Page 51 - Electric Machinery Fundamentals

IN1KUlJUCTIUN TU M ACHINeRY PRINClPLbS 1.1

- ./ "- I \ - - - - - - - - -

1 X ! - X - 1 - - /' - '-.. - -
- ! - /' \ "- X /' - - - - - /'

\ - \ - ./ - - -
"'" '" ---- '" - -
./ X / I • • - - -- - /' '-.. - '-..

t ! \ I 1 / - - - --
'" ---- - "'"

(a) (b)
(
FIGURE 1-12
(a) Magnetic domains oriented randomly. (b) Magnetic domains lined up in the presence of an
external magnetic field.

Notice that if a large magnetomotive force is first applied to the core and
then removed, the flux path in the core will be abc. When the magnetomotjve
force is removed, the flux in the core does not go to zero. Instead, a magnetic field
is left in the core. This magnetic field is called the residual flux in the core. It is in
precisely this manner that permanent magnets are produced. To force the flux to
zero, an amount of magnetomotive force known as the coercive magnetomotive
force (~( must be appJied to the core in the opposite direction.
Why does hysteresis occur? To understand the behavior of ferromagnetic
materials, it is necessary to know something about their structure. The atoms of
iron and similar metals (cobalt, nickel, and some of their alloys) tend to have their
magnetic fields closely aligned with each other. Within the metal, there are many
small regions called domains. In each domain, all the atoms are aligned with their
magnetic fields pointing in the same direction, so each domain within the material
acts as a smal1 permanent magnet. The reason that a whole block of iron can ap-
pear to have no flux is that these numerous tiny domains are oriented randomly
within the material. An example of the domain structure within a piece of iron is
shown in Figure 1- 12.
When an external magnetic field is applied to this block of iron, it causes do-
mains that happen to point in the direction of the field to grow at the expense of
domains pointed in other directions. Domains pointing in the direction of the mag-
netic field grow because the atoms at their boundaries physically switch orientation
r to align themselves with the applied magnetic field. The extra atoms aligned with
the field increase the magnetic flux in the iron, which in turn causes more atoms to
switch orientation, further increasing the strength of the magnetic field. It is this pos-
itive feedback effect that causes iron to have a permeability much higher than air.
As the strength of the external magnetic field continues to increase, whole
domains that are aligned in the wrong direction eventually reorient themselves as