A Combination 5-DOF Active Magnetic Bearing for Energy Storage Flywheels

Conventional active magnetic bearing (AMB) systems use several separate radial and thrust bearings to provide a five-degree of freedom (DOF) levitation control. This article presents a novel combination 5-DOF AMB (C5AMB) designed for a shaft-less, hub-less, high-strength steel energy storage flywheel (SHFES), which achieves doubled energy density compared to prior technologies. As a single device, the C5AMB provides radial, axial, and tilting levitations simultaneously. In addition, it utilizes low-cost and more available materials to replace silicon steels and laminations, which results in reduced costs and more convenient assemblies. Apart from the unique structure and the use of low magnetic grade material, other design challenges include shared flux paths, large dimensions, and relatively small air gaps. The finite-element method (FEM) is too computationally intensive for early-stage analysis. An equivalent magnetic circuit method (EMCM) is developed for modeling and analysis. Nonlinear FEM is then used for detailed simulations. Both permanent magnets (PMs) and electromagnetic control currents provide the weight-balancing lifting force. During the full-scale prototype testing, the C5AMB successfully levitates a 5440-kg and 2-m-diameter flywheel at an air gap of 1.14 mm. Its current and position stiffnesses are verified experimentally.