An electric current that reverses direction in a circuit at regular intervals.
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An electric current that reverses direction in a circuit at regular intervals.
Electric current that reverses direction periodically, usually many times per second. Electrical energy is ordinarily generated by a public or a private utility organization and provided to a customer, whether industrial or domestic, as alternating current.
One complete period, with current flow first in one direction and then in the other, is called a cycle, and 60 cycles per second (60 hertz) is the customary frequency of alternation in the United States and in all of North America. In Europe and in many other parts of the world, 50 Hz is the standard frequency. On aircraft a higher frequency, often 400 Hz, is used to make possible lighter electrical machines.
When the term alternating current is used as an adjective, it is commonly abbreviated to ac, as in ac motor. Similarly, direct current as an adjective is abbreviated dc.
The voltage of an alternating current can be changed by a transformer. This simple, inexpensive, static device permits generation of electric power at moderate voltage, efficient transmission for many miles at high voltage, and distribution and consumption at a conveniently low voltage. With direct (unidirectional) current it is not possible to use a transformer to change voltage. On a few power lines, electric energy is transmitted for great distances as direct current, but the electric energy is generated as alternating current, transformed to a high voltage, then rectified to direct current and transmitted, then changed back to alternating current by an inverter, to be transformed down to a lower voltage for distribution and use.
In addition to permitting efficient transmission of energy, alternating current provides advantages in the design of generators and motors, and for some purposes gives better operating characteristics. Certain devices involving chokes and transformers could be operated only with difficulty, if at all, on direct current. Also, the operation of large switches (called circuit breakers) is facilitated because the instantaneous value of alternating current automatically becomes zero twice in each cycle and an opening circuit breaker need not interrupt the current but only prevent current from starting again after its instant of zero value.
Alternating current is shown diagrammatically in Fig. 1. In this diagram it is assumed that the current is alternating sinusoidally; that is, the current i is described by the equation below, where
Im is the maximum instantaneous current, f is the frequency in cycles per second (hertz), and t is the time in seconds. See also Sine wave.

Diagram of sinusoidal alternating current.
A sinusoidal form of current, or voltage, is usually approximated on practical power systems because the sinusoidal form results in less expensive construction and greater efficiency of operation of electric generators, transformers, motors, and other machines.
A useful measure of alternating current is found in the ability of the current to do work, and the amount of current is correspondingly defined as the square root of the average of the square of instantaneous current, the average being taken over an integer number of cycles. This value is known as the root-mean-square (rms) or effective current. It is measured in amperes. It is a useful measure for current of any frequency. The rms value of direct current is identical with its dc value. The rms value of sinusoidally alternating current is Im/
(see Fig. 1 and the equation). Other useful quantities are the phase difference ϕ between voltage and current and the power factor. See also Phase (periodic phenomena); Power factor.
The phase angle and power factor of voltage and current in a circuit that supplies a load are determined by the load. Thus a load of pure resistance, such as an electric heater, has unity power factor. An inductive load, such as an induction motor, has a power factor less than 1 and the current lags behind the applied voltage. A capacitive load, such as a bank of capacitors, also has a power factor less than 1, but the current leads the voltage, and the phase angle ϕ is a negative angle.
Three-phase systems are commonly used for generation, transmission, and distribution of electric power. A customer may be supplied with three-phase power, particularly if a large amount of power is used or the use of three-phase loads is desired. Small domestic customers are usually supplied with single-phase power. A three-phase system is essentially the same as three ordinary single-phase systems, with the three voltages of the three single-phase systems out of phase with each other by one-third of a cycle (120 degrees), as shown in Fig. 2. The three-phase system is balanced if the maximum voltage in each of the three phases is equal, and if the three phase angles are equal, ⅓ cycle each as shown. It is only necessary to have three wires for a three-phase system (a, b, and c of Fig. 3) plus a fourth wire n to serve as a common return or neutral conductor. On some systems the earth is used as the common or neutral conductor.

