What it shows
The number and angle of blades change how much electricity a small wind rotor makes. Measuring voltage for each design under the same fan shows the best design for a fixed wind speed.
The science
Each blade works like a small aerofoil, turning wind into a turning push. Few blades spin fast, but too few waste wind; many blades give more starting push but block each other. Tilting the blade (pitch) near 15 to 25 degrees gives a good push without too much drag.
Materials
Cost: under Rs 200. Many items come from home or from waste; spare parts such as LEDs, motors and wire are in the store.
- A small DC motor to act as a generator
- A plastic hub or bottle cap
- Stiff card or plastic blades
- A multimeter or LED
- A table fan with three speeds
- A stand made from a ruler and clay
Safety
For this project: Keep fingers and hair away from the fan and spinning blades. An adult must supervise cutting. Test indoors with a battery-powered fan if possible.
Never use mains electricity: power models with cells, batteries or a low-voltage adapter. Sharp and hot tools, flames and lab chemicals need an adult or your teacher beside you. Keep magnets, button cells and small parts away from young children, and wash your hands after making.
All the safety rulesStep by step
8 steps. Do them in order, and test each part before you go on to the next. Write what you notice in a notebook as you go.
Cut identical blades 12 cm long and 3 cm wide from stiff card.
Attach two blades to the hub at equal angles and fix the hub to the motor shaft.
Tilt each blade by the same twist, about 20 degrees, using small bits of tape.
Hold the fan 50 cm from the rotor and connect the multimeter to the motor terminals.
Run the fan on one speed for twenty seconds and write down the peak voltage.
Repeat with three, four and six blades keeping size, pitch and fan speed the same.
Now fix a four blade rotor and vary pitch: 10, 20 and 40 degrees.
Plot voltage against blade number and against pitch.
What you should see
The graph shows the voltage peaking at three or four blades and at a middle pitch. Too many blades or too steep a tilt lowers the output.
Viva questions
A teacher or a judge will ask how it works. Try to answer each question before you open it, then compare.
1Why is a motor used as a generator?
When its shaft is turned it produces a voltage.
2What is blade pitch?
The tilt angle of a blade against the wind.
3Why do big turbines have three blades?
It gives a good balance of efficiency, smoothness and cost.
4Which variables did you keep fixed?
Blade size, fan speed and distance.
Take it further
Test blade shape: flat strips against slightly curved ones from cut plastic bottle.
Change one thing, measure what happens, and add a table or chart of your readings: that is what turns a model into your own investigation.
Free ebooks for this project
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