Section 2
AC Circuits
Chapter 12
Alternating Current
• Objectives
– After completing this chapter, the student
should be able to:
• Describe how an AC voltage is produced with an
AC generator.
• Define alternation, cycle, hertz, sine wave, period,
and frequency.
• Identify the parts of an AC generator.
• Define peak, peak-to-peak, effective, and rms.
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• Explain the relationship between time and
frequency.
• Identify and describe three basic nonsinusoidal
waveforms.
• Describe how nonsinusoidal waveforms consist of
the fundamental frequency and harmonics.
• Understand why AC is used in today’s society.
• Describe how an AC distribution system works.
• Identify and use the math associated with AC.
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• Nikola Tesla
– The father of Alternating Current (AC).
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• AC generator produces an alternating
voltage using the principles of
electromagnetic induction.
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( B ) No voltage
is induced when
the conductor is
moved parallel to
the lines of force.
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( C ) As the loop is
rotated, it passes
through more
lines of force, and
the maximum
voltage is induced
when the loop is
at right angles to
the lines of force.
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( D ) As the loop
continues to
rotate, fewer
lines of force are
cut and the
induced voltage
decreases.
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( E ) Again, the
maximum voltage
is induced when
the loop is at right
angles to the lines
of force.
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( F ) As the loop
returns to its
original position,
the induced
voltage returns to
zero.
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• Cycle
– Each time the AC generator completes one
revolution.
– Its output voltage is referred to as one cycle of
output voltage.
– It produces one cycle of output current in a
complete circuit.
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– The two halves of a cycle are called
alternations.
– Two complete alternations make up a cycle.
– One cycle per second is called a hertz (Hz).
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• Armature
– The rotating loop of wire.
• Sinusoidal waveform
– Also called a sine wave.
– The waveform produced by an AC generator.
– Can be produced by mechanical and electronic
methods.
– Identical to the trigonometric sine function.
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• AC values
– Each point on a sine wave has two numbers
associated with it.
• The degree of rotation.
– The angle to which the armature has turned.
• The amplitude.
– The maximum departure of the value of an alternating
current or wave from the average value.
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• Peak value
– The absolute value of the point on the
waveform with the greatest amplitude.
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• Peak-to-peak value
– The vertical distance between two peaks.
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• Effective value
– The amount that produces the same degree of
heat in a given resistance as an equal amount of
direct current.
– Can be determined by the root-mean-square
(rms) process.
– Also called the rms value.
– Erms = 0.707Ep.
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• Period
– The time required to complete one cycle of a
sine wave.
– Measured in seconds.
– The letter t is used to represent period.
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• Frequency
– The number of cycles that occur in a specific
period of time.
– Expressed in terms of cycles per second.
– Unit of frequency is called a hertz.
– One hertz equals one cycle per second.
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• Nonsinusoidal waveforms
– Generated by specifically designed electronic
circuits.
– Represent either current or voltage.
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• Square waveform
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• Triangular waveform
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• Sawtooth waveform
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• Square waveform
– Useful as an electronic signal because its
characteristics are easily changed.
• Triangular Waveform
– Used primarily as electronic signals.
• Sawtooth Waveform
– Used to sweep the electron beam across the
screen, creating an image, as in television sets.
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• Fundamental frequency
– Represents the repetition rate of the waveform.
• Harmonics
– Higher frequency sine waves that are exact
multiples of the fundamental frequency.
• Odd harmonics are odd multiples of the fundamental
frequency.
• Even harmonics are even multiples of the
fundamental frequency.
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• Square waveforms
– The fundamental frequency and all odd
harmonics.
• Triangular waveforms
– The fundamental frequency and all odd
harmonics, 180 degrees out of phase.
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• Sawtooth waveforms
– Composed of odd and even harmonics, with the
even harmonics 180 degrees out of phase with
the odd harmonics.
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• In Summary
– AC is the most commonly used type of
electricity.
– Cycle
– Alternations
– Hertz
– Sinusoidal waveform or sine wave
– Peak value of a sine wave
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–
–
–
–
–
–
Effective value of AC
Determine effective value by rms process.
The rms value of a sine wave
Period (t)
Frequency (f)
The relationship between frequency and period
is: f = 1/t
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– Nonsinusoidal Waveforms
• Square waves
• Triangular waveforms
• Sawtooth waveforms
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