3000 rpm is the rotor speed of two-pole three-phase, 1500 rpm is the rotor speed of four-pole three-phase alternator, however rotor speed of Wind Turbine is always low, small power-size Wind Turbine turns a few hundred rpm at most; a few dozens of rpm is for middle and MW power-size Wind Turbine, which need a speed-increasing gearbox to take generator speed to rated speed. Figure 1 is showing the structure of horizontal axis Wind Turbine within speed-increasing gearbox.
Figure 1 Horizontal axis Wind Turbine within speed-increasing gearbox
It will reduce the efficiency of Wind Turbine using speed-increasing gearbox, gearbox is quick-wear equipment especially for the high speed gearbox and maintaining gearbox is difficult in nacelle of Wind Turbine and replacing gearbox regularly is a very expensive job. Direct driving generator through rotor system is feasible without gearbox, but low speed alternator must be used in Wind Turbine, which is called Direct driving Wind Turbine. Wind Turbine technology is developing from small power-size to huge power-size in recent years. 5MW permanent magnet direct drive Wind Turbine has been developed in China with independent intellectual property rights. It was reported that the largest Wind Turbine in foreign countries has reached 7MW, which is direct drive Wind Turbine.
Multi-pole structure is for low speed generator, Hydraulic turbine generator is multi-pole, at present direct driving Wind Turbine is mainly multi-pole including outer rotor structure and inner rotor structure, and it just needs to be a little lighter in structure.
High magnetic energy permanent magnet technology developed quickly, In particular, rare earth permanent magnet NdFeB (neodymium iron boron) is widely used in direct drive generator. The direct drive generator with permanent magnet technology will has simple structure and high efficiency. There are both main shaft structure and disk structure applied in permanent magnet direct drive Wind Turbine, the axial magnetic field direction of axial structure is radial air gap flux, which is divided into inner rotor and outer rotor; the magnetic field direction of the disk structure is axial air-gap flux, which is divided into intermediate rotor, intermediate stator and multi-disc. In recent years, new type of double salient pole generator and switch reluctance generator is also being applied in permanent magnet direct drive generator, new type of transverse stator flux permanent magnet generator is being developed and popularized.
Figure 2 below is a schematic diagram of an internal rotor direct drive Wind Turbine. This structure is similar to that of a hydro-generator, with three-phase winding on the stator, and the rotor is composed of several permanent magnets with a salient pole structure. For the specific structure, please refer to the courseware of “Inner rotor permanent magnet direct drive wind turbine ".
Figure 2 Inner rotor-direct drive wind generator
The inner stator winding of the external permanent magnet direct drive wind generator is similar to the ordinary three-phase alternator. On the outside of stator is rotor, the rotor is composed of several permanent magnets fixed on the inner circular surface of the outer magnetic yoke. The section "external direct drive Wind Turbine" is specially introduced. Figure 3 is a structural diagram of an external direct drive Wind Turbine.
Figure 3 External rotor permanent magnet direct drive wind turbine
For disc permanent magnet direct drive Wind Turbine, stator and rotor are both flat disc structure and arranged axially, it can be divided into intermediate rotor, intermediate stator and multi-disc. This column has a special introduction to intermediate rotor, intermediate stator of direct drive Wind Turbine. Figure 4 below is showing a structure of intermediate stator direct drive Wind Turbine.
Figure 4 disc permanent magnet direct drive wind turbine
Follow the video below to watch the spinning animation of the disc permanent magnet direct drive Wind Turbine.
3D animation of disc permanent magnet direct drive wind turbine
There is another semi direct drive Wind Turbine, the structure is similar to these above, just the number of poles is so much, but it still need to use a gearbox to increase rotor speed, Since the generators with fewer poles and the low-speed gearbox with less speed increase are more convenient to manufacture and maintain, and the cost is relatively low, the use of a semi-direct drive generator with a low-speed gearbox is also a compromise solution.
Direct drive and semi-direct drive are widely used in vertical shaft Wind Turbines, and disc type permanent magnet direct drive Wind Turbines are more widely used. Please visit the "straight drive vertical shaft wind generator" courseware.
Another important feature of the structure of the multipole permanent magnet direct drive Wind Turbine is that the number of slots in the windings is not an integer relation to the number of magnetic poles. Because the magnetic force line will has the shortest magnetic circuit when all the magnetic poles and number of slots are neatly corresponded, and the strong attraction between rotor and stator makes it very difficult to start the generator. Therefore, the starting resistance moment becomes an important parameter for the permanent magnet generator, and the design with fractional groove can reduce the starting resistance moment better. In addition, the design of fractional groove can reduce the number of linear groove under the same number of magnetic poles and manufacturing difficulty. How to select the number of poles and slots of permanent magnet generator is a new technology currently under study, and there are many schemes, which will not be introduced in this column. Integer slots are still used in the following structural examples of permanent magnet direct drive Wind Turbine.