Movatterモバイル変換

[0]ホーム

Jump to content

Volt-ampere

Edit links

From Wikipedia, the free encyclopedia

SI unit of apparent power in an electrical circuit

This articlemay be too technical for most readers to understand. Pleasehelp improve it tomake it understandable to non-experts, without removing the technical details.(December 2019) (Learn how and when to remove this message)

volt-ampere
Apparent power ( $\|S\|$ ) is the magnitude of the vector sum ofreal power vector ( $P {\displaystyle P}$ ) andreactive power vector ( $Q {\displaystyle Q}$ ); $\varphi$ is the phase shift between voltage and current.
General information
Unit system	SI units
Unit of	Apparent power
Symbol	V⋅A
Conversions
1 V⋅Ain ...	... is equal to ...

SI base units	1 kg⋅m²⋅s⁻³

Thevolt-ampere (SI symbol:VA,^[1] sometimesV⋅A orV A) is theunit of measurement forapparent power in anelectrical circuit. It is the product of theroot mean square voltage (involts) and theroot mean square current (inamperes).^[2] Volt-amperes are usually used for analyzingalternating current (AC) circuits. Indirect current (DC) circuits, this product is equal to thereal power, measured inwatts.^[3] The volt-ampere is dimensionally equivalent to thewatt: inSI units, 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

[edit]

For a simpleelectrical circuit running ondirect current, the electrical current and voltage are constant. In that case, thereal power (P, measured inwatts) is the product of the current (I, measured inamperes) and the voltage from one side of the circuit to the other (V, measured involts):

P=I\cdot V

Foralternating current, 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 (pk) androot mean square (rms)):

P_{\text{av}}={I_{\text{pk}}\cdot V_{\text{pk}} \over 2}=I_{\text{rms}}\cdot V_{\text{rms}}

More generally, when voltage and current are not in phase, these products no longer represent average power but a new apparent power, measured in volt-amperes:^[4]

|S|=I_{\text{rms}}\cdot V_{\text{rms}}

Usage

[edit]

The relationship between real power and apparent power is described by thepower factor. 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, includinguninterruptible power supplies (UPSs), have ratings both for maximum volt-amperes and maximum watts. A commonprefixed 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 areactive load with a low power factor, neither limit may safely be exceeded.^[5] 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.^[6] 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.^[7]^{[page needed]}

Volt-ampere reactive

[edit]

In electric powertransmission anddistribution,volt-ampere reactive (var) is a unit of measurement ofreactive power. Reactive power exists in an AC circuit when the current and voltage are not in phase. The termvar was proposed by the Romanianelectrical engineer Constantin Budeanu and introduced in 1930 by theIEC inStockholm, which has adopted it as the unit forreactive power.

Special instruments calledvarmeters are available to measure the reactive power in a circuit.^[8]

The unit "var" is allowed by theInternational System of Units (SI) even though the unit var is representative of a form of power.^[9] Per EUdirective 80/181/EEC (the "metric directive"), the correct symbol is lower-case "var",^[1] although the spellings "Var" and "VAr" are commonly seen, and "VAR" is widely used throughout the power industry.

References

[edit]

^^a ^bCouncil 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."
^Ciletti, M. D., Irwin, J. D., Kraus, A. D., Balabanian, N., Bickard, T. A., and Chan, S. P. (1993). Linear circuit analysis. InElectrical Engineering Handbook, edited by R. C. Dorf. Boca Raton: CRC Press. (pp.82–87)
^IEEE 100 : the authoritative dictionary of IEEE standards terms.-7th ed.ISBN 0-7381-2601-2, page 14
^Schavemaker, P.; van der Sluis, L. (2008),Electrical Power System Essentials, Chichester, England ; Hoboken, NJ: Wiley, p. 35,ISBN 978-0-470-51027-8,OCLC 193911699
^Watt Ratings Differs from Volt Amp Ratings APC
^"Transformers and VA Ratings". 2012-06-27. Retrieved13 December 2016.
^"SI Brochure"(PDF) (8th ed.). Archived fromthe original on March 29, 2019.
^Wildi, Theodore (2002).Electrical Machines, Drives and Power Systems. Pearson. p. 137.ISBN 978-0-13-093083-5.
^SI Brochure, 8th ed.

This article incorporatespublic domain material fromFederal Standard 1037C.General Services Administration. Archived fromthe original on 2022-01-22.

Retrieved from "https://en.wikipedia.org/w/index.php?title=Volt-ampere&oldid=1276516812"

Categories:

Hidden categories:

[8]ページ先頭