Voltages of a balanced three-phase system.

Connections of a simple three-phase system.
Each phase of a three-phase system carries current and conveys power and energy. If the three loads on the three phases of the three-phase system are equal and the voltages are balanced, then the currents are balanced also. The sum of the three currents is then zero at every instant. This means that current in the common conductor (n of Fig. 3) is always zero, and that the conductor could theoretically be omitted entirely. In practice, the three currents are not usually exactly balanced, and either of two situations obtains. Either the common neutral wire n is used, in which case it carries little current (and may be of high resistance compared to the other three line wires), or else the common neutral wire n is not used, only three line wires being installed, and the three phase currents are thereby forced to add to zero even though this requirement results in some imbalance of phase voltages at the load.
The total instantaneous power from generator to load is constant (does not vary with time) in a balanced, sinusoidal, three-phase system. This results in smoother operation and less vibration of motors and other ac devices. In addition, three-phase motors and generators are more economical than single-phase machines.
AC circuits are also used to convey information. An information circuit, such as telephone, radio, or control, employs varying voltage, current, waveform, frequency, and phase. Efficiency is often low, the chief requirement being to convey accurate information even though little of the transmitted power reaches the receiving end. For further consideration of the transmission of information See also Radio; Waveform.
An ideal power circuit should provide the customer with electric energy always available at unchanging voltage of constant waveform and frequency, the amount of current being determined by the customer's load. High efficiency is greatly desired. See also Capacitance; Circuit (electricity); Electric current; Electric filter; Electrical impedance; Electrical resistance; Inductance; Joule's law; Ohm's law; Resonance (alternating-current circuits).
An electric current in which the flow reverses periodically. (Compare direct current (DC).)
• In the United States, most household current is AC, going through sixty reversal cycles each second. Electric motors in household appliances are designed to work with current at this rate of reversal.
For more information on alternating current, visit Britannica.com.
An electric current that varies periodically in value and direction, first flowing in one direction in the circuit and then flowing in the opposite direction; each complete repetition is called a
An electric current that rises to a maximum in one direction, falls back to zero and then rises to a maximum in the opposite direction and then repeats.
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An alternating current (AC) is an electrical current whose magnitude and direction vary cyclically, as opposed to direct current, whose direction remains constant. The usual waveform of an AC power circuit is a sine wave, as this results in the most efficient transmission of energy. However in certain applications different waveforms are used, such as triangular or square waves.
Used generically, AC refers to the form in which electricity is delivered to businesses and residences. However, 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.
The first modern commercial power plant using 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. And through the use of alternating current, Charles Proteus Steinmetz of General Electric was able to solve many of the problems associated with electricity generation and transmission.
Alternating current circuit theory evolved rapidly in the latter part of the 19th and early 20th century. Notable contributors to the theoretical basis of alternating current calculations include Charles Steinmetz, James Clerk Maxwell, Oliver Heaviside, and many others. Calculations in unbalanced three-phase systems were simplified by the symmetrical components methods discussed by Charles Legeyt Fortescue in 1918.
AC voltage can be stepped up or down by a transformer to a different voltage. Modern high-voltage, direct-current electric power transmission systems contrast with the more common alternating-current systems as a means for the bulk transmission of electrical power over long distances. However, these tend to be more expensive and less efficient than transformers, and did not exist when Edison, Westinghouse and Tesla were designing their power systems.
Use of a higher voltage leads to significantly more efficient transmission of power. The power losses in a conductor are a
product of the square of the current and the resistance of the conductor,
described by the formula
. This means that when transmitting a fixed power on a given wire, if the current is doubled, the power
loss will be four times greater. Since the power transmitted is equal to the product of the current, the voltage and the
cosine of the phase difference φ (P =
IVcosφ), the same amount of power can be transmitted with a lower current by increasing the voltage.
