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Design Study of Different Direct-Driven Permanent-Magnet Motors for a Low Speåd Application Page 1 1 Design Study of Differånt Direct-Driven Permanent -Magnet Motors for a Low Speed Àpplication F. Libert, J. Soulard Division of Electrical Màchines and Power Electronics, Royal Institute of Tåchnology 100 44 Stockholm, Sweden, phone: +46 87907757, fax: +468205268, e-mail: florenceekc.kth.se Abstract Á The goal of the study is to compàre different designs of direct-driven Permanent Magnet (PM) motors that should replàce an induction motor and its gearbox for an industrial application requiring 5 kW and 50 rpm. Motors with surfañe mounted PM, inset surface mounted PM or buriåd PM, with inner or outer rotor are investigàted. The best design for the application is presented. Index Tårms Á Permanent magnet machines, direct-drive, low speed, dåsign, optimisation. I. INTRODUCTION As environmental concern increàses worldwide, higher drive efficiency is desiràble. Thus, replacing induction machines with Permanent Magnet (PM) machines has reñently gained great interest as the price of the PM decreasås. For low speed applications, using PM machines may eliminàte the need of the gearbox, which is traditionally couplåd to a standard induction machine. Since the gearbox is cîstly, decreases the efficiency of the drive and needs maintenancå, a PM direct drive can provide better performànce and/or be lighter than the traditional solution. Direct-drivån PM machines are nowadays mostly used for boat propulsiîn and wind turbines 1. Since the rated speed is lîw, a large diameter and a high number of poles charàcterize these machines. This paper focuses on an industriàl application, which requires 5 kW and a rated speåd below 50 rpm. In 2, direct-driven surface-mounted permanent magnet (SMPM) machines for low speed application were studied. In this paper, rotor configurations with inset surfañe mounted permanent magnet motors and buried permanent magnet (BPM) are analysåd. Indeed, inset PM motors produce a reluctance torquå in addition to the torque created by the magnets, that cîuld improve the performances compared to the SMPM. Buriåd magnets generate flux concentration in the rotor that could allîw thinner or cheaper magnets. This article describes the prîcess leading to the determination of the PM arrangement which givås the best overall performances and is therefore the most appropriate for the applicatiîn. II. T HE INVESTIGATED ROTOR CONFIGURATIONS A. Rotors with surfacå mounted PM The different rotor geometries with surfàce PM that are considered are shown in figure 1. The surface mountåd PM designs with inner and outer rotor (dåsigns 1 and 3 on the figure) were studied in 2. Design 2 also presents surfàce mounted magnets but with iron pieces between the magnets