High-Frequency Air-Core Permanent-Magnet Motor
In response to global aviation’s growing fuel needs (over $100 billion per year) and carbon impact (780 million tons of CO2 per year), many commercial efforts have been launched in
the last few years to develop more efficient hybrid-electric airplanes. Studies show that commercial transport aircraft with electric propulsion can reduce the mission fuel burn by up to 70%, without compromising payload, range, or cruise speed. An important gap preventing these concepts to be realized is the availability of flight-weight motors and generators at the MW scale. A recent report released by the National Academies of Sciences, Engineering, and Medicine presented electrical system component requirements for a broad range of hybrid
and electric propulsion systems—motors and drives sub-systems at 6.5kW/kg. Today’s state of the art is about 1-2kW/kg. The high specific power (power-to-weight ratio) electrical machine and drive pursued at Illinois can significantly alter system level trade-offs of weight versus efficiency and improve the technical and commercial viability of these vehicles.
High specific power can be achieved using high frequency (up to 10X the current values) to minimize the motor’s magnetic circuit, thereby reducing the amount of heavy metal in its design. Adoption of litz wires for the high-frequency armature coils and elimination of stator teeth alleviate the electrical and magnetic losses associated with high frequency. Cover figures show a high-speed, high-frequency one-megawatt motor being developed by the CEME for NASA’s Advanced Air Transport Technology program. Energy-dense, rare-earth permanent magnets in the rotor arranged in a Halbach array reinforce the magnetic field at the air-gap and effectively cancel the field on the opposite side, eliminating the need for a rotor yoke. Carbon fiber, chosen for its high strength-to-weight ratio, is chosen to retain the magnet at high tip speeds. Furthermore, using outer-rotor topology permits proper thickness of the retaining ring without sacrificing the magnetic air gap between the magnet and the high-frequency coils. The resulting air-core topology allows the heat sink to be designed around the stator’s inner diameter. Together, with a ducted fan that pulls air in from the free end of the cantilevered design, the heat sink makes possible effective extraction of thermal losses from the high frequency coils. Several bench tests, including a full-speed rotor spin test, have been conducted and many risks, such as hotspot temperature and structural integrity, have been retired. The high power density, high specific power of the described motor provides advantages for the aerospace industry as well as for any applications with weight and space constraints, such as the oil and gas industry and renewable energy sector.