Propulsion motor concepts for airplanes

Abstract

Compact, lightweight, highly efficient motors are being sought for airplane electric propulsion. Superconducting motors rated 3 MW, 4500 RPM have been identified as possible candidate propulsion systems for commercial aircraft. Based on a preliminary parametric design analysis, this paper discusses three types of superconducting motors for such applications; • Synchronous motors employing superconducting field winding on the rotor • Induction motors employing superconducting squirrel cage winding • Homopolar AC synchronous motors employing superconducting field winding The performance characteristics and component challenges for each type of machine are discussed. Parameters included in the comparison are size, mass and efficiency. It appears that the superconducting wound rotor motors have characteristics of highest power density and efficiency but have significant challenges in supplying coolant and electric power to the rotating field winding. An induction motor employing superconducting squirrel cage windings only requires cooling of the rotor i.e. no current needs to be transferred to the rotor. The squirrel cage is expected to be carried on the rotor in a copper cylinder which is non-magnetic in nature. Thus, there is high reluctance between the stator winding and the squirrel cage. The larger reluctance considerably weakens the magnetic coupling between the two, which makes the machine bulkier and less efficient than the synchronous machine. It might be possible to build the induction motor with a superconducting stator, but that would require a large amount of superconductor. Lastly, the AC homopolar machine is expected to be less compact and power-dense than the synchronous machine but has the advantage of carrying all windings on the stator which are easy to cool, thus eliminating challenges of supplying power or coolant to the rotor.