Wind Power - Understanding Renewable Energy Systems - page 204

history, wind towers were lattice towers. The advantage of lattice towers is the
lower material demand and costs. Because lattice towers are now considered
to be visually unappealing, they are only rarely used today. Very small wind
generators use simple towers with guys that hold the tower. Larger wind power
plants use tubular towers, which are made of steel or concrete with circular
cross-section. Modern towers can reach heights up to 100 m and even more. A
60-m-high steel tower with a diameter of 3.65 m at the bottom and 2 m at the
top has a weight of 55 t. Hence, transport and final assembly of tower and
rotor blades can become rather difficult for large systems, especially in rough
terrain. Carefully designed foundations must hold these high masses so as to
withstand even severe storms. There are well-documented cases of badly-
designed wind power plants that have fallen during storms.
The nacelle of the wind power plants carries the rotor bearings, the gearbox
and the electric generator. Since most generator designs need high rotational
speed, a gearbox must adapt the rotor speed to the generator speed. However,
a gearbox has a lot of disadvantages: it causes higher costs, friction losses, noise
pollution and higher maintenance efforts. Gearless wind power plant concepts
with specially designed generators try to avoid these disadvantages. These
generators must have a high number of magnetic poles so that the generator
can also produce electricity at low rotor speeds. However, a high number of
poles enlarge the generator significantly and therefore increase its cost.

E LECTRICAL M ACHINES

Electrical generators for converting mechanical energy produced by the wind
to electricity are the heart of a wind power station. Therefore, this section is
dedicated to electrical machines. Before their application in wind power can
be described, some basics must be explained.
The type of current is one important characteristic feature of electrical
machines. Major current types are:

• direct current (DC)

• alternating current (AC)

• impulse current

• three-phase current.

DC machines are the oldest machines, because their principle is easy to
understand. Today they are used for many applications, for instance
windscreen wiper motors in cars. The universal motor that works with AC is
mainly used for household appliances. Stepping motors work with pulsed
current, for instance in computer printers.
However, three-phase machines are the best suited for electricity
generation at wind power plants and therefore only these are described here.
Three-phase machines can be subdivided into asynchronous machines and
synchronous machines .