Inductor in a DC Circuit
An Inductor is a passive device that stores
energy in its Magnetic Field and returns energy to the circuit whenever
required. An Inductor is formed by a cylindrical Core with many Turns of
conducting wire. Figure 1 and Figure 2 are the basic structure and the
schematic symbol of the Inductor.
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Figure 1: Basic
Structure of the Inductor
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When an Inductor is connected to a circuit
with Direct Current (DC) source, two processes, which are called “storing” and
“decaying” energy, will happen in specific conditions.
The Inductor is connected to the DC Power
Supply, Figure 3. The sudden increase of current in the Inductor produces an
Self Induced Electromotive Force, vemf, opposing the Current change, Figure 1.
This appears as a Voltage across the Inductor, vL = - vemf. This - vemf will
slow down the Current change, and in turn, the slow down of the Current change,
will make vL become smaller. When the Current becomes stable, the Inductor
creates no more opposition and vL becomes zero, the Storage Phase is over.
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Figure 3:
Inductor is Storing Energy
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An Inductor is equivalent to a Short
Circuit to Direct Current, because once the Storage Phase has finished, the
Current, iL, that flows through it is stable, iL = V / R, no Self Induced
e.m.f. is produced and vL is zero. The Inductor acts like an ordinary
connecting wire, its Resistance is zero. The Current iL through an Inductor
cannot change abruptly.
When the Inductor is disconnected from the
Power Supply, Figure 4, vL reverses polarity and drops instantaneously from
zero to a negative value, but iL maintains the same direction and magnitude.
The energy stored in the Inductor decays through the Resistor RD. vL rises
gradually to zero and iL drops gradually to zero.
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Figure 4:
Inductor is Decaying Energy
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In Figures 3 and 4, the Resistance of RS and
RD affects the storing rate and the decaying rate of the Inductor
respectively.
The quotient of Inductance L and Resistance
R is called the Time Constant τ, which characterizes the rate of energy storage
and energy decay in the Inductor, Figure 5.
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Figure 5: The
Voltage VL and Current iL during the Storage
Phase and Discharge (Decay) Phase
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The larger the Resistance, the smaller the
Time Constant, the faster the Inductor stores the energy and decays the energy,
and vice versa.
Inductors are found in many electronic
circuits. For example, two Inductors can form a Transformer that is used to convert
between high and low Voltages, and vice versa.
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