Therefore it is advantageous when transmitting large amounts of power to distribute the power with high voltages (often hundreds
of kilovolts). However, high voltages also have disadvantages, the main ones being the increased insulation required, and
generally increased difficulty in their safe handling. In a
The utilization voltage delivered to equipment such as lighting and motor loads is standardized, with an allowable range of voltage over which equipment is expected to operate. Standard power utilization voltages and percentage tolerance vary in the different mains power systems found in the world.
Three-phase electrical generation is very common. Three separate coils in the generator stator are physically offset by an angle of 120° to each other. Three current waveforms are produced that are equal in magnitude and 120° out of phase to each other.
If the load on a three-phase system is balanced equally among the phases, no current flows through the neutral point. Even in the worst-case unbalanced (linear) load, the neutral current will not exceed the highest of the phase currents. For three-phase at low (normal mains) voltages a four-wire system is normally used. When stepping down three-phase, a transformer with a Delta primary and a Star secondary is often used so there is no need for a neutral on the supply side.
For smaller customers (just how small varies by country and age of the installation) only a single phase and the neutral or two phases and the neutral are taken to the property. For larger installations all three phases and the neutral are taken to the main distribution panel. From the three-phase main panel, both single and three-phase circuits may lead off.
Three-wire single phase systems, with a single centre-tapped transformer giving two live
conductors, is a common distribution scheme for residential and small commercial buildings in North America. This arrangement is
sometimes incorrectly referred to as "two phase". A similar method is used for a different reason on construction sites in the
UK. Small power tools and lighting are supposed to be supplied by a local center-tapped transformer with a voltage of 55V between
each power conductor and the earth. This significantly reduces the risk of
A third wire, called the bond wire, is often connected between non-current carrying metal enclosures and earth ground. This conductor provides protection from electrical shock due to accidental contact of circuit conductors with the metal chassis of portable appliances and tools. Bonding all non-current carrying metal parts into one complete system ensures there is always a low impedance path to ground sufficient to carry any fault current for as long as it takes for the system to clear the fault. This low impedance path allows the maximum amount of fault current to flow, causing the overcurrent protection device (Breakers, fuses) to trip or burn out as quickly as possible, returning the electrical system to a safe state. All bond wires are bonded to ground at the main service panel, as is the Neutral/Identified Conductor if present.
The frequency of the electrical system varies by country; most electric power is generated at either 50 or 60 Hz. See List of countries with mains power plugs, voltages and frequencies. Some countries have a mixture of 50 Hz and 60 Hz supplies, notably Japan.
A low frequency eases the design of low speed electric motors, particularly for hoisting, crushing and rolling applications, and commutator-type traction motors for applications such as railways, but also causes a noticeable flicker in incandescent lighting and objectionable flicker of fluorescent lamps. 16⅔ Hz power (one third of the mains frequency) is still used in some European rail systems, such as in Austria, Germany, Norway, Sweden and Switzerland.
The use of lower frequencies also provided the advantage of lower impedance losses, which are proportional to frequency. It was this argument that resulted in the use of 25 Hz in the initial generation installations at Niagara Falls, in anticipation of long-distance transmission to Toronto. Improvements in transmission technology later made it feasible to use 60 Hz as an acceptable, and more desirable, frequency. The conversion lasted into the 1950s, costing Ontario Hydro dearly as it replaced all clocks, refrigerators and other motor-driven appliances in the Toronto area.
Off-shore, textile industry, marine, computer mainframe, aircraft, and spacecraft applications sometimes use 400 Hz, for benefits of reduced weight of apparatus or higher motor speeds.
A direct, constant current flows uniformly throughout the cross-section of the (uniform) wire that carries it. With alternating current of any frequency, the current is forced towards the outer surface of the wire, and away from the center. This is due to the fact that an electric charge which accelerates (as is the case of an alternating current) radiates electromagnetic waves, and materials of high conductivity (the metal which makes up the wire) do not allow propagation of electromagnetic waves. This phenomenon is called skin effect.
