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.
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.
Nikola Tesla The father of Alternating Current (AC).
AC generator produces an alternating voltage using the principles of electromagnetic induction.
( B ) No voltage is induced when the conductor is moved parallel to the lines of force.
( 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.
( D ) As the loop continues to rotate, fewer lines of force are cut and the induced voltage decreases.
( E ) Again, the maximum voltage is induced when the loop is at right angles to the lines of force.
( F ) As the loop returns to its original position, the induced voltage returns to zero.
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.
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).
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.
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.
Peak value The absolute value of the point on the waveform with the greatest amplitude.
Peak-to-peak value The vertical distance between two peaks.
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. E RMS = 0.707E p.
Period The time required to complete one cycle of a sine wave. Measured in seconds. The letter t is used to represent period.
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.
Nonsinusoidal waveforms Generated by specifically designed electronic circuits. Represent either current or voltage.
Square waveform
Triangular waveform
Sawtooth waveform
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.
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.
Square waveforms The fundamental frequency and all odd harmonics. Triangular waveforms The fundamental frequency and all odd harmonics, 180 degrees out of phase.
Sawtooth waveforms Composed of odd and even harmonics, with the even harmonics 180 degrees out of phase with the odd harmonics.
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
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
Nonsinusoidal Waveforms Square waves Triangular waveforms Sawtooth waveforms