A flywheel system includes a rotor and a fixture. The rotor forms an aperture. The fixture includes a bottom support, a top support, and a shaft connecting the bottom support to the top support. The shaft passes through the aperture. The bottom support and the top support are outside opposite ends of the aperture. The rotor is configured to rotate about the shaft. A method for operating a flywheel system includes converting between rotational energy of a rotor and electrical energy in windings of a generator stator that is implemented in a stationary shaft passing through an aperture of the rotor, while the rotor is rotating about the shaft.