Reliable, failsafe, robust, compact, low-cost electric motors are needed for applications where high temperature or intense temperature variations are the norm. Switched-reluctance motors have these characteristics.In addition, they are most often the natural choice of motors for operation under high rotational speeds (refs. 1 and 2). However, because of rotor eccentricity due to fabrication imperfections, conventional switched-reluctance motors suffer from vibration caused by large magnetic attraction forces on the rotor in radial directions (ref. 3). A solution to this problem is to suspend the rotor by magnetic bearings, where the vibration suppression capability of these bearings can be employed.
Methods for simultaneously levitating and rotating a switched-reluctance motor within a single stator are proposed in references 2 and 4. In these motors, the technique of differential stator windings was employed. The studies primarily used a main four-pole winding to rotate the rotor and a two-pole winding to apply radial force to the rotor, with all of the stator poles having both windings thereon. This device consisted of a 12-pole stator and an 8-pole rotor (12/8 configuration). Self-levitation (also called self-bearing) of motors has been achieved for nearly every type of electric motor, but it is very marginal in performance for switched-reluctance motors with low numbers of poles.
At the NASA Glenn Research Center, a novel eight-stator-pole, six-rotor-pole (8/6), hybrid bearingless switched reluctance motor, also known as the Morrison Motor, was developed to address the marginal performance of motors with few poles. The motor employs an alternate mode of operation in which robust, simultaneous levitation and rotation can be attained, not only for 18/12 or 12/8 stator-pole/rotor-pole configurations, but also for 8/6 and 6/4 configurations that employ a single set of coils positioned on each stator pole. The motoring techniques described in references 2 and 4 are not applicable to 8/6 and 6/4 motor configurations because, for many positions of the shaft during rotation, there are no rotor poles in appropriate positions to apply levitating forces. In contrast, the hybrid rotor technique ensures robust bearingless operation in all four of these configurations.
Disassembled eight-stator-pole, six-rotor-pole (8/6) hybrid rotor.
The successful operation of the Morrison Motor (see ref. 5 for a detailed description) hinges on the novel hybrid rotor design depicted in the photograph, in which a portion of the length of the rotor lamination stack is composed of circular laminations (used for levitation) and the remaining portion is composed of castellated laminations (used for motoring).
Inserting circular laminations onto the shaft ensures that levitating force is always present as the rotor spins. The recently formulated one-dimensional magnetic circuit equations, approximating the complex topology of the magnetic field existing in and around the hybrid rotor, were employed to generate the theoretical curve depicted in the graph. The experimental and theoretical curves represent the nonlevitated static radial force on the hybrid rotor.
Theoretical and experimental radial forces.
The prototype achieved a levitated rotor speed of 6000 rpm. Future software and hardware upgrades will engender higher rotational speeds. Before this demonstration, it was not clear whether or not circular and motor laminations could coexist without grossly distorting the magnetic flux distribution, and thus seriously degrading or destroying either the levitating or motoring action. A paper describing the levitated and nonlevitated magnetic field and force characteristics of the motor is in preparation.
Last updated: October 11, 2006
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