Deflection of the wind rotor to the side wind when the wind is enhanced is the anti-strong wind method adopted by some small wind turbines. Figure 1 is a small wind turbine with an upward tilting head. The generator (including the wind rotor) is connected to the tail vane by the wind rotor upright component. The upright component is mounted on the deflection platform by a horizontal transverse axis. It is featured in that the tail vane beam is tilted upwards, and there is a tail vane at the tail end, and the tail vane is perpendicular to the ground or slightly inclined forward.

In the normal working wind, the tail vane only acts as a yaw (facing the wind). The generator rotates around the tower through the bearing under the yaw platform under the action of the tail vane to achieve the wind and the wind rotor runs at normal speed.

There are other ways to deflect the wind rotor: the tail vane is directly fixed on the yaw platform, and only acts on facing the wind. The upright horizontal axis of the generator (including the wind rotor) is below the generator and is installed on the yaw platform. The force of the spring causes the generator shaft to be in a horizontal state, which is also the state at normal wind speed. When the wind is strong, the wind rotor increases the thrust to a certain extent, overcoming the spring force, the wind rotor is inclined backwards, the effective wind receiving area is reduced, and the speed of the wind rotor is limited. After the wind is reduced, the wind rotor returns to the horizontal position under the action of the spring. The working process is similar to that of a wind turbine that depends on the tail vane.

The deflection axis of the generator can also be set on the side. Similar to the above structure, the generator shaft is parallel to the tail vane rod under the action of the spring, and the tail vane rod is fixed on the yaw platform to maintain the windward state. Facing the wind, in normal working condition. When the wind is strong, the wind rotor increases the thrust to a certain extent, overcoming the spring force, the wind rotor is inclined to the side, the effective wind receiving area is reduced, and the speed of the wind rotor is limited. After the wind is reduced, the wind rotor returns to the normal position under the action of the spring.

This method of adjusting the effective wind receiving area by the deflection of the wind rotor is simple and feasible, and is applied in small and micro wind turbine.

In fact, the wind turbine blade itself also has a certain speed regulation capability. When the wind speed increases, the blade enters the stall state to limit the speed increase. Many small wind turbines work in the fixed pitch with variable speed mode.

The blade is fixed on the hub. During normal operation, when the wind is low, the rotation speed is low, and the wind is high, the rotation speed is high. When the wind speed increases and the rotation speed exceeds the rated speed, the speed is limited by the stall to protect the generator. This operation mode is called fixed pitch with variable speed plus stall mode, which is also used in intermediate wind turbines. Let's analyze the basic principle of this stall control speed mode.

Figure 4 is a curve diagram of the lift coefficient and drag coefficient of an airfoil as a function of the angle of attack variation. It can be seen from the curve that the airfoil starts to stall when the angle of attack α is greater than 11 degrees, the lift suddenly drops, and the resistance rises sharply. The value of the angle of attack α of the stall is called the stall angle.

Figure 5 is a cross-sectional (airfoil) force analysis diagram of a wind turbine blade. The fixed angle β between the airfoil string and the rotating plane of the wind rotor is called the pitch angle. For fixed blade pitch angle is constant. The wind speed of the relative airfoil is the relative wind speed w synthesized by the external wind speed v and the airfoil line speed u, and the angle α between the relative wind speed w and the airfoil string is the angle of attack of the airfoil.

The left diagram of figure 5 shows the state of the airfoil operating within the allowable wind speed (rated wind speed). The angle of attack α of the airfoil should be less than the stall angle (11 degrees). The resultant force of the airfoil lift Fl and the resistance Fd is F1, and the projection F of F1 on the plane of rotation of the wind rotor is the force that pushes the airfoil motion. When the wind speed v is low, the angle of attack is small, then the F is small and the rotation speed is low; when the wind speed is high, the angle of attack is large, and the F is high.

When the wind speed exceeds the rated wind speed, the wind speed is unlikely to increase significantly due to the load. The airfoil enters the stall state after the angle of attack α is greater than 11 degrees. This figure is shown in the right figure of figure 2. Although the wind speed v is much increased, the lift Fl is decreased, and the resistance Fd is greatly increased. As a result, the F is reduced, thereby suppressing the increase in the rotational speed.

