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# How to calculate wind speed and rotor rpm

Updated April 17, 2017

Wind speed and revolutions per minute (RPM) are important factors in designing wind turbines for generating electricity. You need to know the wind speed and RPM to calculate the tip-speed ratio of the rotor blades, which must be optimal to maximise the power that the turbine extracts from the wind while rotating. An optimal tip-speed ratio ensures that the turbine spins at just the right speed relative to the wind speed. Slower rotation means that wind passes between the turbine blades without generating electricity, whereas faster rotation creates turbulence and obstructs the wind, reducing the efficiency of the turbine.

Use an anemometer to measure the wind speed. An anemometer is an instrument that has cups attached to an arm that spins as the cups catch the wind. A recording device attached to the anemometer calculates the wind speed based on how fast its arm spins.

Estimate RPM manually by trying to count the number of revolutions the turbine blades make within a five-minute period. You can mark or colour one of the blades to make it easier to count the revolutions. Use a stopwatch to keep track of the time. Divide the total number of revolutions you counted by five to get an estimate of RPM.

Either a tachometer or a bicycle computer offers the advantage of more accurate measurement of RPM. The disadvantage is that these options are more costly than the manual option, which does not require any high-tech equipment other than a stopwatch.

Two tachometer models you can use are the Hangar 9 and Extech Pocket tachometers. To use a bicycle computer, attach the magnet of the computer to the turbine rotor and the computer sensor to a stationary part of the turbine. As a bicycle computer usually gives the speed and distance covered by the bicycle as the output, and not the RPM of the bicycle wheel, you have to use a conversion factor to trick the computer into giving you the rotor RPM by entering the circumference of the wheel as 166.66 cm and setting the speed output display of the computer to kilometres per hour. Multiply the output of the computer by 10 to get the RPM of the wind turbine.

#### Things You'll Need

• Anemometer
• Tachometer
• Stopwatch
• Bicycle computer (optional)