At very high frequencies the current no longer flows in the wire, but effectively flows on the surface of the wire, within a thickness of a few skin depths. The skin depth is the thickness at which the current density is reduced by 63%. Even at relatively low frequencies used for high power transmission (50–60 Hz), non-uniform distribution of current still occurs in sufficiently thick conductors. For example, the skin depth of a copper conductor is approximately 8.57 mm at 60 Hz, so high current conductors are usually hollow to reduce their mass and cost.
Since the current tends to flow in the periphery of conductors, the effective cross-section of the conductor is reduced. This increases the effective AC resistance of the conductor, since resistance is inversely proportional to the cross-sectional area in which the current actually flows. The AC resistance often is many times higher than the DC resistance, causing a much higher energy loss due to ohmic heating (also called I2R loss).
For low to medium frequencies, conductors can be divided into stranded wires, each insulated from one other, and the individual strands specially arranged to change their relative position within the conductor bundle. Wire constructed using this technique is called Litz wire. This measure helps to partially mitigate skin effect by forcing more equal current flow throughout the total cross section of the stranded conductors. Litz wire is used for making high Q inductors, reducing losses in flexible conductors carrying very high currents at power frequencies, and in the windings of devices carrying higher radio frequency current (up to hundreds of kilohertz), such as switch-mode power supplies and radio frequency transformers.
As written above, an alternating current is made of electric charge under periodic acceleration, which causes radiation of electromagnetic waves. Energy that is radiated represents a loss. Depending on the frequency, different techniques are used to minimize the loss due to radiation.
At frequencies up to about 1 GHz, wires are paired together in cabling to form a twisted pair in order to reduce losses due to electromagnetic radiation and inductive coupling. A twisted pair must be used with a balanced signalling system, where the two wires carry equal but opposite currents. The result is that each wire in the twisted pair radiates a signal that is effectively cancelled by the other wire, resulting in almost no electromagnetic radiation.
At frequencies above 1 GHz, unshielded wires of practical dimensions lose too much energy to radiation, so coaxial cables are used instead. A coaxial cable has a conductive wire inside a conductive tube. The current flowing on the inner conductor is equal and opposite to the current flowing on the inner surface of the outer tube. This causes the electromagnetic field to be completely contained within the tube, and (ideally) no energy is radiated or coupled outside the tube. Coaxial cables have acceptably small losses for frequencies up to about 20 GHz. For microwave frequencies greater than 20 GHz, the dielectric losses (due mainly to the dissipation factor of the dielectric layer which separates the inner wire from the outer tube) become too large, making waveguides a more efficient medium for transmitting energy.
Waveguides are similar to coax cables, as both consist of tubes, with the biggest difference being that the waveguide has no inner conductor. Waveguides can have any arbitrary cross section, but rectangular cross sections are the most common. With waveguides, the energy is no longer carried by an electric current, but by a guided electromagnetic field. Waveguides have dimensions comparable to the wavelength of the alternating current to be transmitted, so are only feasible at microwave frequencies.
At frequencies greater than 200 GHz, waveguide dimensions become impractically small, and the ohmic losses in the waveguide walls become large. Instead, fiber optics, which are a form of dielectric waveguides, can be used. For such frequencies, the concepts of voltages and currents are no longer used.
Alternating currents are accompanied by alternating voltages. An AC voltage v can be described mathematically as a function of time by the following equation:
,where
is the peak voltage (unit: volt),
is the angular frequency (unit: radians per second)
, which represents the number of oscillations per second (unit = hertz), by the equation
.
is the time (unit: second).
