Development status of rotating machines employing superconducting field windings

Superconducting rotating machines have looked promising since multifilamentary niobium-titanium (NbTi) superconductors became available in the mid-1960s. Both dc homopolar and ac synchronous machines were successfully tested from the 1970s to the 1990s. Three different 70-MW generators were recently demonstrated by the SuperGM project in Japan. However, economic considerations with respect to competitive cost combined with the requirement for liquid helium cooling did not make these machines commercially attractive. On the other hand, high-temperature superconductors (HTSs) can operate at much higher temperatures (30-40 K), providing much larger thermal margin and simpler cooling systems. This refrigeration advantage has provided new impetus to the development of such machines for commercial applications. In the last few years, a number of superconducting rotating machines with HTS field windings have been demonstrated and several projects are currently transitioning to advanced development stages. HTS machines with ratings from a few kilowatts to several megawatts have been demonstrated in the United States and Europe. Currently, large high-torque ship propulsion motors, large generator prototypes, and synchronous condensers are under development and are expected to be commercially available in the next few years. Prospects for improved life cycle cost, smaller size, less weight, and higher efficiency benefits are providing incentives for the development of these larger rating HTS machines. This paper reviews the past and recent progress on the worldwide development of industrial-grade superconducting rotating machines utilizing lowtemperature superconductor and HTS field windings and provides an outlook on the benefits and opportunities of this new technology.