Residential Wind by Way of Illustration - Wind Power for the World: The Rise of Modern Wind Energy - page 577

The alternator produces a three-phase voltage of variable
frequency and amplitude. In most cases it is rectified and,
depending on application, converted to a diferent level of DC
(battery charging) or to a constant voltage and constant frequency
AC (grid-tie or of-grid AC application). Most small wind turbine
manufacturers use a set of three brushes and three slip rings to
pass the power over from the alternator on the top of tower to the
ground equipment. For Turbine it was decided to install a rectifier
on the tower and use a set of two brushes and two slip rings for
power transfer.

Figure 26.2 The Turbine in the American West.

For its higher efficiency the electrical textbooks usually praise
three-phase AC over DC or single-phase AC. It is indeed better
if compared at the same voltage. However, the output voltage of
three-phase uncontrolled bridge rectifier is higher than the input
one—for the same current density DC requires slightly less copper
of the cables; for the same amount of copper it produces less losses.
The diference between DC and three-phase AC in cable losses
is about 27%. Company has never regretted using two-wire DC
from the tower top down. But there was a period of time when
installation of rectifier on the tower top was questioned—until
its lightning protection was enhanced.