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Chapter 12: Applications of Soft Magnets
1. Losses
2. Materials
3. Static Applications
4. Low-frequency Applications
5. High-frequency Applications
Comments and corrections please:jcoey@tcd.ie
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Further reading
C.W. Chen,Magnetism and Metallurgy of Soft Magnetic Materials, Dover: 1983An excellent monograph which contains a wealth of detailed and reliable information on almost every aspect of soft magnets.
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Ni-Fe/Fe-Co (heads)
Fe-Si
Fe-Si (oriented)
Ni-Fe/Fe-Co
Amorphous
Others
Others
Alnico
Sm-CoNd-Fe-B
Hard ferrite
Co- Fe 2O 3
(tapes, floppy discs)
CrO2 (tapes)
Iron (tapes)
Co-Cr (hard discs)
Soft ferrite
Others
Iron
Soft
Magnets
HardMagnets
MagneticRecording
Magnet applications; A 30 Bmarket
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Figure 12.1 Hysteresis in a soft magnetic material. B(H)
and J(H) are indistinguishable in small fields.
Minimal hysteresis.
High polarization
Largest possible permeability
B = 0rH
102< r
< 106
r
B 0M)
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skin depth
Bd= 0r
H
Electrical steel; = 0.5 m, r = 10,000
s= 0.36 mm at 50 Hz; s= 3.6 m at 500 kHz
rdecreases with increasing frequency; 106 to 102
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Pan
f
Phy
Pan
P/f
Phy
Ped
Hysteresis loss per cycle is the area of the
B(H) loop
H
Reduction of eddy-current losses by lamination
Figure 12.2 Total loss per cycle showing the three
contributions
1. Losses
1/n2
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t
d
H
Figure 12.3 Pry and Bean model for movement of uniformly-spaced domain walls.
Currents are induced in the vicinity of the walls, as shown by the dashed lines.
Reduces to zero as d/t 0
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Figure 12.4 Total loss per kg for permalloy at differentfrequencies. Thickness is 350 m
Figure 12.5 Progress during the 20th Century.
a) Losses in transformer cores
b) Initial static permeability
Losses are double in a rotating field.
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12.1.2 High-frequency losses.
Complex permeability = - i
h= h0exp it
b= b0exp i(t-) real parts are h(t), b(t)
= (b/h) exp -i
= (b/h) [cos - i sin]
Re(h) is the time dependent flux density
b(t) = h0[cost + sint] Losses are proportional to
Quality factorQ = / = cot
Loss angle
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A Fourier integral
The Fourier components are
General time-dependent response
Real and imaginary parts of are related via the Kramers-Kronig relations
Rate of energy dissipation P= h(t) db(t)/dt = h02cost (- sint + cost) sc=0, c2=1/2
P = (1/2)h02
Losses are +ve, hence the - sign in the defn
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Hs
h
M
Msm
Precession of the
magnetization
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Snoeks relation
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Figure 12.6 Real and imaginary parts of the susceptibilities and . The peak is at the ferromagnetic resonance frequency
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Figure 12.7 Global market for soft magnetic materials.
The pie represents about 10 B$ per annum
12.2 Soft Magnetic Materials
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[100] roll direction
(011) plane
Goss texture of grain-oriented silicon steel
Iron-rich edge of the Fe-Si phase diagram
Figure 12.8 Losses as a function of operating inductionfor grain-oriented silicon steel
At % Si
Wt % Si
L
TC
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Figure 12.9 Frequency response of some Ni-Zn ferrites
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Figure 12.10 A laboratory electromagnet
Tapered pole pieces; 55
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Figure 12.11 Types of cores. The powder core has been
sectioned to indicate its internal structure.
Stacked laminations Tape-wound core Powder core
Ferrite E-core
12.4 Low Frequency Applications
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Figure 12.12 Two electric motor designs: a) an induction motor with a squirrel-cage winding and b) a 3/4
variable reluctance motor
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Figure 12.13 A fluxgate magnetometer. a) Schematic b) operating principle
~
V
H
H
B
Hexc
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Figure 12.14 A surface accoustic wave delay line
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a a
b b
A magnetic amplifier
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An assortment of soft magnetic components made from Finemet
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A pulse transformer
Figure 12.15 A wire loop antenna, and am equivalent ferrite rod
with a much smaller cross section.
12.5 High Frequency Applications
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Q = 0/
A C-core with an airgap
Figure 12.16 An LC filter circuit, and the pass band.
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Figure 12.17 Absorption and transmission for left- and
right-polarized radiation
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Figure 12.18 A plane-polarised wave is decomposed into the sum of two, counter-rotating
circularly-polarized waves (a) which become dephased because they propagate at different
velocities (b) through the magnetized ferrite. The Faraday rotation is non- reciprocal - independentof direction of propagation.
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Figure 12.19 A waveguide propagating a TE01mode. Filling the upper half with YIG
magnetized vertically absorbs the microwaves for one direction of propagation, but not the
other.
YIG
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Figure 12.20 A four-port circulator a) illustrates the principle, b) shows the sense of
propagation and c) is the logic table.antennareceiverloadtransmitter.
0100
0010
0001
1000
In out
45
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A resonant microwave filter. The device transmits a
signal in a narrow frequency range, around the
ferromagnetic resonance frequency of the YIG sphere.