How Fast Does a Wind Turbine Spin (and Why Does it Matter?)

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When most people look at wind farms, they often remark that the wind turbines don't seem to be doing much and wonder how they can actually generate electrical energy from so little movement.

So how fast does a wind turbine spin, and how do they actually work?

The quick answer to the first part of this question is:

  • Small wind turbines spin at up to 100mph
  • Large turbines spin at up to 180mph

But as you can probably guess, it's a bit more complicated than that!

There are a few factors that determine how fast wind turbines spin, and today I'm going to explain these to you.

Measuring a Wind Turbine's Speed

When considering the question of how fast do wind turbines spin, it is important to note that there are two ways in which the rotation speed can be measured.

  • RPM (revolutions per minute) is the number of times that a wind turbine's blades complete an entire circle within one minute.
  • Tip speed is the speed at which the tip of the blade is actually moving. The blade tip speed is higher than the speed at the center of the blade because it has to travel a greater distance to complete a full circle.

Therefore, a higher RPM does not necessarily indicate a higher tip speed: this depends on the size of the blades.

What Makes the Blades of a Wind Turbine Rotate?

wind turbine on beach

Wind turbines take kinetic energy from the wind and convert it into electricity. The blades of a wind turbine are what make this possible, as they are what catch the wind and cause the turbine to rotate.

The blades will only rotate once the wind reaches the minimum wind speed that is required to turn them. Known as the "cut in speed," this varies according to the turbine but is generally between 6 and 10 mph.

There is also a maximum speed or "cut-out speed" which, when reached, causes the turbine to shut off automatically to prevent damage to the rotor. For most wind turbines, the maximum wind speed is around 55mph.

When the wind passes through the turbine, it causes the rotor (a large wheel to which the blades are attached) to spin faster. This is because the wind turns the blades, which in turn spin the rotor.

A shaft connects the rotor to a gearbox, which in turn transfers the energy to an electrical generator. This can then transfer the electricity to the grid, or (in the case of a home turbine) your home's battery.

What Factors Affect How Fast a Wind Turbine Spins?

Wind turbines are a great way to generate renewable energy, and statistics show they are an increasing part of the global energy solution. But how fast they spin depends on a variety of factors.

The speed of the wind, the size of the turbine, and the design of the blades all play a role in how fast a wind turbine can spin.

Wind Speed

The most important factor in determining the speed of a wind turbine is the speed of the wind itself. The faster the wind blows, the faster the turbine will spin. In general, turbines can operate at speeds ranging from 6-55 mph.

Size of Turbine

The size of the turbine also plays a role in the speed at which it can spin.

Smaller turbines will have a higher RPM and it may appear that they are spinning faster, but the blades of larger wind turbines spin through a much larger circumference and therefore have a higher blade tip speed.

Air Density

Another factor that can affect the speed of a wind turbine is the density of the air. When the air is denser, it can apply more pressure to the blades of the turbine, which results in a higher power output.

Blade Size and Design

The size and design of the blades on a turbine can also affect its speed. Larger blades can capture more wind, while blades that are designed to be more aerodynamic can spin faster.

modern wind turbine blades

The Tip Speed Ratio (TSR) of Wind Turbines

The Tip Speed Ratio (TSR) is the ratio between the rotational speed of the wind turbine blades and the linear speed of the wind.

A wind turbine with a TSR of 6 would have blades that rotate at 6 times the linear speed of the wind.

The TSR is an important parameter in determining how much power a wind turbine can extract from the wind.

The power output of a wind turbine is proportional to the cube of the wind speed, so doubling the wind speed will increase the output eight-fold.

However, the TSR also has an effect on the efficiency of the turbine, and too high or too low a TSR can decrease the amount of electricity that can be gained from a wind power installation.

What is the Ideal Tip Speed Ratio?

The optimum TSR for a wind turbine depends on the design of the turbine and the wind conditions at the site.

In general, horizontal-axis wind turbines have a TSR that is between 2 and 6, while vertical-axis turbines have a TSR that is between 1 and 3.

Some of the largest wind turbines in the world have a TSR of 6.5. These turbines are designed for use in very high wind speeds, such as those found offshore.

The rotational speed of the blades is usually between 30 and 60 revolutions per minute (rpm). This means that the tips of the blades are moving at between 1800 and 3600 rpm.

The blades of a typical wind turbine are about 50 meters in length, so the tips of the blades are travelling at around 100 to 200 m/s.

The TSR of a wind turbine can be increased by increasing the rotational speed of the blades or by decreasing the length of the blades.

However, there are limits to how fast the blades can rotate and how short the blades can be.

Increasing the rotational speed beyond 60 rpm can cause the turbine to become unstable, and decreasing the blade length too much will decrease the power output.

How is Turbine Speed Calculated?

To calculate how fast the blades on a wind turbine spin, you first need to know how far they travel in a single revolution.

You may remember from your school days that the formula for this is 2∏r.

In this case r, the radius of the circle is equal to the length of the wind turbine blade.

So a typical modern wind turbine with 170ft (52m) blades would have a turning distance of (170 x ∏ x 2) = 1068.14 ft or (52 x ∏ x 2) = 326.73m.

Next, you need to know how long it takes for the blade tip to travel through one complete revolution. Let's say in our example it takes 4.5 seconds.

Therefore the tip speed would be 1068.14 / 4.5 = 237.36 ft per second.

Convert this into miles per hour using the converter here and you can see that in this (fictional example) the wind turbine spin speed is 161.83 mph.

How to Calculate Average Wind Turbine Speed

Most wind turbines have a rotor that spins at around 20 to 30 revolutions per minute (rpm) when the wind is blowing at its rated speed. But the average speed of the rotor can be much lower than this if the wind speed isn't constant.

To calculate the turbine's average speed, you need to know how long it took for the rotor to make one revolution and then divide that by the time interval you're using.

For example, if it took the rotor 2 minutes to make one complete revolution and you're using a 60-minute interval, the average speed would be 30 rpm.

To calculate the wind speed at which the turbine is spinning at its rated speed, you need to know the rotor's diameter and the wind speed.

Use an anemometer to measure the wind speed and then multiply it by two. So, if the rotor is 10 meters in diameter and the wind speed is 10 meters per second, the turbine would be spinning at its rated speed of 20 rpm.

The Final Word

It's a common misconception that wind turbines spin very slowly, but in actual fact, this is a common optical illusion due to their size, and in reality, they are moving faster than most people think.

However, faster wind speeds can sometimes make a wind turbine spin too quickly and when it exceeds its maximum wind speeds a turbine may need to shut down in order to prevent damage.

For maximum efficiency, wind energy is best harnessed when the wind speed is constant, but the reality is that it will always vary to some degree.

For that reason, it is important to be able to calculate the rotation speed, and thus the electrical output, at any given time, to ensure that it is enough to meet demand.

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With her Master of Science in Renewable Energy Engineering (MSREE) from Oregon Tech, Steph is supremely well qualified to write on all aspects of renewable energy. She has already achieved a zero carbon footprint and her goal is to help as many other people as possible do the same. Her other hobbies include music, yoga, swimming and horror movies.
Stephanie Cole
With her Master of Science in Renewable Energy Engineering (MSREE) from Oregon Tech, Steph is supremely well qualified to write on all aspects of renewable energy. She has already achieved a zero carbon footprint and her goal is to help as many other people as possible do the same. Her other hobbies include music, yoga, swimming and horror movies.

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