Page 488 - Basic Electrical Engineering

(b) is shown the B–H characteristic of the material used for making
permanent magnets like alnico (alloy of aluminium, nickel, and cobalt). Here
the residual magnetism is large and the negative magnetizing force required

to bring down the residual magnetism to zero (which is called the coercive
force) is also large.

Figure 5.12 (a) Hysteresis loop for silicon steel; (b) hysteresis loop for alnico

5.4.1 Hysteresis Loss

The energy spent in alternate magnetization of the core appears as heat in the
magnetic material. An emperical formula to calculate hysteresis loss in a

magnetic material has been developed on the basis of experiments.
If the magnetizing force is now reduced, the curve traces a somewhat
different path as shown. The negative current provides magnetization in the

opposite direction as shown. When the magnetizing force is reduced to zero,
there is some magnetism left in the magnetic material, which is known as

residual magnetism. In electrical machines, this residual magnetism plays an
important role, as will be studied in chapters on electrical machines.

When, instead of direct current through the coil, an alternating current
flows, the core will get magnetized in the opposite directions alternately. The