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INDUCTORS
The electronic component known as the inductor is
best described as electrical momentum. In our water
pipe analogy the inductor would be equivalent to a
very long hose that is wrapped around itself many
times (see Figure 18). If the hose is very long it will
contain many gallons of water. When pressure is
applied to one end of the hose, the thousands of
gallons of water would not start to move instantly. It
would take time to get the water moving due to
inertia (a body at rest wants to stay at rest). After a
while the water would start to move and pick up
speed. The speed would increase until the friction of
the hose applied to the amount of pressure being
applied to the water. If you try to instantly stop the
water from moving by holding the plunger, the
momentum (a body in motion wants to stay in
motion) of the water would cause a large negative
pressure (Suction) that would pull the plunger from
your hands.
Since Inductors are made by coiling a wire, they are
often called Coils. In practice the names Inductor
and Coil are used interchangeably. From the above
analogy, it is obvious that a coiled hose will pass
Direct Current (DC), since the water flow increases
to equal the resistance in the coiled hose after an
elapsed period of time. If the pressure on the
plunger is alternated (pushed, then pulled) fast
enough, the water in the coil will never start moving
and the Alternating Current (AC) will be blocked.
The nature of a Coil in electronics follows the same
principles as the coiled hose analogy. A coil of wire
will pass DC and block AC. Recall that the nature of
a Capacitor blocked DC and passed AC, the exact
opposite of a coil. Because of this, the Capacitor
and Inductor are often called Dual Components.
How inductors are made?:
In order to understand how inductors are made, we
have to change our water pipe analogy slightly to
include the effect of magnetic fields. Consider two
pipes filled with water and small magnets attached
to the walls of the pipes with rubber bands as shown
in Figure 19. The moving magnets, due to the
original current, pull the magnets in the second pipe
and force a small current to flow in the same
direction as the original current. When the rubber
bands are fully stretched, the induced current will
stop, even though the initial DC current is still
flowing. If the original current is an AC current
however, it will induce a continuous AC current in
the second pipe because the magnets will move
back and forth, pulling the magnets in the second
pipe back and forth.
When the pipe is stretched out (increased length) as
in coil 1, the adjacent turns have little affect on each
other. In coil 2 (decreased length) the magnets in
each turn of the pipe are linking and the amount of
“apparent mass” in the pipe seems to increase. In
an inductor, pushing the coiled wire closer together
causes the inductance of the coil to also increase,
and stretching the coil out will lower the inductance
of the coil. In other words, the inductance of a coil is
indirectly proportional to its length. If the diameter of the coil is increased, it will take more hose to form a
loop, and the amount of water will therefore
increase. More water means a larger “apparent
mass”. Inductance will also increase in a coil if the
cross sectional area increases. Inductance is
directly proportional to area.
Consider the affect of adding more turns to coiled
pipe. The amount of material to push (mass) is
increased and the amount of linkage is increased
due to more magnets available. This causes the
“apparent mass” to increase at a greater rate than
would be expected. When making an inductor, the
actual inductance is directly proportional to the
square of the number of turns.
The final factor to consider when making a coil is the
core material at the center of the coil. If our pipe
wrapped around a material that contained many
magnets, they would also link to the magnets in the
pipe. This would
increase the “apparent
mass” of the water in
the pipe. The tiny
magnets in the core
would rotate as shown
in Figure 21 and force
the water to keep
moving in the same
direction. Placing an
iron core at the center
of an inductor will
directly increase the
inductance by an
amount equal to the
permeability of the
core material.
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