Section 2

AC Circuits

Chapter 12

Alternating Current

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• 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