Stall is that the airfoil runs in an abnormal state and is unstable; and the stall angle is not constant. It is affected by changes in air humidity, temperature, etc., such as an increase in air humidity or a stall angle when dust is attached to the blade. It will decrease and the lift coefficient will decrease. In addition, the angle of attack before returning to the stall after the stall is not returned along the original curve, so the stall is impossible to control the speed stably, and the range of stall control is limited. In order to start the wind turbine in low wind, the blade should have a certain angle of attack, which will reduce the range of the blade tip speed ratio and affect the efficiency of the wind turbine.
The fixed pitch stall mode can prevent the rapid increase of the rotor speed within a certain range but does not have the ability to resist strong wind. To resist strong winds, the blades must have sufficient strength and the wind rotor has good braking performance. the wind can be effectively reduced to reduce the losses caused by strong winds. When strong wind comes, the side of wind rotor is facing the wind, brakes, which can reduce the loss that strong wind brings effectively.
However, the wind turbine operating in this mode has a simple structure and low cost is its advantage.

When the wind turbine with variable pitch adjustment speed function is lower than the rated wind speed, it works in the fixed pitch with variable speed state (it can also work in the pitch shifting state when necessary); when the wind speed exceeds the rated wind speed, it must work in the pitch constant speed state. Obviously, it is only possible to stabilize the speed of the wind turbine when the rated wind speed or the rated wind speed is exceeded.

Large and medium-sized wind turbines are mostly directly connected to the grid. To ensure that the generator speed is synchronized with the grid frequency, it is difficult to achieve stable wind turbine speed by pitching, unless the wind speed is stable (the wind speed changes slowly) area.

Nowadays, the variable speed pitch mode is popular. The so-called variable speed means that the generator starts to generate electricity at a lower wind speed than at a lower speed. The speed increases with the wind speed until the rated wind speed. At this time, although the stall state is not entered, Select the appropriate pitch angle to make the generator output the maximum power; when the wind speed exceeds the rated wind speed, increase the pitch angle to keep the speed stable near the rated speed, so that the output power is stable within a certain range. At this time, the output frequency and voltage can be the same as the grid frequency and voltage through the doubly-fed generator; or the output frequency and voltage can be the same as the grid frequency and voltage through the inverter, so as to achieve the so-called variable-speed constant-frequency operation mode. Therefore, the variable speed pitch mode allows the wind turbine to generate electricity from low wind to high wind, which greatly improves the efficiency of wind power generation.

The complete pitch adjustment system can adjust the blade to a downwind state when strong wind is encountered, that is, the main segment string of the blade is parallel to the axis of the wind rotor, which is called the full feather state. At this time, the windward area of the blade is the smallest, and the wind rotor is greatly reduced by wind. And the resistance of the blades allows the wind rotor to quickly stop (aerodynamic braking), effectively protecting the wind turbine.

The following describes a simple pitch angle adjustment method. Three pitch bushings are distributed on the wind rotor hub by 120 degrees. The roots of the three blades are mounted in the three pitch bushings via bearings and are free to rotate within the bushings; there are springs and limit devices in the bushing to make the blades of the wind rotor at normal working angle, see figure 7. The left side of the figure is the front view of the wind turbine, the right side is the perspective view of the wind turbine, the figure 8 is the view of the blade axis direction, and the left figure clearly shows the angle of the blade at this time.

When the rated wind speed is below, the centrifugal force of the centrifugal pendulum is not enough to overcome the spring force in the direction around the blade rotation axis, and the blade maintains the normal working pitch angle.

The centrifugal force pitch adjustment device is relatively simple, but it does not stop at high speed when the wind speed exceeds the cut-out wind speed. The blade angle locking mechanism needs to be locked in the feathering state when the wind is strong, so that the wind turbine can be stopped, and the wind turbine safety can be guaranteed. Centrifugal pitch adjustment devices can be used in small wind turbines.

In larger wind turbines, a uniform pitching mechanism or an independent pitching system is adopted, and the pitch is controlled by an electrically or a hydraulically driven pitch, and a real pitch control function can be realized by using a computer. Figure 10 is a picture of the electric pitch system.