Development of a Ring-type Flywheel Battery System for Storage of Renewable Energy

In this study, a small scale prototype of a ring-type flywheel battery system is designed and fabricated. The system consists of 5-DOF magnetic bearing and a built-in motor/generator. The magnetic bearing is of hybrid type, with passive axial magnetic bearing and active radial magnetic bearing. For the passive axial magnetic bearing, a pair of Halbach arrays of permanent magnets are arranged vertically to support the weight of rotor and flywheel. For the active radial magnetic bearing, a set of Halbach array is placed on the rotor side, which will correspond to a coil set on the stator side. The active magnetic bearing can produce both attraction and repulsion forces on the radial direction, depending on the direction of the coil currents. When driven by an active feedback control system operating in conjunction with a position sensor, the actuator force can be used to balance the lateral forces coupled from the passive axial magnetic bearing and to achieve stable levitation. In the radial magnetic bearing, the design of eight-pole and differential winding mode are applied, which can result in a linear force-current relation. Hence, the system can be modeled by a simple linear time-invariant system. For robustness, the controller is designed by integral sliding mode control (ISMC) to overcome the effects of uncertainty and to achieve good steady-state accuracy. Numerical simulation results verify the effectiveness of the controller. To further validate the efficiency of the proposed controller, the experiments of the prototype are also performed.