Page 52 - Electric Machinery Fundamentals

2~ ELECTRIC MACHINERY FUNDAMENTALS

a unit to line up with the field. Finally, when nearly all the atoms and domains in
the iron are lined up with the external field, any further increase in the magneto-
motive force can cause only the same flux increase that it would in free space.
(Once everything is aligned, there can be no more feedback effect to strengthen
the field.) At this point, the iron is saturated with flux. This is the situation in the
saturated region of the magnetization curve in Figure 1- 10.
The key to hysteresis is that when the external magnetic field is removed,
the domains do not completely randomize again. Why do the domains remain
lined up? Because turning the atoms in them requires energy. Originally, energy
was provided by the external magnetic field to accomplish the alignment; when
the field is removed, there is no source of energy to cause all the domains to rotate
back. The piece of iron is now a permanent magnet.
Once the domains are aligned, some of them will remain aligned until a
source of external energy is supplied to change them. Examples of sources of ex-
ternal energy that can change the boundaries between domains and/or the align-
ment of domains are magnetomotive force applied in another direction, a large (
mechanical shock, and heating. Any of these events can impart energy to the do-
mains and enable them to change alignment. (It is for tltis reason that a permanent
magnet can lose its magnetism if it is dropped, hit with a hammer, or heated.)
The fact that turning domains in the iron requires energy leads to a common
type of energy loss in all machines and transformers. The hysteresis loss in an iron
core is the energy required to accomplish the reorientation of domains during each
cycle of the alternating current applied to the core. It can be shown that the area
enclosed in the hysteresis loop formed by applying an alternating current to the
core is directly proportional to the energy lost in a given ac cycle. The smaller the
applied magnetomotive force excursions on the core, the smaller the area of
the resulting hysteresis loop and so the smaller the reSUlting losses. Figure 1~13
illustrates this point.
Another type of loss should be mentioned at this point, since it is also
caused by varying magnetic fields in an iron core. This loss is the eddy current
loss. The mechanism of eddy current losses is explained later after Faraday's law
has been introduced. Both hysteresis and eddy current losses cause heating in the
core material, and both losses must be considered in the design of any machine or
transformer. Since both losses occur within the metal of the core, they are usually
lumped together and called core losses.

1.5 FARADAY'S LAW- INDUCED VOLTAGE
FROM A TIME-CHANGING MAGNETIC FIELD
So far, attention has been focused on the production of a magnetic field and on its
prope11ies. It is now time to examine the various ways in which an existing mag-
netic field can affect its surroundings.
The first major effect to be considered is called Faraday's law. It is the ba-
sis of transformer operation. Faraday's law states that if a flux passes through a
turn of a coil of wire, a voltage will be induced in the turn of wire that is directly