The Rotor

30.1.1.1 The Rotor

The rotor is the part of the wind turbine that transforms the energy from the wind into mechanical energy [1]. The area swept by the rotor is the area that captures the energy from the wind. The parameter measuring the influence of the size of the capturing area is the ratio area/rated power. Thus, for the same installed power, more energy will be delivered if this ratio is greater, and so, more equivalent hours (kWh/kW). Values

for this ratio close to 2.2 m 2 /kW are found today in locations with high average wind speed (>7 m/s), but there is a trend to elevate this ratio above 2.5 m 2 /kW for certain locations of medium and low potential. In this case, the technical limits are the high tangential speed at the tip of the blade, that force to lower the speed of the rotors, hence the variable speed and the technology used are most important. Making a bigger rotor for

a certain wind turbine involves the possibility of using it for a lower wind speed location by compensating wind loss with a bigger capturing area. The rotor consists of a shaft, blades, and

a hub, which holds the fastening system of the blades to the shaft. The rotor and the gearbox form the so-called drive train.

A basic classification of the rotors is between constant pitch and variable-pitch machines, according to whether the type of tie of the blade to the hub is constant or whether it allows rotation to the rotor axis.

The pitch control of a wind turbine makes it possible to regulate energy extraction at high speed wind condition. On the other hand, the use of variable speed makes the systems more expensive to build and maintain.

FIGURE 30.2 Wind turbine in Monteahumada (Spain). Made S.A. The use of variable-speed generators (other than 50 Hz AE-41PV.

of the grid), allows the reduction of sudden load surges.

30 Wind Turbine Applications 793 This condition differentiates between constant speed and vari- order to adapt the generator frequency to the frequency of

able speed generators. The hub includes the blade pitch the grid [3, 6, 7]. The general principles applicable to change controller in case of variable pitch, and the hydraulic brake the speed of an induction generator can be deduced from the system in case of constant pitch. The axis to which the hub is following equation: tied to the so-called low speed shaft is usually hollow which allows for the hydraulic conduction for regulation of the power

·f 1

(30.1) by varying the blade pitch or by acting on the aerodynamic

p · (1 − s) brakes in case of constant pitch.

r =N 1 · (1 − s) =

where N r is the generator speed (rpm), N 1 is the generator synchronous speed, s is the induction generator slip, p is the

pole pair number, and f 1 is the excitation stator frequency (Hz). The function of the gearbox, shown in Fig. 30.1, is to adapt

30.1.1.2 The Gearbox

From Eq. (30.1), it is immediately inferred that the speed

a low rotation speed of the rotor axis to a higher one in the can be controlled in either ways; one way is changing the syn- electric generator [1, 2]. The gearbox may have parallel or chronous speed and the other is changing the slip. The speed planetary axis. It consists of a system of gears that connect is deduced from the number of pole pairs p and the sup-

the low speed shaft to the high speed shaft connected to the plying frequency into the machine f 1 . The slip can be easily electric generator by a coupler. In some cases, using multi-pole, changed when modifying the torque/slip characteristic curve. the gearbox is not necessary.

This modification can be achieved as follows: first, by chang- ing the input voltage of the generator; second, by changing the resistance of the rotor circuit; and third, by injecting a voltage