Basic Electrical Theory
MAGNETIC CIRCUITS
The hysteresis loop is a series of
Figure 28 Hysteresis Loop for Magnetic Materials
c u r v e s
t h a t
s h o w s
t h e
characteristics of a magnetic
material (Figure 28).
Opposite
directions of current will result in
opposite
directions
of
flux
intensity shown as +H and -H.
Opposite polarities are also shown
for flux density as +B or -B.
Current starts at the center (zero)
when unmagnetized. Positive H
values increase B to the saturation
point, or +Bmax, as shown by the
dashed line. Then H decreases to
zero, but B drops to the value of
Br due to hysteresis. By reversing
the original current, H now
becomes negative.
B drops to
zero and continues on to -Bmax. As
the -H values decrease (less
negative), B is reduced to -Br
when H is zero. With a positive
swing of current, H once again
becomes
positive,
producing
saturation at +Bmax. The hysteresis
loop is completed. The loop does
not return to zero because of
hysteresis.
The value of +Br or -Br, which is the flux density remaining after the magnetizing force is zero,
is called the retentivity of that magnetic material. The value of -Hc, which is the force that must
be applied in the reverse direction to reduce flux density to zero, is called the coercive force of
the material.
The greater the area inside the hysteresis loop, the larger the hysteresis losses.
Magnetic Induction
Electromagnetic induction was discovered by Michael Faraday in 1831. Faraday found that if
a conductor "cuts across" lines of magnetic force, or if magnetic lines of force cut across a
conductor, a voltage, or EMF, is induced into the conductor. Consider a magnet with its lines
of force from the North Pole to the South Pole (Figure 29). A conductor C, which can be moved
between the poles of the magnet, is connected to a galvanometer G, which can detect the
presence of voltage, or EMF. When the conductor is not moving, zero EMF is indicated by the
galvanometer.
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