Integrating Equation 5-4, we obtain:

Let us examine this aspect for an example rotor with the moment of inertia J = 7500 kg.m2. Changing this rotor speed from 100 to 95 revolutions per minute in five seconds requires ∆P of 800 kW. The resulting torque of 80 Nm would produce the torsional stress on the rotor structure and the hub components. If the same speed change is made in one second, the required power would be 4,000 kW, and the torque 400 Nm. Such high torque can overstress and damage the rotor parts or shorten the life significantly. For this reason, the acceleration and deceleration must be controlled within the design limits with adequate margins.

The strategy for controlling the speed of the wind turbine varies with the type of the electrical machine used, i.e., the induction machine, the synchronous machine or the DC machine.

5.8Environmental Aspects

5.8.1Audible Noise

The wind turbine is generally quiet. It poses no objectionable noise disturbance in the surrounding area. The wind turbine manufacturers generally supply the machine noise level data in dB versus the distance from the tower. A typical 600 kW machine noise level is shown in Figure 5-8. This machine produces 55 dBA noise at a 50-meter distance from the turbine and 40 dBA at a 250-meter distance. Table 5-2 compares the turbine noise level with other generally known noise levels. The table indicates that the turbine at a 50-meter distance produces noise no higher than the average factory. This

TABLE 5-2

Noise Level of Some Commonly Known

Sources Compared with Wind Turbine

© 1999 by CRC Press LLC