Megavolt Ampere
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The volt-ampere ( SI symbol: VA, sometimes V⋅A or V A) is the
unit of measurement A unit of measurement, or unit of measure, is a definite magnitude (mathematics), magnitude of a quantity, defined and adopted by convention or by law, that is used as a standard for measurement of the same kind of quantity. Any other qua ...
for
apparent power In an electric circuit, instantaneous power is the time rate of flow of energy past a given point of the circuit. In alternating current circuits, energy storage elements such as inductors and capacitors may result in periodic reversals of the d ...
in an
electrical circuit An electrical network is an interconnection of electrical components (e.g., battery (electricity), batteries, resistors, inductors, capacitors, switches, transistors) or a model of such an interconnection, consisting of electrical elements (e. ...
. It is the product of the root mean square voltage (in
volts The volt (symbol: V) is the unit of electric potential, electric potential difference (voltage), and electromotive force in the International System of Units (SI). Definition One volt is defined as the electric potential between two point ...
) and the root mean square current (in
amperes The ampere ( , ; symbol: A), often shortened to amp,SI supports only the use of symbols and deprecates the use of abbreviations for units. is the unit of electric current in the International System of Units (SI). One ampere is equal to 1 c ...
). Volt-amperes are usually used for analyzing
alternating current Alternating current (AC) is an electric current that periodically reverses direction and changes its magnitude continuously with time, in contrast to direct current (DC), which flows only in one direction. Alternating current is the form in w ...
(AC) circuits. In
direct current Direct current (DC) is one-directional electric current, flow of electric charge. An electrochemical cell is a prime example of DC power. Direct current may flow through a conductor (material), conductor such as a wire, but can also flow throug ...
(DC) circuits, this product is equal to the
real power In an electric circuit, instantaneous power is the time rate of flow of energy past a given point of the circuit. In alternating current circuits, energy storage elements such as inductors and capacitors may result in periodic reversals of the ...
, measured in
watt The watt (symbol: W) is the unit of Power (physics), power or radiant flux in the International System of Units (SI), equal to 1 joule per second or 1 kg⋅m2⋅s−3. It is used to quantification (science), quantify the rate of Work ...
s. The volt-ampere is dimensionally equivalent to the
watt The watt (symbol: W) is the unit of Power (physics), power or radiant flux in the International System of Units (SI), equal to 1 joule per second or 1 kg⋅m2⋅s−3. It is used to quantification (science), quantify the rate of Work ...
: in
SI units The International System of Units, internationally known by the abbreviation SI (from French ), is the modern form of the metric system and the world's most widely used system of measurement. It is the only system of measurement with official st ...
, 1 V⋅A = 1 W. VA rating is most used for generators and transformers, and other power handling equipment, where loads may be reactive (inductive or capacitive).


Formulation

For a simple
electrical circuit An electrical network is an interconnection of electrical components (e.g., battery (electricity), batteries, resistors, inductors, capacitors, switches, transistors) or a model of such an interconnection, consisting of electrical elements (e. ...
running on
direct current Direct current (DC) is one-directional electric current, flow of electric charge. An electrochemical cell is a prime example of DC power. Direct current may flow through a conductor (material), conductor such as a wire, but can also flow throug ...
, the electrical current and voltage are constant. In that case, the
real power In an electric circuit, instantaneous power is the time rate of flow of energy past a given point of the circuit. In alternating current circuits, energy storage elements such as inductors and capacitors may result in periodic reversals of the ...
(''P'', measured in
watt The watt (symbol: W) is the unit of Power (physics), power or radiant flux in the International System of Units (SI), equal to 1 joule per second or 1 kg⋅m2⋅s−3. It is used to quantification (science), quantify the rate of Work ...
s) is the product of the current (''I'', measured in
amperes The ampere ( , ; symbol: A), often shortened to amp,SI supports only the use of symbols and deprecates the use of abbreviations for units. is the unit of electric current in the International System of Units (SI). One ampere is equal to 1 c ...
) and the voltage from one side of the circuit to the other (''V'', measured in
volts The volt (symbol: V) is the unit of electric potential, electric potential difference (voltage), and electromotive force in the International System of Units (SI). Definition One volt is defined as the electric potential between two point ...
): :P = I \cdot V For
alternating current Alternating current (AC) is an electric current that periodically reverses direction and changes its magnitude continuously with time, in contrast to direct current (DC), which flows only in one direction. Alternating current is the form in w ...
, both the voltage and current are oscillating. Instantaneous power is still the product of instantaneous current and instantaneous voltage, but if both of those are ideal sine waves driving a purely resistive load (like an incandescent light bulb), average power becomes (with subscripts designating average (av),
peak amplitude The amplitude of a periodic variable is a measure of its change in a single period (such as time or spatial period). The amplitude of a non-periodic signal is its magnitude compared with a reference value. There are various definitions of amp ...
(pk) and
root mean square In mathematics, the root mean square (abbrev. RMS, or rms) of a set of values is the square root of the set's mean square. Given a set x_i, its RMS is denoted as either x_\mathrm or \mathrm_x. The RMS is also known as the quadratic mean (denote ...
(rms)): :P_\text = = I_\text \cdot V_\text More generally, when voltage and current are not in phase, these products no longer represent average power but a new apparent power, measured in volt-amperes: :, S, = I_\text \cdot V_\text


