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Reactance in AC Circuits - Fundamental Concepts
A.
Bhatia
Overview
Reactance is the measure
of the opposition to the flow of alternating current caused by the inductance
and capacitance in a circuit rather than by resistance. Steady electric currents
flowing along conductors in one direction undergo opposition called electrical
resistance, but no reactance. Reactance is present in addition to resistance
when conductors carry alternating current. Reactance also occurs for short
intervals when direct current is changing as it approaches or departs from
steady flow (e.g., when switches are closed or opened).
Reactance is of two types,
inductive and capacitive.
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Inductive reactance (symbol XL) is
associated with the varying magnetic field that surrounds a wire or a coil
carrying a current. The more rapidly the current changes, the
more an inductor resists it. The reactance is proportional to the frequency; a
high-frequency alternating current changes more rapidly than a low-frequency
one, since there are more cycles per second. Thus the higher the frequency and
the larger the inductance, the harder it is for current to flow through the
inductance; it meets more opposition. The unit of reactance is also the ohm.
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Capacitive
reactance (symbol XC)
is associated with the
changing electric field between two conducting surfaces (plates) separated
from each other by an insulating medium. The more
rapidly the applied voltage changes in value, the faster the capacitor stores
energy. This means that a high-frequency alternating voltage will put more
current into a given capacitor than a low frequency voltage could. Thus the
reactance of a capacitor goes down as the frequency increases.
Reactance
produces a phase shift between the electric current and voltage in the circuit.
Reactance is denoted by the symbol X and is measured in ohms.
·
If
X > 0, the reactance is said to be inductive.
·
If
X = 0, then the circuit is purely resistive (i.e. it has no reactance).
·
If
X < 0, it is said to be capacitive.
This 4-hr course provides
a basic overview of Reactance theory and is based
entirely on Naval Education and Training
Materials (NAVEDTRA 14174),
Electricity and Electronic Training Series;
Module-2, Chapter 4
titled “Inductive and Capacitive Reactance”.
The
student must take a multiple-choice quiz consisting of twenty (20) questions at the
end of this course to obtain PDH credits.
Specific Knowledge or Skill Attained
This course teaches the
following specific knowledge and skills:
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The phase relationships
between current and voltage in an inductor and in a capacitor
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The terms
for the opposition an inductor and a capacitor offer to AC
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The formulas for
inductive and capacitive reactances
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The effects of a change
in frequency on inductive and capacitive reactances
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The effects of a change
in inductance on X L and a change in capacitance on XC
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How to compute total
reactance (X) in a series circuit and determine whether the total reactance is
capacitive or inductive
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The formula for total
reactance and impedance
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The Ohm's law formulas
used to determine voltage and current in an ac circuit
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Definition of "true
power", "reactive power", and "apparent power"
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The definition of and
the formula for power factor
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The
difference between calculating impedance in a series ac circuit and in a
parallel ac circuit.
Course
Click on the following
PDF attachment and review the document prior to taking
a quiz for credit.
Inductive and Capacitive Reactance (254 KB)
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