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(A)
(B)
FIGURE 11.66
(A) Small diameter, low inductance; (B) larger diameter, higher inductance.
(A)
(B)
(C)
FIGURE 11.67
(A) Air core, low inductance; (B) powdered iron core, higher inductance; (C) soft iron core,
highest inductance.
Figure 11.67 shows three identical coils. One has an air core, one has a powdered iron core in the
center, and the other has a soft iron core. This figure illustrates the effects of core material on induc-
tance. The inductance of a coil is affected by the magnitude of current when the core is a magnetic
material. When the core is air, the inductance is independent of the current.
Note:
The inductance of a coil increases very rapidly as the number of turns is increased. It also
increases as the coil is made shorter, the cross-sectional area is made larger, or the perme-
ability of the core is increased.
11.7.12.2 Mutual Inductance
When the current in a conductor or coil changes, the varying flux can cut across any other conduc-
tor or coil located nearby, thus inducing voltages in both. A varying current in
L
1
, therefore, induces
voltage across
L
1
and across
L
2
(Figure 11.68; see Figure 11.69 for the schematic symbol for two coils
with mutual inductance). When the induced voltage
e
L
2
produces current in
L
2
, its varying magnetic
field induces voltage in
L
1
; hence, the two coils
L
1
and
L
2
have
mutual inductance
because current
change in one coil can induce voltage in the other. The unit of mutual inductance is the henry, and the
symbol is
L
M
. Two coils have
L
M
of 1 henry when a current change of 1 A/sec in one coil induces 1
E
in the other coil. Factors affecting the mutual inductance of two adjacent coils include the following:
• Physical dimensions of the two coils
• Number of turns in each coil
• Distance between the two coils
• Relative positions of the axes of the two coils
• Permeability of the cores
e
L2
L
2
L
1
Magnetic flux
FIGURE 11.68
Mutual inductance between
L
1
and
L
2
.
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