Directdrive generators

There is considerable interest in the application of generators driven directly by the wind-turbine rotor without a speed increasing gearbox and a number of manufacturers offer such wind turbines. However, the power output of any rotating electrical machine may be generally described by (Laithwaite and Freris, 1980):

P = KD2 Ln where D is the rotor diameter, L is the length, n is the rotational speed, and K is a constant.

Thus it may be seen that if the rotational speed is reduced then it is necessary either to lengthen the generator in proportion or to increase the diameter. It is cheaper to increase the diameter as this raises the power by the square rather than linearly. Thus, direct-drive generators for wind turbines tend to have rather large diameters but with limited length (Figure 6.20).

Induction generators require a rather small radial distance between the surface of the rotor and the stator (known as the air-gap). This is necessary to ensure an adequate air-gap magnetic flux density as all the excitation is provided from the stator. In contrast, synchronous generators have excitation systems on the rotor and so can operate with larger air-gaps. It is difficult to manufacture large diameter electrical machines with small air gaps for mechanical and thermal reasons. Hence direct-drive wind turbines use synchronous generators (either with permanent magnet excitation or, more usually, with a wound rotor and electromagnets providing the field). The use of a synchronous generator, in turn, leads to the requirement for solid-state frequency conversion equipment to de-couple the generator from the network and permit variable-speed operation.

Renewable Energy Eco Friendly

Renewable Energy Eco Friendly

Renewable energy is energy that is generated from sunlight, rain, tides, geothermal heat and wind. These sources are naturally and constantly replenished, which is why they are deemed as renewable.

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