Pollution-free flight is one of NASA’s goals for the 21st century. One method of approaching that goal is hydrogen-fueled aircraft that use fuel cells or turbogenerators to develop electric power to drive the electric motors that turn the aircraft’s propulsive fans or propellers. Hydrogen fuel would likely be carried as a liquid, stored in tanks at its boiling point of 20.5 K (-422.5 °F). The liquid hydrogen could provide essentially free refrigeration to cool electric motor windings before being used as fuel. Either superconductivity or the low resistance of pure copper or aluminum in liquid hydrogen could be applied to greatly increase electric current density and motor power density.
At the NASA Glenn Research Center, a testbed switched-reluctance motor with copper windings was operated in liquid nitrogen at current densities, specific torque, and specific force that would not be possible at room temperature. Coil current capacity measurements and locked-rotor torque measurements were made to guide upgrades in coil geometry and power electronics and to validate analysis methods that can be extended to liquid-hydrogen-cooled motors. These results are reported in references 1 and 2.
Changes in coil geometry and power electronics have since increased the measured torque and power output substantially. Because the phases (and even “half phases”) in a switched reluctance motor operate virtually independently of each other, Glenn researchers excite the windings on just one pole pair and project the measured results to the full-motor performance (12 coils) by multiplying the torque and power produced by 6. The previously reported best projected power output (ref. 2) was 15 kW (20 hp) for the motor, which has an electromagnetic weight of 18 lb. By increasing the drive voltage and current capacity of the power conditioning and rewinding the motor coils to match the new power electronics, Glenn researchers have more than tripled the power output to 47 kW (63 hp) at 12,000 rpm with the motor submerged in liquid nitrogen. The specific power now stands at 3.5 hp/lb-EM, the specific torque at 1.5 ft-lb/lb, and the specific force at 9 lb/lb-EM (all figures based on the electromagnetic weight (EM), as is typically done for switched-reluctance motors). By “specific force” we mean the effective specific circumferential force applied to the rotor surface, a basic measure of motor performance. (It avoids the misjudgment of the relative merits of motors of different radial size which, though possessing equally effective motive features, would have differing specific torques because the rotor radii differ.) Planned upgrades to the power electronics will have only minor effects on the specific torque and force, but they may nearly double the specific power by preserving the motor’s torque to higher speeds.
Last updated: October 11, 2006
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