Page 392 - Wind Energy Handbook

366 CONCEPTUAL DESIGN OF HORIZONTAL-AXIS TURBINES

6.10.2 Direct-drive 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 manufac-
turers offer such wind turbines. However, the power output of any rotating
electrical machine may be generally described by (Laithwaite and Freris, 1980):

2
P ¼ KD 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 gen-
erator from the network and permit variable-speed operation.

6.11 Drive-train Mounting Arrangement Options

6.11.1 Low-speed shaft mounting

The functions of the low-speed shaft are the transmission of drive torque from the
rotor hub to the gearbox, and the transfer of all other rotor loadings to the nacelle
structure. Traditionally the mounting of the low-speed shaft on fore and aft
bearings has allowed these two functions to be catered for separately; the gearbox is
hung on the rear end of the shaft projecting beyond the rear bearing and the drive
torque is resisted by a torque arm. The front bearing is positioned as close as
possible to the shaft/hub flange connection, in order to minimize the gravity
moment due to the cantilevered rotor mass, which usually governs shaft fatigue
design. The spacing between the two bearings will normally be greater than that
between front bearing and rotor hub in order to moderate the bearing loads due to
shaft moment (see Figure 6.15 for an illustration of a typical arrangement).
The opposite approach is to make the gearbox an integral part of the load path
between the low-speed shaft and tower top i.e., an ‘integrated gearbox’. The fore