1.5.1
Soft magnetic materials typically have a very narrow magnetic hysteresis loop to minimize losses and to maximize the permeability ^max, as shown in Fig. 1.22. Equation (1.37) and Fig. 1.6 show that the area within the hysteresis loop represents energy. When ac coils and motors drive the magnetic material around the hysteresis loop, energy is lost in the form of heat. Therefore, minimizing the width of the hysteresis loop (coercive force Hc) will also minimize the hysteresis loss. The maximum slope of the hysteresis loop \imax is approximately equal to the ratio of the remnant flux density Br to the coercive force Hc. Therefore, minimizing the width of the hysteresis loop (coercive force Hc) will also maximize the permeability Umax.
The magnetic permeability nmax reflects the ability of a material to carry magnetic flux. As mentioned, steel materials are composed of uniform magnetic regions called domains. When a magnetic field intensity is applied to a steel material, the magnetic domains rotate to align with the magnetic field. As the magnetic field intensity is increased, additional domains rotate into alignment with the magnetic field. Residual stress adds energy to the crystal structure and prevents the domains from rotating. The result is an increase in coercive force.
Small coercive force values are typically obtained in steels by minimizing the carbon content and by eliminating the residual strain with a full anneal. The properties of various soft magnetic materials are listed in Table 1.1.