Stability analysis of magnetically-levitated large flywheels in energy storage systems

This paper describes a comparison of stability of two large-capacity flywheel energy storage systems (FESSs) supported by active magnetic bearings. We designed and manufactured two systems that can store up to 5 kWh of usable energy at the maximum speed of 18,000 rpm. One of them is optimized to store as much energy per weight as possible, resulting in a flywheel with a strong gyroscopic coupling. The other design has a much smaller gyroscopic coupling for the ease of control. In order to analyze the stability of the systems accurately, we derived dynamic models of the flywheel rotor using finite-element method, and reduced-order models using modal truncation. The rotor model is combined with those of active magnetic bearings, amplifiers, and position sensors, resulting in a system simulation model. This simulation model is validated using experimental measurements. The stability of the system is checked from the pole locations of the closed-loop transfer functions. We also investigated the sensitivity function to quantify the robustness of the systems to the disturbances such as mass unbalance and sensor noises.