The importance of speed in Turbine’s measurements

previous Measurement of speed of rotation

The power output of a wind turbine is proportional the cube of the speed. The speed is the most important parameter in evaluating the power from a turbine. It is often understated by manufacturers or misunderstood by buyers that the power rating of a wind turbine is only true at a given speed, know as the rated speed, which is neither too low nor too high, but somewhere where it is trusted that the wind turbine will be able to withstand the forces of rotation. These speeds are distinguished into four different regimes (Figure 5.13):

1. Start-up Speed – This is the speed at which the rotor and blade assembly begins to rotate.
2. Cut-in Speed –
   Cut-in speed is the minimum wind speed at which the wind turbine will generate usable power. This wind speed is typically between 7 and 10 mph for most turbines.
3. Rated Speed –
   The rated speed is the minimum wind speed at which the wind turbine will generate its designated rated power. For example, a "10 kilowatt" wind turbine may not generate 10 kilowatts until wind speeds reach 25 mph. Rated speed for most machines is in the range of 25 to 35 mph. At wind speeds between cut-in and rated, the power output from a wind turbine increases as the wind increases. The output of most machines levels off above the rated speed. Most manufacturers provide graphs, called "power curves," showing how their wind turbine output varies with wind speed.
4. Cut-out Speed –
   At very high wind speeds, typically between 45 and 80 mph, most wind turbines cease power generation and shut down. The wind speed at which shut down occurs is called the cut-out speed, or sometimes the furling speed. Having a cut-out speed is a safety feature which protects the wind turbine from damage. Shut down may occur in one of several ways. In some machines an automatic brake is activated by a wind speed sensor. Some machines twist or "pitch" the blades to spill the wind. Still others use "spoilers," drag flaps mounted on the blades or the hub which are automatically activated by high rotor rpm's, or mechanically activated by a spring loaded device which turns the machine sideways to the wind stream. Normal wind turbine operation usually resumes when the wind drops back to a safe level.

Figure 5.13 Wind speeds for a wind turbine