Generators - Wind Energy Systems for Electric Power Generation - page 46

3.3.2.5 Permanent Magnet Excitation
Permanent magnet (PM) excitation is used for synchronous machines of increas-
ing rating during the last years and has now been applied for units in the 5 MW
class. The absence of excitation winding losses helps to reach high efficiency val-
ues. Of the different magnet material technologies (ferrite, AlNiCo, rare earth) today
materials of high specific magnetic energy, preferably neodymium-iron-boron, have
become standard because air-gap flux densities no less than in conventional ma-
chines can be realized, albeit at relatively high cost.
Arrangement of the magnets on the rotor can be on the cylindrical rotor surface
(surface-mounted) or in slots (inset magnets) or within the rotor iron (buried mag-
nets), the latter method allowing flux-concentration designs. With surface-mounted
magnets the air-gap reluctance is similar to that of a turbo-type machine. With in-
set and buried magnets there is a variable reluctance torque component; generally
X d < X q .
It is understood that with PM excitation the inductor voltage is not adjustable; of
active and reactive power only one is adjustable in grid operation.
3.3.3 Unconventional Machine Types
3.3.3.1 Direct Driven Generators
In systems with direct driven generator there is no gear box in the drive train.
This concept allows in principle to reduce maintenance cost and improve efficiency.
Closer inspection is needed to evaluate savings in cost and mass.
Systems with synchronous generators for grid supply are always of the speed-
variable type, the machine frequency being decoupled from the utility frequency.
On the other hand for reasonable exploitation of the machine its rated frequency
should not be too much below 50 or 60 Hz. Hence the generator must feature a large
number of poles, especially of large ratings. This leads to decreasing power/mass
ratios, because according to a basic relation the rated torque (and not rated power)
defines the active volume of an electrical machine.
Due to manufacturing considerations, only synchronous machines have found
practical application in systems for direct drive while induction generators are con-
fined to systems with gear boxes.
3.3.3.2 Unconventional Designs
Several unconventional designs of synchronous machines have been taken into con-
sideration for wind energy systems. One of the main reasons is to realize construc-
tions of large number of poles in a given volume with reasonable manufacturing
effort. Some of them are different from the standard alternating pole concept.