High speed PM motor with hybrid magnetic bearing for kinetic energy storage

This paper describes the design essentials of an external-rotor permanent magnet Halbach array motor with a hybrid magnetic bearing, intended for kinetic energy storage. The theoretical estimations are compared to finite element analysis as well as an approximated model using standard shape magnets. A dipole Balbach array produces a uniform flux distribution inside the cylindrical stator, where straight windings on an iron-less stator are placed near the inner boundary. The position of the windings inside this field does not affect the efficiency of the motor. When the motor is operated continuously, these variations become insignificant. The motor is suspended on a hybrid magnetic bearing without active control. Two radial repelling magnetic bearings are used in combination with an axial journal bearing. The theoretical values were found to be consistent to that of a working model. For high speed kinetic energy storage it is essential that the flywheel, which incorporates the motor and hybrid magnetic bearing, is operated in a vacuum. Under these conditions, the motor/bearing system approaches near 100% efficiencies e.g. the approximated Halbach array motor model showed an electrical to mechanical efficiency of 97%. The flywheel target storage capacity was 150 Wh with a minimum power rating of 40 W at a maximum operating speed of 48000 RPM. It is intended that multiple modules of this basic unit can be used in parallel to increase the overall energy storage capacity for rural and isolated power supplies.