Reexamination of superconductive homopolar motors for propulsion

Superconducting homopolar motor concepts with accompanying auxiliary systems have been examined in a quick-look assessment for their impact on ship designs utilizing the Navy's Advanced Surface Ship Evaluation Tool (ASSET) developed by the Naval Surface Warfare Center Carderock Division. An existing ASSET DDG51-FLT2A "like" ship model was used as a convenient means of evaluating the ship impact of the superconducting homopolar, and other advanced electric propulsion systems. For simplicity, all ship impact benefits due to increased power density and efficiency are taken in increased fuel capacity and thus, increased ship operating range. Previous studies have shown that superconducting homopolar motors are well suited to Navy propulsion systems due to their inherent low noise generation, high power density, and efficiency Homopolar motors are the only true direct current electrical motors and produce very smooth torque because they have no magnetic field pulsations or alternating currents. The Annapolis Laboratory of the Naval Surface Warfare Center is reexamining superconducting homopolar motor technology in light of the resurgent interest in a more electric Navy and because of significant advancements in two key homopolar component technologies. Extensive development of mechanical cryogenic refrigerators has resulted in liquid cryogen-free superconducting magnets. Magnet systems utilizing cryogen-free technology have been kept at superconducting temperatures for over two years (18,000 hrs.) with no maintenance. Development of dry copper-fiber and multiple-foil copper brushes has eliminated the need for liquid metal current collectors used in early homopolar motors. Basic wear measurements imply replacement times of tens of thousands of full-power motor hours. Superconductive homopolar motor designs utilizing the new solid collectors are comparable in size to those for previous designs, and show only small increases in machine size as voltage increases from 200V to 2,000V. As a result, the operating voltage of a homopolar machine can now be selected on an overall system basis in the same way as conventional alternating current systems. Advances in these key technologies and the promising results of the ship impact assessments indicate that a closer examination of superconducting homopolar motors for propulsion is warranted.