How Fast Do Wind Turbines Spin?

  • By: Preetam
  • Date: July 20, 2021
  • Time to read: 5 min.

When you see wind turbines, they look like they are spinning at an incredibly slow rate. 

It’s not uncommon to wonder how they can generate any energy when they are moving so slowly. 

The fact is that the blades of the turbines are moving much faster than they appear.

Wind turbines will start to spin when the speed of the wind reaches about 7 miles per hour. 

The braking system of the turbine will start working when the speed of the wind reaches 50 miles per hour. This is to prevent damage as turbine blades have the tendency to spin too fast at high wind speeds.

The speed of the wind turbine will depend on the speed of the wind. Most turbines will turn at a rate of 10 to 20 reps per minute. 

This means that when wind speeds are 12 miles per hour, the blades will turn at a minimum of 120 miles per hour.

Rotations Per Minute

There are a couple of different ways that wind turbine speeds can be measured. One of the most common measurements is rotations per minute. 

Rotations occur as the blades complete a full circle around the axis of the wind turbine.

There are two ways that rotations per minute can be determined. 

First, with a timer and manually counting how many full rotations occur within thirty or sixty seconds. 

If you chose to only do thirty seconds, multiply the number of rotations by two to get the number of rotations per minute.

There is also a tachometer tool that will give you the most accurate results of finding rotations per minute. 

This is usually what professionals will use when they are trying to figure out how fast wind turbines are moving.

What Contributes to Rotations Per Minute?

There are several factors that contribute to how many rotations per minute of a wind turbine. 

This includes wind speed, air density, number of blades, and the size of the blades. 

Wind speed and air density make a difference in the pressure that the blades experience. 

The number of blades and the size of the blades contribute to the surface area that the pressure is exerted on.

Tip Speed Ratio

Another way to determine the speed of wind turbine blades is called Tip Speed Ratio. 

This helps determine how long it takes a blade to make a full rotation around. 

When you determine the Tip Speed Ratio, you will be able to figure out how fast the blades are spinning.

The Tip Speed Ratio can be easily determined by looking at the number of blades that are on the turbine. 

For example, a turbine with two blades will have an approximate Tip Speed Ratio of between six and seven seconds, three blades will be approximately five to six seconds, and five blades will be approximately two to three seconds.

How is the Speed Calculated?

There is a specific formula that is used to determine the speed that wind turbines spin. 

It is important to know how long the blades are and what the size of the circumference is at the tip of the blade.

To determine the circumference of the spin, you are going to multiple the length of the blade by two, then by pi. For a blade that is 120 feet long, the circumference is going to be 753.96 feet.

The next thing you need to know is how long it takes the blade to rotate that distance. 

For example, if it takes five seconds for the blade to rotate, it travels about 150.8 feet each second. 

After multiplying by the number necessary to turn into miles per hour (0.681818182), we get that the speed of this turbine is approximately 102 miles per hour.

What Makes the Blades Rotate?

In addition to the wind speed, there are several parts that all work together to rotate the blades of a wind turbine. There are the blades, the rotor, and the turbine itself. 

As wind passes through the blades, it causes them to spin. 

As the blades spin, it rotates the rotor. 

The rotor is connected to a gearbox that works to increase the velocity that the blades spin. It can increase from as little as 13 rotations per minute to as high as 1800 rotations per minute.

The gearbox transfers the energy produced by the spinning blades through the turbine to a generator. 

In the generator, the energy is transformed into electricity and distributed to surrounding power grids.

What Happens When Wind Speeds are Too High?

As mentioned above, when wind speeds reach a certain speed, the brakes will kick in. 

This helps control the blades of the turbine and prevents them from spinning too fast and out of control. There are two different braking methods used within wind turbines to protect the blades.

First, there is the automatic cut-off brake system. When the wind speeds are so incredibly high, the brakes will engage and completely stop the blades from rotating. 

This prevents anything dangerous from happening.

The second braking system is referred to as the feather braking system. 

These feather brakes fit to the outside edge of the blade and reduce any unwanted torque from high speeds.

As the wind speeds reduce, the brakes will unfeather and allow the blades to rotate as normal.

Summary

From a distance, wind turbines look like they are moving at snail speed. 

In reality, the blades can be rotating at over 100 miles per minute. 

It can be tricky to determine just how fast the blades are moving if you don’t know how to figure it out.

There are a couple of different ways to determine the speed at which wind turbine blades are moving.

Speed can be determined by figuring out the rotations per minute of the blades or the Tip Speed Ratio of the tips of the blades. 

The average speed of a wind turbine can reach as high as 180 miles per hour when the wind speeds are fast enough.

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