Coreless HTS synchronous generator operating at liquid nitrogen temperatures

In this paper we report on our new design of a liquid nitrogen/air cooled 100 KW synchronous generator with core-less rotor. This follows our successful completion of 100 KW generators with a 9 wt% Ni steel core operating at 77 K. In the new design, we demonstrate that a coreless rotor using commercial BSCCO tape is a realistic choice while maintaining the cooling at 57-77 K rather than 25-30 K and still achieving reasonable air-gap flux density. This is made possible by a combination of improved HTS wire technology and a careful optimization of HTS winding and flux diverters. The maximum flux density normal to the broad face of the tape is kept below 0.13 T, even when the air-gap density is raised to 0.5 T. The superconducting winding consists of 22 pancake coils with a total length of around 1250 meters of HTS tape. The required temperatures down to 57 K are achieved by a purpose-designed refrigeration system where liquid cryogen is circulated via a network of well insulated pipes and a stationery-rotating liquid coupling junction.