Planetary Backup Bearings for Flywheel Applications

High-speed flywheels can be used as kinetic energy storages for electrical energy. To increase the energy density of the flywheel, an outer rotor type flywheel can be utilized. The fly-wheels are supported by magnetic bearings and operated in vac-uum conditions to provide a high efficiency. Backup bearings are needed to support the rotor in case of a malfunction of the mag-netic bearings. However, since conventional backup bearings are not available for the high surface velocities above 200 m/s of the outer rotor type flywheels, a planetary backup bearing design was introduced. The present study investigates the experimental results of two test series conducted with a planetary backup bearing test rig. The rotor having similar properties to the outer rotor type fly-wheel was consecutively delevitated at rotational speeds of 1 – 20 krpm. In order to assess the severity of the delevitation events po-sition, rotational speed, force, and thermal data were evaluated. The advancing wear of the planetary backup bearing was as-sessed with multiple methods. During the first test series, the ro-tor was mostly relevitated a few seconds after the drop-down. The second test series consisted only of full run-downs and led to a bearing failure. The results suggest that the planetary backup bearing is fea-sible in the proposed application. Multiple full run-downs were possible with neither unwanted rotor-stator contact nor damage to the rotor, magnetic bearings or electric machine. The proposed indicators were found to be applicable in the planetary backup bearing life-time assessment. The future work includes the exper-imental investigation of the planetary backup bearing in a full-scale outer rotor type kinetic energy storage.