Volt-amperes reactive

In alternating-current power transmission and distribution, volt-ampere reactive power (VAr) is a unit used to measure reactive power in an AC electric power system. 1 VAr = 1 V·A. [http://www.iec.ch/zone/si/si_elecmag.htm SI units – Electricity and magnetism] , IEC webpage.]

Electric power (watts) is transmitted by the simultaneous product of electric voltage and current in a wire. If large amounts of current are present when there is less voltage, the wires, transformers and other power equipment are heated, but less power is transmitted by the equipment. Since equipment is designed to remain cool up to a certain amount of current, VArs waste some of the power unnecessarily as excess heat

Since AC power has a varying voltage, efficient power systems must therefore vary the current in synchrony with the voltage. VArs measure unsynchronized "leading" or "lagging" currents. These currents are usually caused by the side effects of powering equipment that behaves like coils (e.g. motors) or capacitors (e.g. arc welders).

Technically, VArs are the product of the rms voltage and current, or the apparent power, multiplied by the sine of the phase angle between the voltage and the current. Or in mathematical terms, the reactive power $Q$, (measured in units of volt-amperes reactive or VAr), is given by

:$Q\; =\; V\_mathrm\{rms\}I\_mathrm\{rms\}sin\; left(phi\; ight),$,where $phi$ is the phase angle between the voltage and current.

The term "VAR" was introduced in 1930 by the IEC in Stockholm, which has adopted it as the unit for reactive power (name: var, symbol: var [and not VAr] , conversion factor: 1 var = 1 V·A)

"Note 1:" The unit VAr represents the power consumed by a reactive load, i.e. when there is a phase difference between the applied voltage and the current.

"Note 2:" Only effective power, i.e. the actual power delivered to or consumed by the load, is expressed in watts. Imaginary power is properly expressed in volt-amperes reactive.

"Note 3:" To maximize transmission efficiency, VArs must be minimized by balancing capacitive and inductive loads, or by the addition of an appropriate (off-setting) capacitive or inductive reactance to the load.

Source: from Federal Standard 1037C

"Note 4:" Apparent power, or the product of the rms voltage and current, measured in volt-amperes (VA), graphically presented as the hypotenuse of the power triangle, is typically the power rating measure of transmission or distribution equipment. Except in purely resistive circuits, apparent power is always larger than effective power (watts), hence the use of VA as the power rating measure in alternating circuits.

Notes and references