(c) i s the critical current density and R is the rotor radius. This corresponds to a penetration that is 27% greater than that which reaches the centre of the rotor. An examination of the torque density distributions across the ro tor reveal that for situations where the field is less than optimal, a sign ificant increase in the performance can be achieved by removing an inner cy linder from the rotor.