A switched-reluctance cryogenic motor (ref. 1) is being developed for pollution-free flight, which is one of NASA’s goals for the 21st century. Before a super-high-power-density motor can be developed for the next-generation all-electric propulsion system, a noncontact rotor-bearing system is necessary to circumvent the poor bearing life that ordinarily accompanies cryogenic operation.
Recently, a variety of bearingless motors--including permanent magnet, induction, and reluctance--were introduced. These motors have different characteristics and different suitable applications. Among them, switched-reluctance motors can be good candidates for the electric propulsion system because they have inherent fault-tolerance and rotor robustness at high rotational speeds (no coil windings on the rotor). In addition, this type of motor generates a high radial force in the air gap that we might be able to use for rotor levitation (bearing function).
At the NASA Glenn Research Center, a 12-stator-pole, 8-rotor-pole (12/8) switched-reluctance cryogenic motor (ref. 1) was studied for the bearingless motor. This motor does not have separate magnetic bearing coils, but only motor coils, as in a conventional motor configuration. The motor coil windings on the stator generate a radial force (bearing function) and torque (motor function) at the same time. On a stator with superimposed differential magnetic bearing coil windings on the motor coil windings, the radial force is a function of the radial force winding current and the motor current, as well as of the rotor rotational position.
Maxwell three-dimensional model of a 12/8 switched-reluctance motor in one phase.
In this work, the theoretical equations of radial forces were derived from finite-element model (FEM) analysis results to express fringing fluxes neglecting magnetic saturation. This was developed early by Takemoto et al. (ref. 2) and Shuang et al. (ref. 3). We used the fringing effects based on the Maxwell FEM model shown in the preceding illustration to derive the relationships between the radial force and current around unaligned positions. The radial equations developed using Takemoto’s method and Shuang’s method are compared with three-dimensional FEM results in the following graph.
Radial force equations developed using Takemoto’s method and Shuang’s method are compared with Maxwell three-dimensional FEM results. Radial force in the x-axis, Radial forcex, x-axis levitation current in phase a, Ima, 8 A; motor current in phase a, Isa1, 2 A; y-axis levitation current in phase a, Isa2, 2 A.
In the near future, we plan to develop a bearingless controller based on this radial force equation and to further investigate the radial force derivation in the magnetic saturation region. This work is supported by the Alternate Energy Foundation Technologies Project of the Low Emissions Alternative Power Project (Mark D. Klem, manager).
Last updated: October 11, 2006
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