Page 30 - Electrical Equipment Handbook _ Troubleshooting and Maintenance

INTRODUCTION TO MACHINERY PRINCIPLES

INTRODUCTION TO MACHINERY PRINCIPLES 2.3

H represents effort exerted by current to establish a magnetic field
represents relative ease of establishing a magnetic field in a given material
In SI, the units are as follows: H ampere-turns per meter; henrys/meter (H/m); B
2
webers/m , known as teslas (T). And is the permeability of free space. Its value is
0
4 10
7 H/m
0
The relative permeability compares the magnetizability of materials. For example, in mod-
ern machines, the steels used in the cores have relative permeabilities of 2000 to 7000. Thus,
for a given current, the flux established in a steel core is 2000 to 7000 times stronger than in a
corresponding area of air (air has the same permeability as free space). Thus, the metals of the
core in transformers, motors, and generators play an essential part in increasing and concen-
trating the magnetic flux in the device. The magnitude of the flux density is given by
Ni
B H
l
c
Thus, the total flux in the core in Fig. 2.1 is

NiA
BA
l
c
where A is the cross-sectional area of the core.

MAGNETIC BEHAVIOR OF FERROMAGNETIC
MATERIALS

The magnetic permeability is defined by the equation
B H
The permeability of ferromagnetic materials is up to 6000 times higher than the perme-
ability of free space. However, the permeability of ferromagnetic materials is not constant.
Suppose we apply a direct current to the core shown in Fig. 2.1 (starting with 0 A and
increasing the current). Figure 2.2a illustrates the variation of the flux produced in the core
versus the magnetomotive force. This graph is known as the saturation curve or magneti-
zation curve. At first, a slight increase in the current (magnetomotive force) results in a sig-
nificant increase in the flux. However, at a certain point, a further increase in current results
in no change in the flux. The region where the curve is flat is called the saturation region.
The core has become saturated. The region where the flux changes rapidly is called the
unsaturated region. The transition region between the unsaturated region and the saturated
region is called the knee of the curve.
Figure 2.2b illustrates the variation of magnetic flux density B with magnetizing inten-
sity H. These are the equations:
Ni
H
l
c

B
A

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