Environmental Engineering Reference
In-Depth Information
5.1.2 What happened to Alnico?
The real concern with Alnico, a very high flux permanent magnet having typical
remanence flux of the order of the highest grade rare earth NdFeB, is that its
coercive force is very low, less than that of ferrite magnets. This means that in
electric machine applications where demagnetizing force via armature reaction
appears through the magnet, it will easily become demagnetized.
Alnico 5, for example, has an energy product of 5 MGOe that is stable and
flat from
100 to
รพ
300
C, and its intrinsic coercivity,
H
ci
, is flat at a low value of
~1 kOe over this temperature range. In comparison with other magnet materials,
Alnico was introduced relatively early on and found widespread application in
D'Arsonval meter movements as a flux source. Figure 5.3 illustrates the types of
permanent magnets by introduction date and energy capability.
Improvements in magnet strength
60
480
440
400
360
320
280
240
200
160
120
80
40
0
OTHER IMPORTANT CHARACTERISTICS
REQUIRED MAGNETIZING FIELD
THERMAL STABILITY
MECHANICAL PROPERTIES
CORROSION RESISTANCE
MANUFACTURABILITY
COST
50
40
NdFeB
30
SmCO 1-5 and 2-17
Bonded isotropic NdFeB
Sintered ferrite
Columnar alnico
20
10
Mk steel
Ks steel
Alnico 5
0
1900
1920
1940
1960
1980
2000
Year
Figure 5.3 Illustration of permanent magnet types (Arnold Magnetics)
Figure 5.3 also shows by scale the phenomenal energy product of NdFeB
relative to other types of magnets. The magnitudes of energy are truly phenomenal,
with NdFeB reaching close to the theoretical limits of magnetic energy storage in a
PM.
Not all electric machines require, or use, rare earth magnets. Some examples of
various novel electric machines are shown here for perspective. The investigation
of alternative stator steels has also been ongoing over the years with amorphous
iron of particular interest. Figure 5.4 provides some examples of such novel electric
machines.
