Wind Energy Systems - Wind Energy Systems for Electric Power Generation - page 94

Fig. 5.17 Demagnetization curves of different magnetic materials
appearing in the B ( H ) and J ( H ) curves, respectively. Obviously the coercitive
field strength absolute value is decreasing with increasing temperature. Machine
design must take care that the “knee” is not touched at any load or short-circuit,
regarding magnet temperature and armature current reaction (producing m.m.f.
in demagnetizing direction). Otherwise the well-known inreversible magnet de-
magnetization would take place, with a decreased flux and machine capability as
consequence.
In case of the material Fig. 5.18 the manufacturer specifies a maximum tem-
perature of 160 ◦ C for application. This is a rather low value, regarding modern
machines e.g. of temperature class F. Hence the magnet chosen for example may
serve for small machines of moderate utilization only. For wind generators in the
MW-class magnets of higher temperature stability are required; the choice may
then be the quality VACODYM 677 AP [VAC], with properties a little lower at