Page 32 - Electrical Equipment Handbook _ Troubleshooting and Maintenance

INTRODUCTION TO MACHINERY PRINCIPLES

INTRODUCTION TO MACHINERY PRINCIPLES 2.5

In the saturated region, the permeability drops to a very low value. Electric machines and
transformers use ferromagnetic material for their cores because these materials produce
much more flux than other materials.
Table 2.1 lists the characteristics of soft magnetic materials including the Curie tem-
perature (or Curie point) T . Above this temperature a ferromagnetic material becomes
c
paramagnetic (weakly magnetized). Figure 2.3 shows several B-H curves of some soft
magnetic materials.
Permalloy, supermendur, and other nickel alloys have a relative permeability greater
5
than 10 . Only a few materials have this high permeability over a limited range of opera-
tion. The highest permeability ratio of good and poor magnetic materials over a typical
4
operating range is 10 .
Energy Losses in a Ferromagnetic Core

If an alternating current (Fig. 2.4a) is applied to the core, the flux in the core will follow
path ab (Fig. 2.4b). This graph is the saturation curve shown in Fig. 2.2. However, when
the current drops, the flux follows a different path from the one it took when the current
increased. When the current decreases, the flux follows path bcd. When the current
increases again, the flux follows path bed.
The amount of flux present in the core depends on the history of the flux in the core and
the magnitude of the current applied to the windings of the core. The dependence on the
history of the preceding flux and the resulting failure to retrace the flux path is called hys-
teresis. Path bcdeb shown in Fig. 2.4 is called a hysteresis loop.
Notice that if a magnetomotive force is applied to the core and then removed, the flux
will follow path abc. The flux does not return to zero when the magnetomotive force is
removed. Instead, a magnetic field remains in the core. The magnetic field is known as the
residual flux in the core. This is the technique used for producing permanent magnets. A
magnetomotive force must be applied to the core in the opposite direction to return the flux
to zero. This force is called the coercive magnetomotive force .
c
To understand the cause of hysteresis, it is necessary to know the structure of the
metal. There are many small regions within the metal called domains. The magnetic
fields of all the atoms in each domain are pointing in the same direction. Thus, each
domain within the metal acts as a small permanent magnet. These tiny domains are ori-
ented randomly within the material. This is the reason that a piece of iron does not have
a resultant flux (Fig. 2.5).
When an external magnetic field is applied to the block of iron, all the domains will line
up in the direction of the field. This switching to align all the fields increases the magnetic
flux in the iron. This is the reason why iron has a much higher permeability than air.
When all the atoms and domains of the iron line up with the external field, a further
increase in the magnetomotive force will not be able to increase the flux. At this point, the
iron has become saturated with flux. The core has reached the saturation region of the mag-
netization curve (Fig. 2.2).
The cause of hysteresis is that when the external magnetic field is removed, the domains
do not become completely random again. This is so because energy is required to turn the
atoms in the domains. Originally, the external magnetic field provided energy to align the
domains. When the field is removed, there is no source of energy to rotate the domains. The
piece of iron has now become a permanent magnet.
Some of the domains will remain aligned until an external source of energy is supplied
to change them. A large mechanical shock and heating are examples of external energy that
can change the alignment of the domains. This is the reason why permanent magnets lose
their magnetism when hit with a hammer or heated.

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