Abstract
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 5kWh 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.
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