TecKnow of the Day
DC homopolar machines are among the oldest electric machines. Michael Faraday made one in 1831. Superconducting DC homopolar machines use superconductors in their stationary field windings and normal conductors in their rotating pickup winding. In 2005 the General Atomics company received a contract for the creation of a large low speed superconducting homopolar motor for ship propulsion. Superconducting homopolar generators have been considered as pulsed power sources for laser weapon systems. However, homopolar machines have not been practical for most applications.
In the past, experimental AC synchronous superconducting machines were made with rotors using low-temperature metal superconductors that exhibit superconductivity when cooled with liquid helium. These worked, however the high cost of liquid helium cooling made them too expensive for most applications.
More recently AC synchronous superconducting machines have been made with ceramic rotor conductors that exhibit high-temperature superconductivity. These have liquid nitrogen cooled ceramic superconductors in their rotors. The ceramic superconductors are also called high-temperature or liquid-nitrogen-temperature superconductors. Because liquid nitrogen is relatively inexpensive and easier to handle, there is a greater interest in the ceramic superconductor machines than the liquid helium cooled metal superconductor machines.