Generators - Wind Energy Systems for Electric Power Generation - page 49

Transversal Flux Machines

The principle of transversal flux machines offers advantages producing high force
densities and allows realization of large pole numbers [Weh86]. The name illus-
trates the characteristic flux path which, in a rotating machine, is partly led in axial
direction, thus decoupling the space for the armature winding cross-section and the
pole pitch. With permanent magnet excitation, transversal flux machines can be de-
signed to force densities up to 100 kN / m 2 at linear current densities of 150 kA/m,
the achievable values increasing at decreasing pole pitch. The concept is considered
of advantage for direct drive applications.
On the other hand the 3D construction is mechanically complicated. Small air-
gaps are required due to the variable reluctance torque component, and the construc-
tion tends to give rise to large leakage fluxes; consequently low power factor values
are characteristic. Hence up to now their application is scarce due to mechanical
reasons and manufacturing cost.
Principal geometric arrangements are depicted in Fig. 3.24. Part (a) illustrates a
single-sided version; the permanent magnets are arranged surface-mounted on a soft
iron rotor member. The stator ring winding representing one phase is surrounded by
U-shaped soft iron elements, placed two pole pitches apart. Flux-leading elements
are provided for a closed magnetic path [Hen97]. This version is less challenging
in manufacture than the original double-sided concept shown in part (b) of the fig-
ure, using an intermediate rotor positioned between two stator elements. Here the
permanent magnets are in concentrator arrangement, with alternating polarity and
separated by soft iron pieces.
Transversal flux machines were taken into consideration as motors for traction
applications; as generators they were proposed for wind energy systems of lower
rating already years ago. Recently a concept for a 5 MW direct-driven wind genera-
tor was described [Svech06].

a) b)

Fig. 3.24 Principle of transversal flux machines ( a ) single-sided component of polyphase machine;
( b ) Double-sided with intermediate rotor