Alternating current

City lights viewed in a motion blurred exposure. The AC blinking causes the lines to be dotted rather than continuous.

An alternating current (AC) is an electric current of which magnitude and direction vary, unlike direct current, whose direction remains constant.

The usual waveform of an AC power circuit is a sine wave, because this leads to the most efficient transmission of energy. However, in certain applications different waveforms are used, such as triangular or square waves.

When one speaks of alternating current one mostly refers to the form in which electricity is delivered to businesses and residences. But audio and radio signals carried on electrical wire are also examples of alternating current. In these applications, an important goal is often the recovery of information encoded (or modulated) onto the AC signal.

History

William Stanley, Jr. designed one of the first practical devices to transfer AC power efficiently between isolated circuits. Using pairs of coils wound on a common iron core, his design, called an induction coil, was an early precursor of the modern transformer. The system used today was devised in the late nineteenth century, largely by Nikola Tesla. Contributions were also made by George Westinghouse, Lucien Gaulard, John Dixon Gibbs,Wilhelm Siemens and Oliver Shallenger. AC systems overcame the limitations of the direct current system used by Thomas Edison to distribute electricity efficiently over long distances.

The first modern commercial power plant that used three-phase alternating current was at the Mill Creek hydroelectric plant near Redlands, California in 1893. Its designer was Almirian Decker, a brilliant young engineer. Decker's innovative design incorporated 10,000 volt three phase transmission and established the standards for the complete system of generation, transmission and motors used today.

Further reading

• Willam A. Meyers, History and Reflections on the Way Things Were: Mill Creek Power Plant - Making History with AC, IEEE Power Engineering Review, February 1997, Pages 22-24

Other pages

• Direct current
• Electricity

Other websites

• "AC/DC: What's the Difference?". Edison's Miracle of Light, American Experience. (PBS)
• "AC/DC: Inside the AC Generator". Edison's Miracle of Light, American Experience. (PBS)
• Kuphaldt, Tony R., "Lessons In Electric Circuits : Volume II - AC". March 8, 2003. (Design Science License)
• Nave, C. R., "Alternating Current Circuits Concepts". HyperPhysics.
• "Alternating Current (AC)". Magnetic Particle Inspection, Nondestructive Testing Encyclopedia.
• "Alternating current". Analog Process Control Services.
• Hiob, Eric, "An Application of Trigonometry and Vectors to Alternating Current". British Columbia Institute of Technology, 2004.
• "Introduction to alternating current and transformers". Integrated Publishing.
• "Wind Energy Reference Manual Part 4: Electricity". Danish Wind Industry Association, 2003.
• Chan. Keelin, "Alternating current Tools". JC Physics, 2002.
• "Measurement -> ac". Analog Process Control Services.
• Williams, Trip "Kingpin", "Understanding Alternating Current, Some more power concepts".
• "Table of Voltage, Frequency, TV Broadcasting system, Radio Broadcasting, by Country".
• Professor Mark Csele's tour of the 25 Hz Rankine generating station
• 50/60 hertz information
• AC circuits Animations and explanations of vector (phasor) representation of RLC circuits
• Blalock, Thomas J., "The Frequency Changer Era: Interconnecting Systems of Varying Cycles". The history of various frequencies and interconversion schemes in the US at the beginning of the 20th century