A magnetic material with small coercivity and low hysteresis loss, such as electrical steel, ferrites and amorphous alloys, used for transformer and motor cores.
Japanese name: 軟磁性材料
Soft magnetic material appears in 1 practice question across 1 topic, including Electronic, magnetic and optical materials.
"Soft" in soft magnetic material does not mean mechanically soft; it means the material magnetizes easily in a weak field and loses its magnetization quickly when the field is removed. The requirements are low coercivity, high permeability, high saturation flux density and low loss in AC use (core loss). The higher the saturation flux density, the smaller the core can be, and the lower the core loss, the higher the efficiency and the less the heating. No material maximizes all of these at once, however, so materials are chosen according to the frequency of use.
Core loss is divided into hysteresis loss, proportional to the area of the hysteresis loop (and to frequency f), and eddy-current loss, from currents induced in the material by the changing flux. The classical eddy-current loss of a sheet of thickness t is π²t²f²B²/(6ρ), proportional to the square of the thickness and inversely proportional to resistivity ρ. For silicon steel sheet 0.35 mm thick with a resistivity of about 0.48 µΩ·m used at 50 Hz and 1.5 T, this formula gives about 0.3 W/kg; the actual core loss is larger because hysteresis loss and other losses are added. Si is therefore added to raise resistivity, and thin insulated sheets are stacked.
Silicon steel sheet (electrical steel) has a high saturation flux density of about 2 T, is inexpensive, and is the mainstay of transformers and motors at power frequencies. Fe-based amorphous alloys have a somewhat lower saturation flux density of about 1.5 T but very low core loss and are used in distribution transformers. Nanocrystalline alloys of Fe–Si–B with Cu and Nb added (developed in Japan in 1988) combine high permeability with low loss. Ferrites such as Mn–Zn types have low saturation flux densities of about 0.5 T or less, but their very high resistivity keeps losses low at high frequencies.
Large 50/60 Hz transformers use grain-oriented electrical steel, which magnetizes easily in the rolling direction, and motors, which see rotating fields, use non-oriented electrical steel. For switching power supplies from a few kHz to a few MHz, ferrites, nanocrystalline alloys and dust cores are chosen. Soft magnetic materials are sensitive to stress and strain, and punching or bending raises coercivity and increases core loss. Stress-relief annealing is therefore performed after processing, and laminations are not clamped too tightly when stacked.
Answer: B. Small coercivity and a narrow hysteresis loop with a small area
In an AC core the magnetization reverses every cycle, so energy proportional to the area of the hysteresis loop is lost each cycle. A soft magnetic material with small coercivity and a narrow loop is therefore needed. High permeability and saturation flux density are also required, as is high electrical resistivity to reduce eddy-current losses; grain-oriented electrical steel (silicon steel), ferrites and amorphous alloys are used. Large coercivity and a large loop area are characteristics of hard magnetic materials (permanent magnets) such as Nd-Fe-B and ferrite magnets. For permanent magnets, both a large remanence and a large coercivity are important.
It means magnetically "soft": the magnetization direction changes easily in a weak field, and almost no magnetization remains when the field is removed. It has nothing to do with mechanical hardness; hard electrical steel sheet is also a soft magnetic material.
Grain-oriented steel has the easy <001> magnetization direction of its crystals aligned with the rolling direction (the Goss orientation) and is used in transformers, where the flux direction is fixed. Non-oriented steel has nearly random crystal orientations and is used in motors, where the flux direction rotates.
Soft ferrites are spinel-type oxides of the Mn–Zn or Ni–Zn type with small coercivity, used in high-frequency transformers and inductors. Hard ferrites are hexagonal magnetoplumbite-type oxides containing Ba or Sr with large coercivity, used for permanent magnets.
A soft magnetic material with small coercivity and a narrow loop is therefore needed.
Which correctly describes a soft magnetic material suitable…Coercivity Eddy-current loss Curie temperature Indirect-bandgap semiconductor Matthiessen's rule