The NASA Glenn Research Center has been developing high-power-density motors (refs. 1 and 2) for possible use for NASA and aerospace missions. One design would use turbogenerators to develop electric power for motors that rotate a hydrogen-fueled aircraft’s propulsive fans or propellers. If hydrogen was carried as a liquid, it could provide essentially free refrigeration to cool electric motor windings before being used as fuel. Recently, we demonstrated improved performance of a switched-reluctance motor, stemming mainly from cryogenic operation and coil design, surpassing (we believe) previous specific torque and specific tangential force records for the motor type. We anticipate further increases in motor specific power from upgrades to electric power conditioning and coil windings.
However, cryogenic operation at higher rotational speeds markedly shortens the life of mechanical rolling element bearings. Even without cryogenics, conventional bearing life may be limited at the high speeds possible with switched-reluctance motors. Thus, a noncontact rotor-bearing system is a crucial technology for demonstrating the practical feasibility of using this high-power-density motor. During the last decade, a variety of bearingless motors were introduced, including permanent magnet, induction, and reluctance types. The switched-reluctance motor is a favored candidate for future airborne systems because it has inherent fault tolerance and has rotor robustness and reliability at high rotational speeds (no coil windings on the rotor).
Since the switched-reluctance motor has a doubly salient1 structure, an exact analytical expression for its plant model cannot be obtained. Numerous authors have addressed this problem with solutions using the Maxwell stress tensor, the finite-element method (FEM), a flux tube, and so forth. In 2001, Takemoto et al. published a successful controller demonstration of a 12-8 (12 poles in the stator and 8 poles in the rotor) bearingless switched-reluctance motor up to 2500 rpm (ref. 3). They developed a much simplified mathematical expression of the radial bearing force equation based on results of FEM analysis to express fringing fluxes and neglect magnetic saturation. However, they added a separate magnetic bearing coil winding to each stator pole motor winding for the rotor levitation.
Glenn’s Self-Levitation Team began by describing a model-based controller, mainly following the procedure developed by Takemoto et al., but modifying the controller somewhat on the basis of some three-dimensional finite-element results. The resulting controller was successfully demonstrated experimentally. Then, we demonstrated a much simpler observation-based proportional-derivative controller for levitation, which does not require any mathematical plant models and is advantageous at high motor speeds because it is less computationally intensive. However, the demonstration was at room temperature with the coil currents limited to the linear region to avoid magnetic saturation.
We ran the motor from 0 to 6500 rpm (maximum allowable speed at that time) with small rotor orbits. The graphs show the experimental rotor orbit (within the backup bearing clearance circle), the command signal from the controller, and the actual current applied to the pulse-width-modulated amplifier for each phase. The rotor was quite stable and stayed within less than 10 percent of the backup bearing clearance (±10 mils). The required levitation current was less than 10 percent of the motoring current.
Rotor orbit within backup bearing clearance circle, command signals, and actual currents of different coils. Top: 1000 rpm. Center: 2000 rpm. Bottom: 5000 rpm.
This technology could significantly reduce overall system weight and increase system reliability and specific net power, preparing the way for an all-electric, quiet, pollution-free aircraft propulsion system. This work is supported by the Noncombustion Based Propulsion Project.
Last updated: December 15, 2007
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