Permanent magnet synchronous generators (PMSGs) are widely used in small and medium-sized wind turbines due to their simple structure, lack of excitation winding, and high efficiency. With advancements in the manufacturing processes of high-performance permanent magnet materials, large-capacity wind power systems are also increasingly adopting PMSGs. PMSGs are typically used in variable-speed, constant-frequency wind power systems, where the turbine rotor is directly driven by the wind turbine, resulting in very low speeds. Eliminating the speed-increasing gearbox increases the reliability and lifespan of the unit. Utilizing numerous high-performance permanent magnets to form the poles, unlike electrically excited synchronous motors which require complex and bulky excitation windings, improves air gap magnetic flux density and power density, reducing the motor size for the same power rating.
Permanent magnet synchronous generators are structurally divided into external rotor and internal rotor types.
In a typical external rotor permanent magnet synchronous generator structure, the inner circumference of the outer rotor has magnetic poles made of high-energy-product permanent magnet materials, while the inner stator contains three-phase windings. The external rotor design allows for more space to accommodate the permanent magnet poles, and the centrifugal force during rotor rotation makes the magnetic poles more firmly fixed.
Because the rotor is directly exposed to the outside, its cooling conditions are relatively good. The problems with external rotors are the cooling of the stator, the main heat-generating component, and the transportation of large-sized motors.
An internal rotor permanent magnet synchronous generator consists of a rotor with permanent magnet poles that rotates with the wind turbine, and an external stator core. In addition to the advantages of conventional permanent magnet motors, the internal rotor permanent magnet synchronous generator can utilize natural wind conditions outside the frame, effectively improving the cooling conditions of the stator core and windings. The airflow generated by the rotor rotation also has a cooling effect on the stator. Furthermore, if the motor's outer diameter exceeds 4 meters, it often presents transportation difficulties. Many wind farms are designed in remote areas, and the journey from the motor's factory to the installation site may involve crossing bridges and culverts. If the motor's outer diameter is too large, it often cannot pass smoothly. The internal rotor structure reduces the motor's size, often facilitating transportation.
In internal rotor permanent magnet synchronous generators, there are four common types of rotor magnetic circuits: radial, tangential, and axial. Compared to other rotor magnetic circuit structures, the radial magnetization structure has a small leakage flux coefficient because the magnetic poles directly face the air gap, and its yoke is a single piece of magnetic conductor, making it easy to manufacture. Moreover, in the radial magnetization structure, the air gap magnetic flux density is close to the operating point magnetic flux density of the permanent magnet. Although it is not as large as the air gap magnetic flux density of the tangential structure, it is not too low either. Therefore, the radial structure has obvious advantages and is the rotor magnetic circuit structure most commonly used in the design of large wind turbine generators.
