Why do windmills typically have 4 blades, yet all modern wind turbines have 3?

Question

Why do windmills typically have 4 blades, yet all modern wind turbines have 3?

In: 9124

Answer 1

The old farm multi-blade wind mills for pumping water were designed for a high start up torque to get lift the column of water up the well pipe, with a bit of simplicity thrown in for the realities of maintenance and repairs in the rural field. That’s why they had a lot of “rotor solidity” – looking straight on at the rotor, the disc it sweeps had a large proportion of blade area to the total swept area. That gave them the high start up torque in low wind speeds.

The modern electrical generation wind turbine is designed for efficiently extracting as much as it can from the available wind, which means high torque at typical operating wind speeds. In reality though, there isn’t much power to be extracted at low wind speeds, since the power available is related to the square of the wind speed. If you have too many blades, you’re beginning to slow down the wind, which lowers the available power. Theoretically, a two bladed turbine would be more efficient, but a three bladed design isn’t that bad and has fewer balancing and stability issues.

See [this website](https://interestingengineering.com/the-scientific-reason-why-wind-turbines-have-3-blades) for a bit more discussion of the issues.

Answer 2

The old farm multi-blade wind mills for pumping water were designed for a high start up torque to get lift the column of water up the well pipe, with a bit of simplicity thrown in for the realities of maintenance and repairs in the rural field. That’s why they had a lot of “rotor solidity” – looking straight on at the rotor, the disc it sweeps had a large proportion of blade area to the total swept area. That gave them the high start up torque in low wind speeds.

The modern electrical generation wind turbine is designed for efficiently extracting as much as it can from the available wind, which means high torque at typical operating wind speeds. In reality though, there isn’t much power to be extracted at low wind speeds, since the power available is related to the square of the wind speed. If you have too many blades, you’re beginning to slow down the wind, which lowers the available power. Theoretically, a two bladed turbine would be more efficient, but a three bladed design isn’t that bad and has fewer balancing and stability issues.

See [this website](https://interestingengineering.com/the-scientific-reason-why-wind-turbines-have-3-blades) for a bit more discussion of the issues.