Electrical Fundamentals - Concepts of Alternating Current

A. Bhatia, B.E.

Course Outline

The electricity you get from your electric outlet is alternating current (AC). There are many appliances such as computer and TVs that actually work on DC while other electrical appliances, such as refrigerators, air-conditioners, lighting etc… can be designed for both AC and DC.

Since some kinds of loads only require DC to power them and others can easily operate on either AC or DC, the question naturally arises, "Why not dispense entirely with AC and just use DC for everything?" This question is augmented by the fact that in some ways AC is harder to handle as well as to use. Nevertheless, there is a very practical reason, which overrides all other considerations for a widely distributed power grid. It all boils down to a question of cost.

This 3-hr course material provides insight to the basic concepts of alternating current and is based entirely on Naval Education and Training Materials (NAVEDTRA 14173), Electricity and Electronic Training Series; Module-2 "Concepts of Alternating Current" and covers Chapter 1.

The course includes a multiple-choice quiz at the end, which is designed to enhance the understanding of course materials.

Learning Objective

At the conclusion of this course, the student will be able to:

• State the difference between AC and DC voltage and current;
• State the advantages of AC power transmission over DC power transmission;
• State the Left hand rule for a conductor;
• State the relationship between the current and magnetism;
• State the methods by which AC power can be generated;
• State the relationship between frequency, period, time and wavelength;
• Compute peak-to-peak, instantaneous, effective and average values of voltage and current;
• Compute the phase difference between sine waves; andExplain how to use Ohm's law in AC circuits.

Intended Audience

This course is aimed at students, professional engineers, service technicians, energy auditors, operational & maintenance personnel, facility engineers and general audience.

Course Introduction

AC stands for Alternating Current. The current flows in one direction for a period of time and then switches direction, going the opposite way. It switches direction over and over again continuously. In the United States the AC current in power lines goes switches direction, forward to backward, then backward to forward, 60 times each second. This is a frequency of 60 cycles and is called 60Hertz AC electricity.

The usual waveform of an AC power circuit is a sine wave, which results in the most efficient transmission of energy. However in certain applications different waveforms are used, such as triangular or square waves. The alternating voltage and current have a number of properties associated with any such waveform. The most important of these properties are frequency and amplitude, since some types of electrically powered equipment must be designed to match the frequency and voltage of the power lines. Wavelength is not generally important in this context, but becomes much more important when we start dealing with signals at considerably higher frequencies.

Course Content

In this course, you are required to study Naval Education and Training Materials (NAVEDTRA 14173), Electricity and Electronic Training Series; Module-2 "Concepts of Alternating Current" Chapter 1:

Concepts of Alternating Current (Chapter 1, NAVEDTRA 14173)

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Course Summary

Alternating current, or sine wave, is produced by an alternating voltage source that reaches a maximum in one direction (+), decreases to zero, reverses itself, and continues in the opposite direction until a maximum is reached. The cycle repeats continuously. The sine wave is the most common type of waveform. It is so named because it changes in value at the ramp rate, as the trigonometric function known as the sine.

AC transmission uses voltages ranging from 200 to 600 thousand volts. To meet customer demands the power company installs a transformer at different points along the AC power transmission line to lower the output voltage. Various potentials are involved. For homes and commercial buildings, it is lowered to the 220/120 volt level and for industrial use it is 220 volts and above. The AC voltage is usually transmitted at higher voltages which mean lower current for the same power, and less resistive loss. This is the major benefit AC provides over DC for large distance transmissions.

This course familiarizes you with the definitions of the following referenced terms.

• Amplitude = the magnitude of a wave and is represented by a vector arrow whose length indicates the magnitude and direction.
• Peak value = instantaneous maximum value for both the positive and negative alternations. This value may be considered as maximum signal amplitude. Peak value = one half peak-to-peak value.
• Peak-to-peak value = the value between positive and negative maximums of either voltage or current. It is twice the peak value of the same waveform. Vpp = peak x 2.
• Root mean square (rms) = the effective (DC) value equivalent of ac. Rms = 0.707 × peak value. Variations are peak = rms/0.707, and peak = rms x 1.414. AC voltages are always given in rms, and from this value peak and peak-to-peak values may be mathematically obtained. This value tells us how well a sine wave will do its job in terms of dc current. Since maximum values are instantaneous, ac voltage or current cannot supply the same power as these values if they were dc. However, 70.7% of the ac amplitude is available for this.
• Average value = the vector length of each 1 degree interval of either the positive or negative alternation. This calculates out to be 0.637. The formula for an ac or pulsating DC peak is: Vaverage = 0.637 x peak. The average of a complete cycle is zero, because the positive and negative averages cancel.
• Frequency = number of repetitions of a periodic wave in one second. Its symbol is f, and the unit of measure is hertz. Frequency is the reciprocal of time, where f (hertz) = 1/t. When time is known, frequency may be calculated.
• Period = the time, t (seconds) = 1/f, taken to complete one full cycle of a repeating waveform. A cycle is the change from zero to the positive peak, to zero, to the negative peak, and then to zero.
• Wavelength = physical length of one complete cycle measured in meters. Velocity/frequency determines length (lambda). Because electromagnetic waves travel at the speed of light in air, or at 300,000,000 meters per second, a high frequency means a short wavelength. Lambda = 3 x 108/f (Hz). For sound wave length, Lambda = 344.4 ms/f, since sound is much slower than electromagnetic waves.
• Phase relationship = angular relationship between two waves. With an AC circuit, it is normally between voltage and current. A phase shift is a phase angle change. It is between two points. Waveform phase differences are expressed in degrees of lead or lag.

Quiz

Once you finish studying the above course content, you need to take a quiz to obtain the PDH credits.

DISCLAIMER: The materials contained in the online course are not intended as a representation or warranty on the part of PDH Center or any other person/organization named herein. The materials are for general information only. They are not a substitute for competent professional advice. Application of this information to a specific project should be reviewed by a registered architect and/or professional engineer/surveyor. Anyone making use of the information set forth herein does so at their own risk and assumes any and all resulting liability arising therefrom.