, the power drawn from the wind is proportional to the cube of the wind speed. This means that if the wind speed doubles, there is 8 times as much power available from it [7] . p> A further important parameter is the tip speed ratio. The tip speed ratio is defined as the ratio of the tip of the blade to that of the undisturbed wind velocity entering the blades [11] . The ratio is given by [7] :
(Eq 2.2)
where R is the radius of the blades, П‰ r is the rotor speed in rad/s and W the wind speed (m/s). p> Multi bladed rotors operate at low tip speed ratios of 1 or 2, where else, one, two or three bladed rotors operate at higher tip speeds of 6 to 10. The power coefficient in equation 2.1 depends on tip speed ratio as shown in figure 2.2. For a particular wind rotor design there exists a tip speed ratio which will produce the maximum value of power coefficient [11] .
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Fig. 2.2 Power coefficient C p versus tip speed ratio [11]
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2.2.2 Permanent magnet generator
Using permanent-magnet generators for small wind turbines is very commonly used world wide. Usually an AC generator with many poles operates between 10-100 Hz. Many configurations use surface mounted three phase permanent magnet synchronous generators with a rectifier connected to the generator terminals. [16]
A simple PM generator consists of the stator, magnet rotor disk and a shaft. The magnet rotor disk is mounted on a bearing hub so that it can rotate on the shaft due to the rotating blades of the wind generator. p> The stator has coils of copper wire wound around them, which are accommodated in the slots. Electricity is then generated when the magnets on the rotor disks rotate past the coils embedded in the stator. The magnetic field that is created induces a voltage in the coils [6] .
2.2.3 Rotor design
There are two types of rotor configurations commonly used world wide, these are the disk and the cup as shown in figure 2.3 below [20] . br/>В
Fig. 2.3 Disk and cup rotor designs
The radius of the rotor primarily depends on the power expected from the turbine and the strength of the wind regime in which it operates [5] . br/>
2.2.4 Tower
The main function of the tower is to raise the blades and the generator to a height where the wind is stronger and smoother than the ground level. The wind speed increases with height because of the earth surface [9] . The tower should be high enough to avoid any obstacles such as trees, building, etc. Practical considerations such as expense, safety and maintenance limit the tower to between 10m to 20m [6] above ground level.
2.3 Design of a wind turbine for Ga-Rampuru village
In this section a wind generator that is designed specifically for Ga-Rampuru village will be discussed. The generator will be designed using recyclable materials such as car brake plates, cables and drums found in the village [See appendix A]; this will clearly ensure a cost effective design. The wind turbine will be designed in such a way that the local people can easily assemble and manufacture it themselves.
All the recyclable materials that will be used in this design will be discussed below and an artist impression of the wind generator will be sketched.
2.3.1 The drum
The output of the wind generator depend on the amount of wind swept by the blades, therefore the wind extracting materials in a wind generator are very significant. A plastic drum will be used in this design to extract the wind since it can be easily shaped and carefully balanced to run smoothly. Also, it is resistant to fatigue braking and has a very light weight. p> The drum will be assembled as follows:
1. The top and the bottom part of the drum will be cut carefully by using a knife or pair of scissors to make a cylinder with open ends.
2. The cylindrical drum is then cut length-wise into two equal halves.
3. The two halves are then glued together similar to the drum shown in figure 2.4.
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Figure 2.4 An S-shaped drum
To prevent the over speeding of the drum, the permanent magnet generator should always be connected to a battery or other electrical load. If this is not done the wind turbine will become noisy and may vibrate so much that some parts come loose and fall to the ground [6] . <В
2.3.2 Magnet rotor disk
After a tour around the village neighbourhood dumpsites it was discovered that there are many discarded loud-speakers that are no longer in use in the village. These loud-speakers have permanents mounted to their back. Since the PM generator requires magnets, these loud-speakers wi...
