Gasch R., Twele J. (Eds.) Wind Power Plants: Fundamentals, Design, Construction and Operation

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3.1 Rotor

Return spring

Fig. 3-18 Wind turbine SÜDWIND 1237, downwind rotor with flapping hinge

Fig. 3-19 Three-dimensional wind velocity field in front of the wind turbine, [6] modified

3 Wind turbines - design and components

= 0

Fig. 3-20 Equilibrium of forces at the flapping hinge rotor

The flapping hinge reduces in the blade root the bending stress around the profile

chord where the section modulus is small due to the slender profile sections. This

way, the rotor blade weight may be reduced by up to 75%. At least for the design

point, rotors with a rigid hub may also benefit from the effect of the partial com-

pensation of thrust forces and the centrifugal forces, used at flapping hinge rotors.

A fixed flapping angle may be introduced, which is then called cone angle.

A design which was developed specifically for to the two-bladed rotor is the

teetering hub (Figs. 3-17 and 3-21). It reduces loads arising from spatial fluctua-

tions of the wind. The rotor shaft is primarily relieved of the corresponding bend-

ing stress. At the rotor blade root, only the dynamic portion of the flapwise bend-

ing moment is reduced. The design principle of the teetering hub has been applied

mainly for large downwind turbines, e.g. GROWIAN (Fig. 3-21) and WTS-3. Due

to the large size of the turbine, the atmospheric boundary layer led to a signifi-

cantly different wind speed at the top and bottom position of the blade thus chang-

ing flow conditions during the rotation. Due to the downwind position of the rotor,

3.1 Rotor

this asymmetry is even increased. This is primarily because, at its bottom position,

the blade passes through the velocity deficit of the tower wake.

The effect of the teetering hub even improves if coupled with blade pitching.

This was successfully done at the Maglarp wind turbine (WTS-3) and in the 1940s

at the Smith-Putnam turbine (Fig. 2-15).

The coupling of teetering and pitch movement - which is called G

-coupling in

accordance with the corresponding principle in helicopter design - is also used for

hubs of one-bladed rotors. Its design is similar to that of the teetering hub. How-

ever, the physical principle is a combination of flapping hub and pitch hub. Fig.

3-22 shows that it is relatively easy to build such a complex hub by using a car-

danic suspension. However, the cardanic suspension is imperative for this design,

due to its dynamic particularities [8, 9].

Allowing an additional degree of freedom in the connection between rotor

blade and hub has mainly the effect of stress reduction in the whole drive train, on

the one hand. On the other, in the case of wind turbines in the MW range, the re-

quired construction is costly and susceptible to faults due to the heavy rotor

weight and the big loads. Therefore, the concept of the rigid hub (Figs. 3-15 and

3-23) is preferred in practice, apart from some wind turbines for research and

some other prototypes. There are two types of mounting the blades to a rigid hub:

x fixed blade angle, i.e. no blade pitching (stall concept) and

x variable blade angle by pitching the rotor blades (pitch concept).

Fig. 3-21 Teetering hub of GROWIAN [26]

3 Wind turbines - design and components

Flapping

Pitching

Pitch

mechanism

Flapping

Pitching

Pitch

mechanism

Fig. 3-22 Flapping pitch hub with cardanic suspension of FLAIR [8]

Fig. 3-23 350 kW wind turbine with rigid hub and integrated drive train (Suzlon)

The weight of the hub increases significantly with the growing size of the wind

turbine. Due to their size, casting of the hub is limited to a few foundries. The

material is nodular graphite cast iron (e.g. EN-GJS-400-18-LT, formerly known as

GGG-40.3). A spherical hub has a comparatively small hub weight, but it is dis-

advantageous as far as stress-minimising design is concerned. Some hub designs

include an extender to reach larger higher rotor diameters with the same hub and

rotor blade.

3.1 Rotor

For hub design optimisation FEM software is used. This results in topologically

shaped hubs and achieves weight and stress reduction, cf. Fig. 9-9 in chapter 9.4.

By pitching the blade around its blade axis (Figs. 3-16, 3-17 and 3-22), the

blade pitch angle and consequently the angle of attack and the aerodynamic forces

are changed. Pitching influences the power output, as described in Figs. 3-6 and

3-7, and is therefore suitable for power limitation.

Pitching the blade in the direction of the feather position not only reduces the

driving forces but all forces at the rotor blade and therefore also the resulting

stress. By ‘pitching to feather’ the quasi static loads from the mean aerodynamic

forces are reduced at strong wind conditions and during storm. The construction of

the blade pitch system is described in the following section.

3.1.3 Blade pitch system

A significant effort is needed to build a rotor with a pitchable blade, regardless of

whether it is to be pitched to feather (reduction of lift force) or to flow separation,

i.e. stall (‘active stall’). The blade bearing and the pitch drive have to be designed;

moreover the blade has to be positioned precisely and firmly.

The blade bearing, Fig. 3-24, transfers the driving torque and the thrust as well

as the centrifugal force and the bending moment from the blade weight. Single or

double row four-point ball bearings are common.

The blade pitch system is driven by mechanical, hydraulic or electrical energy.

In normal operation, a synchronous pitching has to be assured, no matter which

drive concept is applied, else the aerodynamic forces become asymmetrically

causing an aerodynamic unbalance.

Fig. 3-24 Section of a blade pitch four point ball bearing (INA)