As a clean and renewable energy source, wind energy, with its gradually maturing technology, will have lower operating costs than hydropower and thermal power in the future, and its development prospects are very broad. Currently, with the continuous increase in the single-unit capacity of wind turbines, variable-speed constant-frequency and variable-pitch wind turbines are gradually becoming dominant. Gearboxes are currently the component with the highest overload and premature failure rates in MW-level wind turbine generator sets. Foreign countries have begun to develop a direct-drive type of wind turbine generator set (also known as a gearless wind turbine engine). This type of unit uses a multi-stage asynchronous motor directly connected to the impeller for drive, eliminating the traditional gearbox component (structural features are shown in the figure below). It has many advantages, including low noise, increased unit lifespan, reduced unit size, lower operating and maintenance costs, lower noise, and high efficiency at low wind speeds, and has great potential for future development in wind turbines [1-4].
In 1995, a research institution in New York State, USA, designed a new type of variable reluctance generator, in which the magnetic device in the wind turbine replaced the mechanical gearbox. The feature of this design is that the large number of poles has a winding structure that is cheaper than that of a 6-pole generator. The pole structure of the variable reluctance motor can withstand omnidirectional operation without increasing the cost [1].
In 2000, Dr. M.eng.M.Dubois of Canada proposed that placing the gearbox between the motor and the rotor in a wind turbine is detrimental to improving efficiency under certain workloads and is more susceptible to wear. He suggested that using a motor with the same speed as the wind turbine would eliminate the need for the gearbox. In fact, the application of direct-drive low-speed rotating motors in hydropower stations is not new. However, there are still some issues to be studied when applying them to wind turbines. For example, universities such as the University of Durham, Chalmers University of Technology in Goteborgan and Darmstadt University of Technology have studied the appropriate weight of motors in wind turbines; the selection of the most suitable model (synchronous, permanent magnet, variable reluctance, etc.); the maximum torque density caused by fluctuations in current and pressure; the selection of converters for grid connection; what measures to take to achieve the required noise level; whether it is possible to design a motor without additional losses when the current recommendation is to use a motor composed of multiple modules, which is convenient for transportation and can still operate normally when a component fails; permanent magnet motors are increasingly being used due to their high efficiency and high torque density. Will this lead to excessive iron loss? The selection of magnetic materials, how to magnetize these materials, and how to prevent demagnetization in the event of operation or failure [4].
In 1997, wind turbines featuring gearless operation and variable pitch appeared on the market. These advanced models with high capacity and low operating and maintenance costs included the E-33, E-48, and E-70, with capacities ranging from 330kW to 2MW. They were manufactured by ENERCONGmbH in Germany, and their production began in 1992 [4]. In 2000, ABB in Sweden successfully developed a 3MW giant variable speed wind turbine generator set, including the Windformer high-voltage wind turbine generator with a permanent magnet rotor structure, with a capacity of 3MW, a height of about 70m, and a fan diameter of about 90m [5, 6]. The MWT-S2000 wind turbine, which began operation at Okinawa Electric Power Company in 2003, was the first 2MW wind turbine completely manufactured by Mitsubishi Heavy Industries in Japan. It adopted a small-sized variable speed gearless permanent magnet synchronous motor and new lightweight blades [12]. The table below shows the relevant patents for direct-drive wind turbines applied for by some wind turbine manufacturers:
It can be seen that (direct-drive) gearless wind turbines originated more than 20 years ago, but in recent years they have once again aroused great interest among researchers, who are actively applying the technology to products and promoting them to the wind power market. Germany, the United States, and Denmark are among the leading countries in this field. Among them, the (direct-drive) gearless synchronous generator developed by Siemens of Germany has been installed in the world's largest wind farm in Norway, and its efficiency is said to reach 98% [15].