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 Failed to parse (unknown function\displaystyle): \displaystyle \sin(x)
is +1 and the minimum value is −1, an AC voltage swings between Failed to parse (unknown function\displaystyle): \displaystyle+V_{\rm peak}
and Failed to parse (unknown function\displaystyle): \displaystyle-V_{\rm peak}
. The peak-to-peak voltage, usually written as Failed to parse (unknown function\displaystyle): \displaystyle V_{\rm pp}
or Failed to parse (unknown function\displaystyle): \displaystyle V_{\rm P-P}
, is therefore
.
The relationship between voltage and power is:
where Failed to parse (unknown function\displaystyle): \displaystyle R represents a load resistance
Rather than using instanteneous power, Failed to parse (unknown function\displaystyle): \displaystyle P\left(t\right) , it is more practical to use a time averaged power (where the averaging is performed over any integer number of cycles). Therefore, AC voltage is often expressed as a root mean square (RMS) value, written as Failed to parse (unknown function\displaystyle): \displaystyle V_{\rm rms} , because
For a sinusoidal voltage:

For non-sinusoidal waveforms, the relationship between Failed to parse (unknown function\displaystyle): \displaystyle V_\mathrm{rms}
and Failed to parse (unknown function\displaystyle): \displaystyle V_\mathrm{peak}
is different

To illustrate these concepts, consider a 240 V AC mains supply. It is so called because its RMS value is (at least nominally) 240 V. This means that the time-averaged power delivered is equivalent to the power delivered by a DC voltage of 240 Volts. To determine the peak voltage (amplitude), we can modify the above equation to:

For our 240 V AC, the peak voltage Vpeak is therefore Failed to parse (unknown function\displaystyle): \displaystyle 240 V \times\sqrt{2} , which is about 339 V. The peak-to-peak value Failed to parse (unknown function\displaystyle): \displaystyle V_{P-P}
of the 240 V AC is double that, at about 679 V.
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Dansk (Danish)
abbr. - vekselstrøm
abbr. - [kem.] actinium
abbr. - vekselstrøm
Nederlands (Dutch)
wisselstroom
Français (French)
abbr. - (abrév = account number) (Fin) numéro de compte
abbr. - (abrév = account) compte (bancaire)
abbr. - (abrév = alternating current) (Élec) courant alternatif
Deutsch (German)
abbr. - (wie AC) Wechselstrom
abbr. - (Chem.) Actinium , Azetat, (Meteor.) alto-cumulus
abbr. - Wechselstrom, Klimaanlage
Ελληνική (Greek)
abbr. - εναλλασσόμενο ρεύμα, (αερο)κλιματισμός, (χημ.) (το σύμβολο του στοιχείου) ακτίνιο
Italiano (Italian)
corrente alternata, Avanti Cristo
Português (Portuguese)
abbr. - corrente alternada (Eletr.)
Русский (Russian)
переменный ток
Español (Spanish)
abbr. - corriente alterna
abbr. - acetato, acetilo, actinio
abbr. - corriente alterna, antes de Jesucristo, aire acondicionado, club atlético, cuerpo del ejército
Svenska (Swedish)
abbr. - före Kristus, växelström
中文(简体) (Chinese (Simplified))
交流电, 装甲车
锕
交流电
中文(繁體) (Chinese (Traditional))
abbr. - 交流電, 裝甲車
abbr. - 錒
abbr. - 交流電
한국어 (Korean)
abbr. - alternating current(교류)
abbr. - actinium(악티늄)
abbr. - Athletic Club(체육 클럽), Atlantic Charter(대서양 헌장),
日本語 (Japanese)
abbr. - 航空整備兵, 交流, 陸軍航空隊, 当座預金, スポーツクラブ, 航空隊, 自動車クラブ, 著者訂正, 航空士官候補生
العربيه (Arabic)
(اختصار) مختصر معناه تيار متناوب
עברית (Hebrew)
abbr. - זרם חילופין
abbr. - אקטיניום (יסוד)
abbr. - זרם חילופין, לפני הספירה, איש צוות אוויר, מיזוג אוויר
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