Wind Energy Systems - Wind Energy Systems for Electric Power Generation - page 93

generator G1 has a field winding which is supplied from a coupled a.c. exciter gener-
ator G2 having a rotating three-phase armature winding and a stationary field wind-
ing. The a.c. excitation power is rectified by means of rotating diodes in a bridge
circuit V1. The resistor R11 is a simple protective device on the d.c. side.
A classical extension is the self-controled compound excited generator, where a
further auxiliary a.c. winding G3 provides a voltage dependent component which is
vectorially added to an armature current component in order to create approximately
the required main machine field current. A small electronic controller may be used
for improved voltage control.

5.3.4.3 Machines with Permanent Magnet Excitation

Permanent magnet machines feature higher efficiencies than machines with excita-
tion windings (absence of field winding losses), less weight and the advantage of
having no slip-rings and brushes. Machines above kilowatt range (and most below)
employ high-specific energy density PM material, preferably of Neodymium-iron-
boron (NdFeB). Though prices have steadily gone down, the cost of PM material
constitutes a considerable part of overall machine cost. A challenge in construction
is the installation of the permanent magnets on the rotor, which are mostly mounted
polewise in magnetized state on iron supporting elements, for mounting in axial
direction. Occuring magnet forces reaching extraordinary high values require rigid
auxiliary constructions at the manufacturers.
The material properties of permanent magnets are given in the demagnetization
curve; characterized by the remanence flux density B r , die coercitive force H c and
the maximum remanent energy density ( B

H ) max . Magnetic polarization J ( H ) and
Magnetization M ( H ) are connected with flux density B ( H ) by:

·

B = μ 0 · H + J = μ 0 · ( H + M ) (5.8)

The coercitive field strength is given by B H C (at B = 0) or J H C (at J = 0); here
H C = B H C is used.
Typical curves B ( H ) of PM materials are shown in Fig. 5.17 [VAC]. Ferrite mag-
nets are of low specific energy values, but cheap when mass produced; they are still
standard in small d.c. motors for consumer or automotive auxiliary drives. AlNiCo
magnets, though of high remanence, suffer from low coercitive force values and are
currently restricted to very small motors. The properties of rare earth magnets have
made remarkable progress in the last years, and they are widely applied in present
PM machines. They come as Samarium-Cobalt magnets, especially used in servo
motors, and in Neodymium-iron-boron magnets which are favourites, among other
applications, for wind turbine synchronous generators.
An example of a NdFeB magnet material, suitable for machine excitation, is
VACODYM 655 AP; its catalogue magnetization properties are shown in Fig. 5.18.
Typical values are B r = 1 , 2T , H c = 915 kA / m , ( BH ) max = 275 kJ / m 3 . The de-
pendence on temperature is evident, with the special phenomenon of the “knee”