Alternating current

Alternating current
Zureks · Public domain · source
Name	Alternating current
Caption	A sine wave, the fundamental waveform of AC
Symbols	$\backslash sim$, AC

Alternating current. It is an electric current that periodically reverses direction and changes its magnitude continuously with time, in contrast to direct current which flows only in one direction. The usual waveform of alternating current in most electric power circuits is a sine wave, whose positive half-period corresponds with positive direction of the current and vice-versa. This form of electricity is used to deliver power to homes, businesses, and industry because it can be easily transformed to higher or lower voltages, enabling efficient long-distance transmission.

Overview

The primary characteristic of alternating current is its periodic oscillation, defined by its frequency and amplitude. In most of the world, including Europe and much of Asia, the standard frequency for power grids is 50 hertz, while in North America and parts of South America like Brazil, the standard is 60 hertz. The voltage level also varies by region, with common standards including 230 volts in the United Kingdom and 120 volts in the United States. The development of practical AC systems was a pivotal achievement of the late 19th century, largely driven by the work of inventors and engineers like Nikola Tesla, Mikhail Dolivo-Dobrovolsky, and George Westinghouse.

Generation and transmission

Alternating current is predominantly generated by electromechanical devices called alternators, which are driven by prime movers such as steam turbines in coal-fired power stations, gas turbines, or water turbines in hydroelectric plants. A key advantage of AC for power systems is the ability to use transformers to step voltage up to very high levels for transmission over long distances via high-voltage direct current or AC power transmission lines, significantly reducing energy losses compared to low-voltage DC. Major grid interconnections, such as the Eastern Interconnection in the U.S. or the Union for the Coordination of Transmission of Electricity in Europe, rely on this technology. The final distribution to consumers involves stepping the voltage down again through a series of substations and distribution transformers.

Mathematics of AC

A sinusoidal alternating current can be described mathematically by the equation $I(t)\; =\; I\_\backslash text\{peak\}\; \backslash sin(\backslash omega\; t)$, where $I\_\backslash text\{peak\}$ is the peak current and $\backslash omega$ is the angular frequency. Analysis of AC circuits often utilizes complex number representations, such as phasor calculus, pioneered by Charles Proteus Steinmetz of the General Electric company. Important concepts include root mean square voltage, which provides the equivalent heating effect of a DC voltage, and power factor, which relates the real power used in a circuit to the apparent power supplied. The behavior of AC in circuits containing inductors and capacitors is governed by differential equations, leading to phenomena like electrical resonance.

Applications

Beyond power distribution, alternating current is fundamental to the operation of the vast majority of electric motors, including the ubiquitous induction motor invented by Tesla. It is essential for the functioning of the transformers that power everything from mobile phone chargers to the National Grid. In electronics, AC signals form the basis of radio transmission, as demonstrated by Guglielmo Marconi, and are processed in devices like amplifiers and oscillators. The alternating current power supplied to homes runs major appliances like refrigerators and air conditioners, and is rectified to DC to power devices such as televisions and personal computers.

Comparison with direct current

The late 19th century featured a period of intense industrial competition known as the War of the Currents, primarily between the AC systems advocated by Westinghouse and Tesla and the DC systems championed by Thomas Edison and the Edison Electric Light Company. While DC is superior for applications requiring stable voltage, such as electrochemistry and most modern semiconductor electronics, AC historically won for large-scale power distribution due to the efficiency of voltage transformation. However, advances in power electronics have led to a resurgence of high-voltage DC for specific long-distance transmission projects, like the Pacific DC Intertie or interconnectors managed by ABB Group.

History

Early pioneers of alternating current systems include William Stanley Jr., who built an early practical transformer, and Lucien Gaulard and John Dixon Gibbs, who demonstrated AC power transmission. The pivotal moment came with the 1893 World's Columbian Exposition in Chicago, where the Westinghouse Electric & Manufacturing Company successfully lit the fair using AC, and the subsequent winning of the contract to harness Niagara Falls for hydroelectric power using Tesla's polyphase system. This established AC as the global standard, a decision later reinforced by the expansion of major utilities like the Tennessee Valley Authority and the construction of continent-spanning grids such as the Synchronous grid of Continental Europe.

Category:Electric power Category:Electrical engineering