Oriented (anisotropic) silicon-steel
laminations.
The iron cores of conventional
transformers consist of anisotropic silicon-steel laminations with
lamination thickness ranging from 0.1 mm to 0.4 mm. In a transformer,
the flux travels mostly within the limbs in the with-grain direction,
and in the cross-grain direction only near the corners and lamination
joints of transformer cores; thus oriented steel sheets are used.
The with- and cross-grain structure of
oriented steel is determined by the rolling direction of the sheets
during manufacture. Each side of a lamination is coated with
insulating material so that no eddy currents can flow between
laminations.
The coating does not significantly
interfere with the passage of flux. The magnetic resistance, or
reluctance, is only slightly increased and is taken into account via
the iron-core stacking factor ϕFe = #(iron cross section of all
laminations of core)/(cross section of entire core including
insulation between laminations).
The stacking factor is in the range of
0.93 ≤ ϕFe ≤ 0.97 for 60 Hz units. For anisotropic electrical
silicon steel the relative permeability is larger (and thus the
magnetization required is smaller) in the with-grain direction
(direction of rolling) than in the cross-grain direction. Similarly,
the core losses are small in the with-grain direction and relatively
large in the cross-grain direction.
Amorphous (glass-type) cores.
Amorphous magnetic materials either are
obtained by quenching the molten material at high cooling rates or
are manufactured by deposition techniques in a vacuum. The quenching
process does not permit the forming of a crystalline structure, and
therefore amorphous magnetic materials have a structure similar to
glass.
The cores of transformers with
amorphous alloy (AMTs) can be fabricated in the same manner as those
made of oriented-silicon-steel. METGLAS (trademark of the Allied
Signal Co) cores are 30% heavier than comparable oriented
silicon-steel cores, but the no-load losses in amorphous alloy wound
cores are only 30% of those in comparable oriented-siliconsteel wound
cores.
However, the rated power efficiencies
of present-day designs of AMTs and silicon-steel pole transformers
with wound cores are about the same. For example, the rated power
efficiencies of 20 kVA and 50 kVA wound-core AMTs at unity power
factor are ηpower = 98.26% and 98.59%, respectively, while that of a
25 kVA oriented-silicon-steel wound core (10) at unity power factor
is ηpower = 98.31%. The fabrication cost for AMTs with wound cores
is higher than that for oriented silicon-steel wound cores.
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