The Alternating current reference article from the English Wikipedia on 24-Jul-2004
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Alternating current

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An alternating current (AC) is an electrical current, where electrical charge oscillates (i.e., moves back and forth), rather than flowing continuously in one direction as is the case with direct current. The desired waveform of the oscillation is generally that of a perfect sine wave, as this results in the most efficient transmission of energy.

Table of contents
1 History
2 Distribution and Domestic Power Supply
3 AC frequencies by country
4 Mathematics of AC voltages
5 External links

History

Alternating-current electric power is a form of electrical energy that uses alternating currents to supply electricity commercially as electric power. William Stanley Jr designed one of the first practical coils to produce alternating currents. His design was an early precursor of the modern transformer, called an induction coil. From 1881 to 1889, the system used today was devised by Nikola Tesla, George Westinghouse, Lucien Gaulard, John Gibbs, and Oliver Shallenger. These systems overcame the limitations imposed by using direct current, as found in the system that Thomas Edison first used to distribute electricity commercially.

The first long-distance transmission of alternating current took place in 1891 near Telluride, Colorado, followed a few months later in Germany. Thomas Edison strongly advocated the use of direct current (DC), having many patents in that technology, but eventually alternating current came into general use (see War of Currents). Charles Proteus Steinmetz of General Electric solved many of the problems associated with electricity generation and transmission using alternating current.

Distribution and Domestic Power Supply

Unlike DC, AC can be stepped up by a transformer to a higher voltage. Because of Ohm's law, electrical energy losses are dependent on current flow, not on energy flow. By using transformers, the voltage of the power can be stepped up to a high voltage so that the power may be distributed over long distances at low currents and hence low losses. The voltage can then be stepped down again so that it is safe for domestic supply.

Three-phase electrical generation is very common and is a more efficient use of conductors. Three-phase electricity distribution is common only in industrial premises and many industrial electric motors are designed for it. Three current waveforms are produced that are 120 degrees out of phase with each other. At the load end of the circuit the return legs of the three phase circuits can be coupled together at the neutral point, where the three currents sum to zero. This means that the currents can be carried using only three cables, rather than the six that would otherwise be needed. Three phase power is a kind of polyphase system.

In many situations only a single phase is needed to supply street lights or residential consumers. When distributing three-phase electric power, a fourth or neutral cable is run in the street distribution to provide a complete circuit to each house. Different houses in the street are placed on different phases of the supply so that the load is balanced, or spread evenly, across the three phases when a lot of consumers are connected. Thus the supply cable to each house usually only consists of a live and neutral conductor with possibly an earthed armoured sheath.

For safety, a third wire is often connected between the individual electrical appliances in the house and the main electric switchboard or fusebox. The third wire is known in Britain and most other English-speaking countries as the earth wire, whereas in America it is the ground wire. At the main switchboard the earth wire is connected to the neutral wire and also connected to an earth stake or other convenient earthinging point (to Americans, the "grounding point") such as a water pipe. In the event of a fault, the earth wire can carry enough current to blow a fuse and isolate the faulty circuit. The earth connection also means that the surrounding building is at the same voltage as the neutral point. The commonest form of electrical shock occurs when a person accidentally forms a circuit between a live conductor and ground. A residual-current circuit breaker is designed to detect such a problem and break the circuit before electric shock causes death. As many parts of the neutral system are connected to the earth, balancing currents, known as earth currents, may flow between the generator and the consumer and other parts of the system, which are also earthed, to keep the neutral voltage at a safe level. This system of earthing the neutral point to balance the current flows for safety reasons is known as a multiple earth neutral system.

AC frequencies by country

Most countries in the world have standardised their electricity supply systems to one of two frequencies: 50 hertz or 60 hertz. The list of 60 hertz countries, most of them in the New World, is shorter, but this is not to say that 60 hertz is less common. The 60 hertz countries are: American Samoa, Antigua and Barbuda, Aruba, Bahamas, Belize, Bermuda, Canada, Cayman Islands, Colombia, Costa Rica, Cuba, Dominican Republic, El Salvador, French Polynesia, Guam, Guatemala, Guyana, Haiti, Honduras, South Korea, Liberia, Marshall Islands, Mexico, Micronesia, Montserrat, Nicaragua, Northern Mariana Islands, Palau, Panama, Peru, Philippines, Puerto Rico, Saint Kitts and Nevis, Suriname, Taiwan, Trinidad and Tobago, Turks and Caicos Islands, United States, Venezuela, Virgin Islands (U.S.), Wake Island.[1]

The following countries have a mixture of 50 Hz and 60 Hz supplies: Bahrain, Brazil (mostly 60 Hz), Japan (60 Hz used in provinces).[1]

Most countries have chosen their television standard to match their mains supply frequency. The NTSC standard was developed to work with 60 Hz mains, while PAL and SECAM were designed for 50 Hz mains, but 60 Hz versions of PAL also exist, e.g. in Brazil PAL-M, offering the high resolution of PAL and the low flicker of NTSC.

It is generally accepted that Nikola Tesla chose 60 hertz as the lowest frequency that would not cause street lighting to flicker visibly. The origin of the 50 hertz frequency used in other parts of the world is open to debate.

Mathematics of AC voltages

Alternating currents are usually associated with alternating voltages. An AC voltage v can be described mathematically as a function of time by the following equation:

where
A is the amplitude in volts (also called the peak voltage),
ω is the angular frequency in radians/second, and
t is the time in seconds.

Since angular frequency is of more interest to mathematicians than to engineers, this is commonly rewritten as:

where
f is the frequency in hertz.

The peak-to-peak value of an AC voltage is defined as the difference between its positive peak and its negative peak. Since the maximum value of sin(x) is +1 and the minimum value is -1, an AC voltage swings between +A and -A. The peak-to-peak voltage, written as VP-P, is therefore (+A)-(-A) = 2×A.

The size of an AC voltage is also sometimes stated as a root mean square (rms) value, written Vrms. For a sinusoidal voltage:

Vrms is useful in calculating the power consumed by a load. If a DC voltage of VDC delivers a certain power P into a given load, then an AC voltage of Vrms will deliver the same power P into the same load if Vrms = VDC.

To illustrate these concepts, consider the 240 V AC mains used in the UK. It is so called because its rms value is (at least nominally) 240 V. This means that it has the same heating effect as 240 V DC. To work out its peak voltage (amplitude), we can modify the above equation to:

For our 240 V AC, the peak voltage VP-P or A is therefore 240 V × √2 = 339 V (approx.). The peak-to-peak value of the 240 V AC mains is even higher: 2 × 240 V × √2 = 679 V (approx.)

External links