Generators - Wind Energy Systems for Electric Power Generation - page 47

Axial Field Machines
Axial filed synchronous machines with permanent magnet excitation have been rec-
ommended for high torque to volume ratios compared with radial field types. They
come as double systems with inner stator or inner rotor. Windings can be ring or
trapecoidal coil windings
Figure 3.20 shows principal arrangements. Indicated are in (a) and (b) the rotor-
stator-rotor (RSR) and in (c) the stator-rotor-stator (SRS) types. The windings are
realized in (a) as ring wound (toroidal) and in (b) and (c) in form of coils; trapecoidal
one- or two-layer or concentrated windings. Figure 3.21 indicates, for the RSR-type,
typical field lines. In the symmetrical version (a) same magnet polarities face each
other, and the stator yoke provides the flux linkage to toroid windings. In (b) is
depicted the version suitable for coil windings. The different types have their pros
and cons.
Stators can be slotted, with the winding conductors placed in the slots, or slotless
with air-gap windings. The latter solution does inherently not produce any cog-
ging torques, which is of advantage in view of application in wind energy systems
for starting at low wind speeds. At least for low ratings this may be a preferable
solution; see an example of a generator integrated in a small wind systems in
Fig. 3.22 [Okl95]. An extension to a machine consisting of a number of modules
as shown in Fig. 3.23 has been proposed for large ratings of low-speed synchronous
machines, such as ships propeller drives or wind generators [Cari99]. In spite of
the claimed advantages design and mechanical problems have hitherto prevented
introduction into practical use.
Fig. 3.20 Principal types of axial field machines