Analysis of starting resistance moment of direct drive wind turbine

Permanent magnet motor in the process of running, the permanent magnets provided the magnetic field , the core will have a strong attraction, produce the cogging torque and torque ripple, vibration and noise and other disharmony factors, seriously affect the control precision of the motor system.Therefore, in the design of permanent magnet wind turbine generator, weaken the cogging torque is very important.


Preface
Wind energy is a kind of renewable energy,and it is also one of the most abund antenergy for human development and utilization.There are various ways people use wind energy, and modern society is the main way of using wind power, the wind energy is converted into easy to transport and control of power to use.Traditional doubly-fed induction generator excitation system is complex, with frequent gear transmission failure, maintenance difficulties, greatly limits the development of wind turbine applications.
The direct-drive permanent magnet wind turbine is not only remove the gear box that is easy to be damaged , also use permanent magnets to replace electric excitation, greatly improve the reliability of the system operation, efficiency is also increased.Direct-drive permanent magnet wind turbine, however, there are some technical problems need to solve.The permanent magnet make permanent magnet generator produce the cogging torque, the motor starting performance and running stability of the proposed the new challenge, as far as possible the extinction of the cogging torque, become a direct-drive major research topic of permanent magnet wind turbine .
1 The cogging torque generating mechanism and analysis method open core with a notch, regardless of whether or not with current, will produce a magnetic force.
Once the rotor starts to rotate, the magnetic force on the formation of torque, to maintain the original state, which is the physical embodiment of cogging torque.In the process of rotation of the rotor, permanent magnet track to the region week changes greatly, which led to the magnetic field energy changes, expand the influence of cogging torque.

1.2the cogging torque analysis method
Finite element analysis method Finite element method in electromagnetic calculation, have used the very mature, the development of commercial software such as Ansys, Ansoft has high accuracy.With finite element method can get the magnetic field distribution, the torque by the node to take.It is concluded that magnetic volume and surface force as: The magnetic fields of the Maxwell's tension The virtual displacement work as: For the NTH node forces: w ³ : (1.6) The effect of each node and, get the whole volume on the torque: u ¦ (1.7)

The effective methods to reduce the cogging torque
With the constant improvement of the permanent magnet material performance, remanent magnetism of magnetic steel is also attractive to alveolar, the greater the interaction between permanent magnet and slotted core, produce bigger the cogging torque.When the rotating permanent magnet direct-drive generator, produce torque ripple, vibration and noise, they will directly affect the permanent magnet direct drive wind turbine running stability and low speed startup performance.
In the design requirements through winding magnetic flux is changing, all dt d / ) everywhere is not zero.But the magnetic flux of the total rate of change is very small, is the best way to pole magnetic flux is invariable, the cogging torque are as small as possible.To this end, we use the following methods to weaken the cogging torque.

Slit width
Under the condition of wire to make notch become smaller, the tooth and the magnetic flux become as small as possible, if necessary, can use magnetic slot wedge.
As shown in figure 1 generator adopts chute, craft is not easy toachieve, and using inclined extremelyeasily, but the magnetic material is used up big, the price is higher.
Motor stator chute design is a common way to weaken the torque ripple, and after the chute motor as inaxial direction can be divided into multiple layers, each layer can be treated as a unit of motor, the cogging torque of each layer unit motor phasedue to the chute, added togetherto weaken the cogging torque peaks.
Because of the chute Angle is small, generally 0 ~ 5°, the cogging torque waveform trend does not produce big changes.Can be seen from the figure 5

Brief summary
Starting torque of permanent magnet direct-drive generator are analyzed, and the mechanism of the cogging torque is studied.Puts forward some methods to weaken the cogging

Figure 1 .
Figure 1.Different slit width the cogging torque curve peak change Figure 2 ~ 3 for CAE calculation results, the cogging torque cycle after the chute and waveform trend is almost the same, less peak.Zero cogging torque ripple when the value of 21.073 kNm, chute after 0.42 degrees torque ripple value stable in 6.637 kNm, width decrease fluctuation is bigger.The size of the wind turbine is bigger, this example USES the outer rotor of the generator stator outer diameter is 4310 mm, chute to weaken the torque ripple effect is obvious.

Figure 4 .
Figure 4.Different arc coefficient of the cogging torque curve peak change the cogging torque fluctuation decreased from 12.252 to 6.637KNm, reduced 47%;The motor structure design parameters of fine-tuning have played an important role in to weaken the cogging torque, effect is obvious.

Figure 5 .
Figure 5.Peak torque contrast before and after optimization To sum up, the local structure by adjusting the motor parameters can be found outside the wind turbine rotor cogging torque ripple minimum parameters combination, fundamentally change the motor cogging structure and size, eliminate or greatly weakened the cogging torque, so by fine-tuning the motor structure parameters is of great significance to the design of the wind turbine, improve the external rotor torque performance of wind turbines.
Owned by the authors, published by EDP Sciences, 201 1.1 The reason of cogging torqueThe cogging torque is the inherent DOI: 10.1051/ C

Table 1 .
84 pole 84 slot motor cogging torque characteristics of optimal parameters