IEEE TRANSACTIONS ON MAGNETICS, VOL. 42, NO. 10, OCTOBER 2006 3461 Design and Analyses of a Coreless-Stator-Type Bearingless Motor/Generator for Clean Energy Generation and Storage Systems M. Ooshima , S. Kitazawa , A. Chiba , T. Fukao , Fellow, IEEE, and D. G. Dorrell Department of Electronic Systems Engineering, Tokyo University of Science, Suwa College, Nagano 391-0292, Japan Department of Electrical Engineering, Tokyo University of Science, Chiba 278-8510, Japan Department of Mechanical Systems Engineering, Musashi Institute of Technology, Tokyo 158-8557, Japan Department of Electronics and Electrical Engineering, University of Glasgow, Glasgow G12 8LT, U.K. This paper presents a design for a new bearingless motor/generator suitable for use in clean energy generation and storage systems. The machine is an outer-rotor type with a coreless stator. By using this type of bearingless drive in an energy generation or storage system, it is possible to obtain high efficiency, maintenance-free operation, and a long life cycle. In this paper, the rotational torque, back-emf, and suspension force are computed by the Finite Element Method (FEM). Through the analysis, a set of design parameters for the proposed machine is optimized (the rotor core thickness and the permanent magnet thickness) to meet the torque and suspension maximums. Index Terms—Bearingless motor/generator, coreless-stator, magnetic bearing, permanent-magnet (PM) synchronous motor/ generator. I. INTRODUCTION T HERE has been great progression in clean energy systems such as photovoltaic and wind energy as well as increasing interest in sea wave and tidal energy. Hydroelectric systems (es- pecially small-scale) are also being refocused upon. These sys- tems often require short-term energy storage, so the flywheel is now attracting attention as a possible storage system [1], [2]. These systems are often in remote or inaccessible locations and are required to be low maintenance or maintenance-free with a long life cycle, or they are close to domestic dwellings. In either case, they should have low vibration and acoustic noise. In ad- dition, for an effective flywheel, the mechanical bearing losses and windage losses have to be low. In this paper, the authors propose a bearingless drive system to satisfy the requirements above. The bearingless motor is being actively researched and developed around the world [3]. In this type of machine, the functions of the electric motor and magnetic rotor levitation are integrated so that the shaft is shorter and the number of inverters and controllers is less; hence, the cost is reduced. A two-stage outer-rotor type of bear- ingless motor/generator for flywheel energy storage has already been proposed by the authors [4]. Here permanent magnets were mounted on the inner surface of the outer rotor core. The motor (four-pole) and suspension (two-pole) winding distribu- tions were divided between the outer and inner surfaces of the stator core as air-gap windings. However, it was later concluded that the suspension force was insufficient to support the rotor using the topology proposed. It was found that the suspension force acting on the inner rotor was in the opposite direction of the suspension force acting on the outer rotor. In order to avoid Digital Object Identifier 10.1109/TMAG.2006.879071 Fig. 1. Cross-section of the proposed coreless-stator-type bearingless motor/generator. this, a new coreless-stator-type bearingless permanent-magnet (PM) motor/generator is proposed. The structure, winding arrangements, and principle of suspension are described in detail here. The rotational torque, back-emf, and suspension force are computed by Finite Element Method (FEM), and a set of design parameters (such as the rotor core thickness and PM thickness) is discussed and determined through analysis of the computed FEM results. The optimized design meets the torque and magnetic rotor levitation maximums. II. STRUCTURE AND PRINCIPLE OF THE PROPOSED BEARINGLESS MOTOR/GENERATOR Fig. 1 shows a cross-section of the proposed bearingless motor/generator. It consists of an outer-rotor core, with magnets mounted on the inner surface, and a coreless stator, with two sets of windings. The four-pole motor/generator windings are , and , and the two-pole suspension windings 0018-9464/$20.00 © 2006 IEEE