How Fast Do Wind Turbines Spin?

white, and a elegant, from a distance wind turbines seem to rotate quite slowly; even gracefully. But if you get up close, you would see that they’re actually spinning pretty fast. That’s why the anti-wind crowd thinks they’re so dangerous to birds. The rotational speed of the turbine depends on the wind speed, air density, and the size of the blade. Engineers must tweak the aerodynamics and gear ratios of the blade to ensure they have the optimal tip speed ratio, or the ratio between the turbine’s rotational speed and the wind velocity. The tip speed ratio has an effect on the turbine’s efficiency, its structural integrity, and how much noise it makes.

Despite their seemingly slow speed from a distance, the rotors of a wind turbine may exceed speeds of 100 miles per hour during steady winds, with large turbines topping out at 180 miles per hour. The blade tip speed is directly tied to the wind speed and length of the blades.

Rotational Speed

The speed of a wind turbine’s rotation can be measured either in absolute velocity or in revolutions per minute (RPM). Wind turbines generally make between 10 and 20 revolutions per minute, depending on wind speed. Blade tip speed may differ depending on the size of the blades. Smaller blades may spin at 75 to 100 mph, while larger blades may easily top speeds of 150 mph.

Why Do They Spin So Fast?

Wind turbines seem to rotate slowly from a distance, so how are the blades spinning so quickly? Rotating objects move faster the further out from the center you go. Imagine if you were on a carousel. If you stood near the center, it would feel as if you were moving slowly, but as you walked out to the edge, you would feel as if you were moving faster. You reach the highest rotational speed at the edge of the carousel. This is called the orbital velocity, or the speed at which a certain point on a rotational object orbits around its center.

When measuring the speed of a rotating object, it’s more common to use the angular velocity, or the speed at which any given angle on the object moves through space. This is usually expressed as RPMs. But in the case of wind turbines, the orbital velocity of the blade tip, or the tip speed, is an important measure for engineers.

The same concept applies to wind turbines. The rotor hub doesn’t rotate very quickly, and the center of the rotor hub technically has a velocity of zero. The highest speeds are at the tip of the blades. Every point from the center radiating outward from the hub is in sync, though they travel at different velocities. The edge of the blade must travel further to make a full rotation, and so must move faster to stay in sync with the center. While the entire rotor has the same angular velocity, different points on the rotor plane have different orbital velocities. Using the blade length and the tip speed, one can calculate the speed at different points along the blade.

There are three factors that influence the rotational speed of the blade.

Wind Speed

The wind velocity has the biggest effect on the rotational speed of the rotor. After all, wind turbines are meant to rotate in response to the wind! Faster wind speeds mean faster rotation. The wind turbine begins to react, thus generating electricity, at wind speeds of around 6 miles per hour. They reach their maximum rated capacity at around 35 miles per hour. At this point, they don’t generate any extra electricity no matter how much faster the wind blows. They reach the cut-off point at 55 miles per hour, when the wind turbines shut down to prevent damage to the internal components.

Blade Length

Blade length has a direct effect on blade speed. Longer blades have higher tip speeds. They also capture more wind, so they capture more power. This power is translated into a higher rotational speed, and so, higher quantities of electricity are generated. They’re also heavier, so they carry more momentum. Think of a longer blade as a larger carousel. Since the diameter of the carousel is longer, there is a larger distance from the tip to the center, giving it more space to reach higher speeds at the edge. The tip of a large blade can reach a higher speed compared to a shorter blade spinning at the same angular velocity.

Air Density

Denser air carries more energy because there are more air particles per unit volume of wind. This gives the wind more mass, which translates into more power. Wind turbines in areas with dense air generate more electricity for the same wind speed. They also spin faster because the heavier air exerts more force on the blades.

Tip Speed Ratio

The ratio between the tip speed and the wind velocity is called the tip speed ratio. Commercial wind turbines have a tip speed ratio between 4 and 8. If the tip speed is 140 miles per hour with a wind velocity of 20 miles per hour, that results in a tip speed ratio of 7. If the tip speed is 75 miles per hour with a wind velocity of 15 miles per hour, that results in a tip speed ratio of 5.

The tip speed ratio factors into the efficiency of the turbine. If the rotor spins too slow for a given wind speed, then the turbine won’t generate power at its full potential, and may even stall out. If the rotor spins too quickly, then there will be a loss of efficiency as the blade will leave turbulence in its wake, which disturbs the aerodynamic efficiency of the next blade in succession. This effect is why turbines rarely have more than three blades, as the shorter distance between blades will cause too much turbulence. Engineers must carefully calculate the speed of rotation to balance efficiency with power output. 

High tip speed ratios can also cause stress on the turbine during strong winds, especially on the blades. The leading edge of the blade can become eroded by particulate matter like dirt and sand if it rotates too fast for too long. The high speeds can also lead to brake malfunctions and catastrophic failures, like the infamous Hornslet Incident in 2008.

Noise is also a factor, as the blades produce more sounds at higher speeds. This can be remedied with aerodynamic engineering. More aerodynamic blades can quietly cut through the air at a high speed.

Aerodynamics

The aerodynamic properties of a blade must be taken into account when tip speed and tip speed ratios are being calculated. Better crafted blades can begin rotating at lower wind speeds and move through the air more easily. They can also reach higher speeds much more safely than less aerodynamic blades. While lift and drag are the main aerodynamic forces affecting the blade speed, the torque, or rotational force of the blades also have an effect. Then there’s thrust, which is always trying to push the turbine over. 

At the center of the rotor hub, the local wind speed is similar to the actual wind speed and direction. Although, at the tip of the blade, the rotational speed has a bigger effect on the local wind speed and direction. Engineers must account for this phenomenon when they design the blades, as this can affect the efficiency of the turbine.

Conclusion

Don’t be fooled by the seemingly slow rotation of a wind turbine. Those blades pack a punch! Rotating objects reach higher speeds at their edges, and so the blades of a wind turbine may reach speeds of over 100 miles per hour at the tip, with the largest blades breaking 150 miles per hour on especially windy days. Longer blades have higher tip speeds, as the larger diameter gives the blade more room to reach higher speeds. Engineers actually design the turbines to rotate at a given speed depending on the wind velocity. This is called the tip speed ratio. This affects the turbine’s power generation, as well as several other factors. The blades are made of specialized materials, and are crafted to be aerodynamically efficient so they can more easily cut through the air.