The NASA Glenn Research Center conducts aeronautics research on improving the fuel efficiency and reducing the noise and emissions of aircraft engines. Glenn researchers are considering the use of a hybrid drive with propulsors driven by electrical motors and primary power provided by a turbogenerator or fuel cell that could burn traditional fuel or hydrogen. A hybrid drive may be more efficient with lower emissions than a typical aircraft engine; however, the mass penalty is too high when standard electrical motors and drives are used.
Glenn’s Structural Mechanics and Dynamics branch has begun research in increasing the power-to-mass ratio (specific power) of several types of motors for consideration in different size categories of aircraft. The cryogenic switched-reluctance motor (cryo-SRM) described by Brown et al. (refs. 1 to 4) has experimentally demonstrated higher specific power than any other SRM we are aware of, most recently exceeding 7 hp/lb-EM (horsepower per pound of electromagnetic weight). The enhanced motor coil design and the increased current capability of the power electronics were the source of these improvements.
The cryo-SRM coils are designed to carry maximum current by increasing heat transfer through the use of a finned end-turn design. The coils are wound so that each layer of end turns forms a passage several wire diameters wide for cryogenic fluid to pass through; as the cryogen passes through the coil, it boils and carries heat away. This design allows an 18-gauge copper wire to carry approximately 70 Arms, which equates to a 100-A-peak square current waveform for the SRM motor. Initially our coil design was four layers thick with 80 turns, which provided 8000 A-turn capacity. Improvements in winding methods resulted in a progression to a six-layer, 18-turn coil with six parallel conductors. The improved coil has a capacity of 10,800 A turns, with 35 percent more copper and 1/20th of the inductance. These changes allow the motor to be run at higher field strengths and allow the current to be switched more quickly, resulting in a current waveform that is much closer to ideal (square). These improvements result in a higher generated torque at high shaft speeds and hence to an increased power-to-mass ratio.
Front view of “self-finned” coil for cryogenic switched-reluctance
motor stator showing a six-layer coil with space between the end turns.
The drive power electronics, switching at 20 kHz, were also enhanced. Initially a 2.5-kW pulse-width-modulated drive capable of 170 V and 15 A continuous was used on each coil. Several more iterations resulted in a pair of inverters specially configured to drive an SRM motor, a capacitive direct-current bus filter, and a custom analog control circuit to power each coil. This 126-kW system, switching at 20 kHz, is capable of 300 V and 420 A continuous. We have been unable to test the limits of this system because we have reached our dynamometer capability, but we are developing new test procedures to allow higher speed operation. Future work in the power electronics area is planned to support a second-generation SRM motor that is designed to operate at higher specific power than the present motor.
Last updated: November 26, 2007
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