A Low-Speed, High-Torque, Direct-Drive Permanent Magnet Generator For Wind lhrbines

There is a market for small, efficient and costeffective wind generators for mini-grid and remote power systems. Direct-drive permanent magnet generators have become very attractive for this application. This paper describes the improvements achieved in an outer-rotor direct-drive permanent magnet generator by using finite element analysis and optimisation techniques. The starting torque of the generator is studied. An optimisation routine for the design, including magnetic finite element analysis and lumpedparameter thermal model, is presented. A prototype for 20 kW, 211 rpm generator was built. The test results with a resistive load confirm the satisfactory operation of the generator. Compared with the previous prototype, the new design has lower mass, lower starting torque and improved efficiency. Traditionally wind turbine generators have used gearboxes and pitch control to allow constant high-speed generation under varying wind speed conditions. In recent years contemporary power electronics of high efficiency, high reliability and decreasing cost offers the option to change the power fi-equency out of the generator to match the system frequency, which leads to the idea of variable speed direct-drive generators. A number of alternative concepts have been proposed for direct-drive elecmcal generators for use in gridconnected or stand-alone wind turbines [1,2]. Compared to a conventional gearbox-coupled wind turbine generator, a direct-drive generator has reduced overall size, lower installation and maintenance cost, a flexible control method and quick response to wind fluctuations and load variation. A direct-drive generator must be light and efficient to minimise the requirements for the tower structure and to maximise elecmcal power extracted 6-om the wind. For small wind turbines, direct-drive permanent magnet generators have become very attractive because of their high efficiency, high power density and robust rotor structure. The attractiveness of direct-drive permanent magnet generators is further enhanced by improvements of permanent magnet characteristics and decrease of material prices. Some directdrive examples are Enercon (E12,30 kW), Proven (2.5 kW), LMW (2.5-10 kW) and Venco-Westwind (2.5-10 kW) [3]. A joint effort to develop a 20 kW low-speed, high-torque, directdrive permanent magnet generator for wind turbines was initiated by the University of Technology Sydney (UTS) and Commonwealth Scientific and Industrial Research Organisation (CSIRO) in conjunction with the Australian Cooperative Research Centre for Renewable Energy (ACRE) and Venco-Westwind. A nonoptimised, 48-pole, 170 rpm prototype was constructed by VencoWestwind earlier [4]. It features a radial-flux, slotted-stator topology with outer-rotor and surface-mounted Nd-Fe-B magnets, as shown in Fig. 1. The magnets are bonded to the inner surface of a steel drum that rotates around a stationary stator with conventional threephase windings. An advantage of this arrangement is that the centrifugal force of the rotating magnets applies a pressure to the bonding media, therefore increasing the reliability of the glued joint. Also the blades of the wind turbine are directly mounted on the fi-~nt surface of the outer-rotor drum, which leads to a simple assembly process, as shown in Figs. 15 and 16. This paper describes the improved design of the second prototype by using finite element analysis and computer search techniques. Section I1 considers starting torque. The analysis of the direct-drive permanent magnet generator is given in Section III. Section IV discusses the design optimisation and compares designs for several numbers of poles, and several lamination and magnet materials. In Section V, test results of the second prototype are compared with predicted performance. The conclusions are summarised in Section W. The starting torque of a permanent magnet generator is the total torque including the peak cogging torque, hysteresis torque, and the torque necessary to overcome the bearing and seal friction of the generator. Hysteresis torque arises 6-om the hysteresis loss of the generator. The cogging torque is a dorninent component, which'is inherently generated from the interaction of the magnets with the stator teeth. For a direct-drive wind generator, the starting torque is an important design issue because high starting torque prevents operation at cut-in wind speed. As a consequence, it is necessary to reduce the starting torque to acceptable values. Outer-rotor d ~ m Nd-FeB magnets 1 Stator laminalion Wmdings Fig. 1 Layout of the direct-drive permanent magnet generator 0-7803-6401 -5/001$10.00