A Superconductor Flywheel Energy Storage system (SFES) is used as an electro-mechanical battery which
transforms electrical energy into mechanical energy and vice versa. A 35 kWh SFES using hybrid bearing
sets was developed in KEPRI. Mechanical properties of HTS bearings are the critical factors for stability of
the flywheel and the main parameter in designing the system. The properties of the HTS bearing were
studied. The stiffness and the damping coefficient of the HTS bearing were 346.6 ± 6.7 kN/m and
1255 ± 102 N s/m, respectively. The damping coefficient was twelve times bigger than the estimate value.
Therefore, we concluded that there were some external damping mechanisms. Finally, the 35 kWh SFES
was test operated up to 4000 rpm. After passing two predicted critical speeds, the amplitude of vibration
decreased and reached a stable rotation state.