Why do modern windmills only have three blades?
Answer:
The main reason was not yet given in any of the above answers.
It's the limit of the hub.
Each blade has to be attached. The attachment has to carry the four forces a) weight b) outward force due to rotation c) bending moment due to wind load d) torque = the force we really want to have.
So all this has to be strong and requires a good size.
Then each blade must have a mechanism to rotate the blade around its long axis to optimize the efficiency in various wind speeds. It even turns the blade to "feather" position to idle it in case of hurricane force winds.
It is not feasible to have enough space available to fit more than three such strong mechanisms into a small hub.
Large windmills cannot be efficient due to stress on the metal. Larger size requires fewer blades, while the number of blades determines efficiency.
Small windmills for homeowners could be multi blade and highly efficient, but three blade systems are used.
Efficiency is important for windmills, because the entire cost is in the technology, not the source of energy, which is wind.
The most significant factor determining the efficiency of windmills is the number of blades. This is because more blades capture more energy.
But large windmills must use few blades due to stress on the metal. When the diameter of rotation is 300 ft (100 m), two blade systems must be used. When the diameter is 150 ft, three blade systems can be used.
Horizontal, which has blades like airplane propellers.
Horizontal-axis wind machines are used the most. They make up 95 percent of all wind machines. A typical horizontal wind machine stands as tall as a 10-story building and has two or three blades that span 60 feet across. The largest wind machines in the world have blades longer than a football field! Wind machines stand tall and wide to capture more wind.
Both types of rotors are turned by air flowing over their wing shaped blades. Vertical axis blades lose energy as they turn out of the wind, while horizontal-axis blades work all the time. Also, at many sites, the wind increases as you go higher-above the ground, giving an advantage to tall horizontal-axis turbines. The small tower and ground mounted generators on vertical-axis turbines make them less costly and easier to maintain.
Like the other guy said, the more blades the better the energy capture from the wind. However the gain from more blades is very small and three is a good approximation to "many". You can even get single blade windmills obviously with a counterbalanced weight.
The main factor in efficiency is the size as the power gained is proportional to the square of the radius. This means if you double the radius you get four times the power!
Old windmills used "drag" blades while modern ones use aerodynamic lift. Drag is good for low speed with high torque which was great for milling corn. Lift blades are more efficient at capturing energy so those are used for electricity generating.
i am not sure but i think its because more blades need more power and they need to collect power not use it?
sorry if im wrong!
I have wondered this as well. It seems as if the tower is one limitation, from some of the above answers.
Is it a possibility that too many blades interfere with each other, by causing "used" wind to follow around with the blades? (even so, air is free, and I cant see the torque going down as you add blades).
If the vertical tower is the limit, why couldnt they add diagonal braces in the downwind direction?
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