Wind Energy Systems - Wind Energy Systems for Electric Power Generation - page 92

- MULTIBRID concept [Multibrid Entwicklungsgesellschaft] using a one-stage
gear for speed ratios approximately 10, and
- LIBERTY concept [Clipper Windpower] where a special gear drives four syn-
chronous machines sharing the power production.
Axial flux generators, though frequently proposed for wind energy systems, have
only rarely found practical application in the kW range and prototypes of higher
rating.
5.3.4.2 Machines with Excitation Winding
In synchronous machines the magnetic flux is provided by the inductor, generally
the rotor. Standard generators are provided with a field winding in the rotor, for
feeding the d.c. excitation current via slip-rings and brushes. The excitation power
is supplied by the exciter which may be a d.c. generator coupled with the main
machine. Mostly a static exciter is used which is fed from the generator terminals,
as indicated by exciter E in Fig. 5.15. Self-controled generators of smaller ratings
for island grids are frequently equipped with compound excitation where the field
current is derived from both terminal current and voltage, their values combined
vectorially to take phase displacement of the load into account. To start up, self-
excitation can be effected by the remanent machine flux.
Slipringless excitation, well-known from turbine generators, is also applicable
to other synchronous machines. Fig. 5.16 shows the principle circuitry. The main
Fig. 5.16 Synchronous generator with slipringless excitation