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University of California, Berkeley, California, USA
Steels, Silicon Iron-based: Magnetic
Properties
Electrical steels are used in the cores of electromag-
netic devices such as motors, generators, and trans-
formers because of the ability of ferromagnetic
materials to magnify the magnetic effects of cur-
rent-carrying coils (see Magnets Soft and Hard: Mag-
netic Domains). Of the available ferromagnetic
materials, iron and its alloys offer the best cost ben-
eficial performance. The torque of a motor is pro-
portional to B
2
, where B is the intensity of
magnetization operating between its stationary and
moving parts. Because of this square law relationship
even small gains in B lead to useful increases in
torque and thus output power.
In the case of transformers, the large magnetiza-
tions available from iron enable voltage transforma-
tions to be carried out by windings of an acceptable
size. The voltage appearing at a transformer winding
is related to the rate of flux change dF/dt for the core
and is normally of sine form so that operation at high
peak flux levels is important.
It is clear that for steels to be of the most use for
electrical machines a high working induction is de-
sired, as close as practicable to the approximately 2 T
at which iron becomes saturated.
At the same time a high permeability (ratio of B to
H) or flux magnifying property is desired. The effec-
tive permeability of iron reduces as saturation is ap-
proached leading to heavier demands on magnetizing
current.
Additionally the duties of core steel must be exe-
cuted without serious wastage of energy within the
metal (called power loss or core loss) due to the pe-
riodic magnetic reversals involved in use.
The range of electrical steels available has arisen
out of the appropriate compromises struck between
these factors. Different types of steel suit different
applications (see Table 1).
1. Reduction of Power Loss
One of the primary sources of power loss within an
electrical steel is eddy current loss (see Magnetic
Losses). If a solid iron core is placed within a mag-
netizing coil the iron core itself provides short-
circuited current paths in which so-called induced
‘‘eddy currents’’ can flow. These waste energy as heat
and also produce magnetic fields which oppose the
magnetization (Lenz’s law effects) so that penetration
of flux to the centre of the core is inhibited. Eddy
currents can be radically reduced by splitting up the
core into laminae which restricts the flow of eddy
currents (Fig. 1). There are limits to the degree of
lamination which can be applied set by the cost of
rolling steel to reduced thickness and the complexity
of handling this material for core building. Inevitably
as thinner steel is used the effective space occupancy
of the metal reduces since a pile of plates can never
have the same mass as solid metal of the same su-
perficial dimensions.
Further the need to insulate laminae from each
other by applying coatings reduces the effective space
occupancy. Overall the effect is to reduce the appar-
ent saturation induction of the core. Lamination cre-
ates much more metal surface, and surfaces produce
power loss due to domain wall pinning.
A second means of restraining eddy currents is the
use of alloying elements added to iron. Adding some
3% of silicon to iron raises its resistivity fourfold.
Many other elements have been tried as resistivity
raisers (e.g., aluminum) but silicon has proved to be
the most useful. Adding silicon certainly reduces eddy
currents (and so power loss) but the resulting alloys
are more difficult to roll and harder to punch into
laminations. Further, added silicon dilutes the iron
1123
Steels, Silicon Iron-based: Magnetic Properties