Why do wind turbines always have 3 blades?

Most have 3 blades because of bending moments caused by having an even number of blades--for example 2 blades. With 2 blades, when one blade is at the top of the cycle the opposite blade is at the bottom of the cycle. The top blade is receiving the greatest force of the wind and the bottom blade is in the shadow of the tower, which shelters it from the wind. This sets up a bending torque on the blades which wears out the bearings and also causes undue stresses.

Hey ?, Years ago, the Europeans built what today is still the most efficient turbine, it had one blade. On the opposite side of the hub was a bowling ball shaped counterweight to keep it in balance. They could generate power in very light wind, but had turbulence problems and didn't adjust well to shifting winds. The reason the single blade turbine is most efficient is because each subsequent blade has to fly through the wake formed by the previous one, so each blade generates a bit less than the first, but each blade creates the same amount of drag either way. Then the 2 bladed model became popular. It was almost as efficient as the single blade, but worked better in high winds, particularly gusty winds. It had the same problem with shifting winds. The 3 blade turbine solves all these problems, with yet a slightly larger loss in efficiency. They need a little bit more wind to start up, but can withstand tremendous loads, rapidly changing winds, and a great deal more turbulence than the one and two blade turbines. When a wind turbine is running, like any rotating mass, it has resistance to changing orientation in the revolving plane. If you take a frisbee, set it carefully on one fingertip, and use the other hand to spin it, it tends to stay on your fingertip. But then if you push up on one side of the frisbee, you can see the effect of gyroscopic precession. The frisbee tilts in unpredictable directions and becomes unstable. The problem with a one or two bladed turbine is that plane only exists when the blade is perpendicular to the tower. It has maximum resistance to changing direction for a moment, then when the blade is inline with the tower, it has minimum resistance. This happens very quickly, and in the end, the tower experiences tremendous vibrations and changing loads if the wind changes direction while the turbine is spinning. With a 3 blade unit, this problem goes away, because if any one blade is in line with the tower, the other two are not, and vice versa. So now when the wind shifts, the turbine can rotate smoothly to the correct direction to capture it. There is a loss in efficiency as you add blades, since each additional blade adds drag, but captures a smaller and smaller percent of the wind because it has been altered by the wake of the preceding blade. This, "Blade numbering," problem has been around for years, and the three bladed turbine is generally the best trade off point in cost of materials, efficiency, complication and maintenance. This is why virtually all commercially manufactured turbines today use 3 blades. If you were able to add blades, with the same diameter rotor and wind speed, you would actually be producing less electricity because of the added drag of the extra blades, and the fact that they create wakes that the other blades have to fly through. There are some good sources to look into on this subject, I will list some below. In particular, look for information by Mick Sagrillo, he is considered today to be the modern day guru on wind turbines. We had the privelege of going to one of his seminars years ago. Since then we've converted our home over to wind and solar power with the help of a wind turbine, it also is a 3 blade model, with a rotor diameter of about 6 feet, a little bit bigger than a ceiling fan. They are still making that same model today, and many more larger ones just like it. The engineers do have this worked down to a science, the only type turbines you'll see commercially made today wih more than 3 blades are smaller units designed for very extreme conditions, like an artic weather station, or a mountain top unit. In these cases, efficiency is not important at all, with all the high winds, there is more than enough energy available. What is more important is that they can withstand the conditions, that is the reason they add blades, for strength. I want to add that I am not a big fan of Wikipedia, they have interesting discussions on subjects, but don't always use what I consider to be reliable sources, but this discussion on blade numbering I've added the link to is pretty accurate. In my experience using and teaching wind power, I've learned that there are two things in vast supply, wind and misinformation. Check out the sources below, and try not to put too much investment in stuff you are getting from hacks like me online, anyone can post an answer here, even people who have never laid a hand on a wind turbine. Take care ?, Rudydoo

Wind turbines are being scaled up so fast no one has had time to decide if three is the best number or some other. (On small, fast rotating turbines three are needed for balance.) Efficiency is a complex issue and the shortcomings of the power curve of one turbine in a wind farm are made up for by the others present. Air is heavy and can be damaging from its weight alone. A space that is the size of a cube 30 feet on a side contains no less than one ton of air at standard conditions, making the force generated when in motion something fearsome. Blades, of course, can be much longer than 30 feet.

Blame it on the Danish Turbine manufacturers!

Usually three-bladed, sometimes two-bladed or even one-bladed (and counterbalanced), and pointed into the wind by computer-controlled motors. The rugged three-bladed turbine type has been championed by Danish turbine manufacturers. These have high tip speeds of up to 6x wind speed, high efficiency, and low torque ripple which contributes to good reliability. This is the type of turbine that is used commercially to produce electricity.

Current three-bladed upwind rigid designs, although well understood, may place limitations on loads reductions for future machines. A wide range of alternate design approaches have been proposed that alter one or many aspects of rotor configuration, including number of blades, downwind operations, teetering, flapping, flexing, and a myriad of system control and feedback approaches designed to reduce peak and fatigue loads

It provides the most efficent design. Wind turbines work best with a constant speed and three blades provides that efficency for them

Not enough wind for 4 blades.

Its just a matter of which design is the most efficient.

probably just reaches it's peak velocity quickly and more efficiently than 4.

3 blades optimise energy distribution.