Page 289 - Electrical Properties of Materials

Hard magnetic materials (permanent magnets) 271

Table 11.1 Major families of soft magnetic materials with typical properties

Category B s (T) ρ(μ –m) μ max Typical core loss, W kg –1 Applications, notes
measured at f(Hz)

A. Steels
lamination (low C) 2.1–2.2 0.4 2.0 (60) Inexpensive fractional hp motors
non-oriented (2% Si) 2.0–2.1 0.35 2.7 (60) High efficiency motors
convent. grain oriented 2.0 0.48 5 000 0.9 (60) 50/60 Hz distribution
(CGO M-4) transformers
high grain oriented (HGO) 2.0 0.45 1.2 (60) 50/60 Hz DTs: high design B max
B. Fe–(Ni, Co) alloys
40–50 Ni 1.6 0.48 150 000 110 (50 k)
77–80 Ni (square permalloy) 1.1 0.55 150 000 40 (50 k) High μ, used as thin ribbon
79 Ni–4 Mo (4–79 Mo 0.8 0.58 10 6 33 (50 k) Highest μ/lowest core loss of
permalloy, supermalloy) any metallic material
49 Co–2 V (permendur, 2.3 0.35 50 000 2.2 (60) Highest B s of commercial soft
supermendur) magnetic material
C. Ferrites
MnZn 0.5 2 × 10 6 6 000 35 (50 k) Power supply inductors,
transformers
NiZn 0.35 10 10 4 000 MHz applications

δ

Flux lines

O O

Fig. 11.10
(a) Magnetic field lines inside a
permanent magnet. (b) The same
) a ( ) b (
magnet with a narrow gap.

presence of the gap as in its absence? Without the gap, B = B r (Fig. 11.11). If
the value of flux density is denoted by B = B r1 in the presence of the gap, the
magnetic field in the gap will be H g = B r1 /μ 0 .
If you can remember Ampère’s law, which states that the line integral of
the magnetic field in the absence of a current must vanish for a closed path, it
follows that

H g δ + H m l = 0. (11.31)
H m is the magnetic field in the ma-
From the above equations we get
terial, and δ and l are the lengths
of the paths in the gap and in the
μ 0 l
B r1 =– H m . (11.32) material, respectively.
δ