Usage

The relationship between real power and apparent power is described by the
power factor In electrical engineering, the power factor of an AC power system is defined as the ratio of the ''real power'' absorbed by the electrical load, load to the ''apparent power'' flowing in the circuit. Real power is the average of the instantaneou ...
. With a purely resistive load, they are the same: the apparent power is equal to the real power. Where a reactive (capacitive or inductive) component is present in the load, the apparent power is greater than the real power as voltage and current are no longer in phase. In the limiting case of a purely reactive load, current is drawn but no power is dissipated in the load. Some devices, including
uninterruptible power supplies An uninterruptible power supply (UPS) or uninterruptible power source is a type of continual power system that provides automated backup electric power to a load when the input power source or mains power fails. A UPS differs from a tradition ...
(UPSs), have ratings both for maximum volt-amperes and maximum watts. A common prefixed derived unit is "kilovolt-ampere" (symbol kVA). The VA rating is limited by the maximum permissible current, and the watt rating by the power-handling capacity of the device. When a UPS powers equipment which presents a
reactive Reactive may refer to: *Generally, capable of having a reaction (disambiguation) *An adjective abbreviation denoting a bowling ball coverstock made of reactive resin *Reactivity (chemistry) *Reactive mind *Reactive programming See also *Reactanc ...
load with a low power factor, neither limit may safely be exceeded. For example, a (large) UPS system rated to deliver 400,000 volt-amperes (400 kVA) at 220 volts can deliver a current of 1818 amperes (these are RMS values). VA ratings are also often used for transformers; maximum output current is then VA rating divided by nominal output voltage. Transformers with the same sized core usually have the same VA rating. The convention of using the volt-ampere to distinguish apparent power from real power is allowed by the SI standard.


Volt-ampere reactive

In electric power
transmission Transmission or transmit may refer to: Science and technology * Power transmission ** Electric power transmission ** Transmission (mechanical device), technology that allows controlled application of power *** Automatic transmission *** Manual tra ...
and
distribution Distribution may refer to: Mathematics *Distribution (mathematics), generalized functions used to formulate solutions of partial differential equations *Probability distribution, the probability of a particular value or value range of a varia ...
, volt-ampere reactive (var) is a unit of measurement of
reactive power In an electric circuit, instantaneous power is the time rate of flow of energy past a given point of the circuit. In alternating current circuits, energy storage elements such as inductors and capacitors may result in periodic reversals of the ...
. Reactive power exists in an AC circuit when the current and voltage are not in phase. The term ''var'' was proposed by the Romanian
electrical engineer Electrical engineering is an engineering discipline concerned with the study, design, and application of equipment, devices, and systems that use electricity, electronics, and electromagnetism. It emerged as an identifiable occupation in the l ...
Constantin Budeanu Constantin Budeanu (28 February 1886 – 27 February 1959) was a Romanian electrical engineer who contributed to the analysis of electric networks states and the SI system of units. Life and work He was born in Buzău. In 1903 he was admitted ...
and introduced in 1930 by the
IEC The International Electrotechnical Commission (IEC; ) is an international standards organization that prepares and publishes international standards for all electrical, electronic and related technologies. IEC standards cover a vast range of ...
in
Stockholm Stockholm (; ) is the Capital city, capital and List of urban areas in Sweden by population, most populous city of Sweden, as well as the List of urban areas in the Nordic countries, largest urban area in the Nordic countries. Approximately ...
, which has adopted it as the unit for
reactive Reactive may refer to: *Generally, capable of having a reaction (disambiguation) *An adjective abbreviation denoting a bowling ball coverstock made of reactive resin *Reactivity (chemistry) *Reactive mind *Reactive programming See also *Reactanc ...
power. Special instruments called ''varmeters'' are available to measure the reactive power in a circuit. The unit "var" is allowed by the
International System of Units The International System of Units, internationally known by the abbreviation SI (from French ), is the modern form of the metric system and the world's most widely used system of measurement. It is the only system of measurement with official s ...
(SI) even though the unit var is representative of a form of power.SI Brochure, 8th ed.
/ref> Per EU
directive 80/181/EEC As of 2009, the European Union had issued two units of measurement directives. In 1971, it issued Directive 71/354/EEC, which required EU member states to standardise on the International System of Units (SI) rather than use a variety of CGS ...
(the "metric directive"), the correct symbol is lower-case "var",Council Directive on units of measurements 80/181/EEC
Chapter 1.2.3., p. 6: "Special names for the unit of power: the name volt–ampere (symbol ‘VA’) when it is used to express the apparent power of alternating electric current, and var (symbol ‘var’) when it is used to express reactive electric power."
although the spellings "Var" and "VAr" are commonly seen, and "VAR" is widely used throughout the power industry.


References

* {{FS1037C Electricity Units of power