Field optimisation in a synchronous generator with high temperature superconducting field winding and magnetic core

A successful design of an electromechanical device depends crucially on the ability to predict its performance. In the case of a high temperature superconducting (HTS) synchronous generator two particular areas of concern can be identified associated with losses generated in a ´cold´ region and harmonic content of stator voltage. In this paper we consider the problem of voltage harmonics. As the superconducting machines have large air gaps, the synchronous reactance is low and consequently a small voltage harmonic can drive significant currents in the external circuit. The design of this machine is based on an existing stator, which has 48 slots and a balanced 2-pole, 3phase star-connected winding. Since the raw material of the 9% nickel steel is usually supplied in plates, it is necessary to use through bolts. However, these bolts may distort the field distribution. It is known that for a balanced 3-phase winding most harmonic voltages cancel and only fields of order 6n±1 remain to be considered. Thus, it is most important to carry out studies on the effect of the bolts and make sure that they do not increase harmonic voltages. The 2D model is used to analyse and evaluate the harmonic voltages without the bolts or their holes. The results are then compared with a 3D model which includes the bolts. Further 3D analysis is performed to optimise the design.