Electric motors for aircraft must have very high specific power, and the most effective technique for raising specific power is cryogenic cooling. The NASA Glenn Research Center has been developing cryogenic switched-reluctance motor technology and analysis methods in-house for possible future application on aircraft that may utilize liquid hydrogen fuel. Motor coil current capacity measurements and locked-rotor torque measurements have been made to guide a series of upgrades in coil geometry and power electronics and to validate analysis methods that can be extended from our present liquid-nitrogen-cooled motors to liquid-hydrogen-cooled motors. Motorpower tests have also been made with a progression of coil configurations, power electronics capacities, and motor speed. These results were reported previously (refs. 1 to 3).
We reported in reference 3 that the specific power of a switched-reluctance motor was tripled to 3.5 hp/lb-EM (horsepower per pounds electromagnetic), or 5.8 kW/kg-EM, from the previous report (ref. 2). That specific power number was somewhat underreported; with appropriate drag corrections, it was 4.6 hp/lb-EM (7.6 kW/kg-EM) produced at 12,000 rpm. Improvements in the motor windings and power electronics upgrades (described in ref. 4) have increased the specific power to 7.1 hp/lb-EM (11.7 kW/kg-EM) at 19,000 rpm. This specific power is 50-percent higher than that produced by switched-reluctance motors operating at room temperature or above. The dynamometer-measured shaft-power output, projected to the full motor and corrected appropriately for bearing and fluid drag, was 126 hp (94 kW). The motor has an EM weight (i.e., the weight of the magnetic core laminations and the copper windings) of 18 lb (8.2 kg). With a factor of 1.5 to convert from an EM weight to a total weight, the specific power would be 4.7 hp/lb-total (7.7 kW/kg-total).
This testbed switched-reluctance motor with copper windings is operated in liquid nitrogen at current densities, specific torque, specific force, and specific power much higher than would be possible at room temperature. The stator and windings of the motor are shown in the photograph. The recent tests demonstrated the development of the magnetic forces and torques at higher speed than in previous tests, and the increased performance is the result of lower coil inductance, higher power electronics voltage and current capacity, and higher motor shaft speed. The low-speed specific torque of the motor reached 3.1 ft-lb/lb-EM(9.2 N-m/kg-EM), and the specific torque at maximum power was 2.1 ft-lb/lb-EM (6.3 N-m/kg-EM). These are at least twice the best specific torque numbers produced by switched-reluctance motors operating at room temperature or above.
Switched-reluctance motor stator showing a variety of coils with “self-finned” construction for enhanced heat rejection.
To reduce the cost of power electronics, we energized the coils on only 2 of the 12 stator poles for the high-power tests. The reported results are the projections of those test results to the entire motor. Electric motors are often capable of significant overloads for short times. We must perform further testing to assure that steady-state performance has been obtained.
Further improvements will involve careful equalization of the inductances and mutual inductances of paralleled coils on each pole, a further reduction in coil inductance, and still higher power electronics capacity. A further 50-percent increase in specific power is possible from these improvements, plus operation nearer the calculated safe rotor structural limit of 30,000 rpm.
Last updated: November 26, 2007
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