Electromagnetic Forces in a Hybrid Magnetic-Bearing Switched-Reluctance Motor

We have performed analysis and experimental measurement of the electromagnetic force loads on the hybrid rotor in a novel hybrid magnetic-bearing switched-reluctance motor (MBSRM). An MBSRM has the combined characteristics of a switched-reluctance motor and a magnetic bearing. The MBSRM discussed in this paper has an eight-pole stator and a six-pole hybrid rotor, which is composed of circular and scalloped lamination segments. The hybrid rotor is levitated with only one set of four stator poles, while a second set of four stator poles imparts torque to the scalloped portion of the rotor, which is driven in a traditional switched reluctance manner by a processor. We performed static torque and radial force analysis of rotor poles oriented to achieve maximum and minimum radial force loads on the rotor. The objective was to assess whether simple one-dimensional magnetic circuit analysis is sufficient for preliminary evaluation of this machine, which may exhibit strong three-dimensional electromagnetic field behavior. We employed two magnetic circuit geometries, approximating the complex topology of the magnetic fields in and around the hybrid rotor, in formulating the electromagnetic radial force equations. The experimental and the theoretical radial force load predictions agreed reasonably well with typical magnetic bearing derating factors applied to the predictions.