Volt-ampere

This page was last edited on 3 December 2017, at 05:36.

A volt-ampere (VA) is the unit used for the apparent power in an electrical circuit, equal to the product of root-mean-square (RMS) voltage and RMS current.[1] In direct current (DC) circuits, this product is equal to the real power (active power) [2] in watts. Volt-amperes are useful only in the context of alternating current (AC) circuits (sinusoidal voltages and currents of the same frequency).

With a purely resistive load, VA = Power in watts. Where a reactive (capacitive or inductive) component is present in the load, VA > Power (watts) 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 (UPSs), have ratings both for maximum volt-amperes and maximum watts. 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 load with a low power factor, neither limit may safely be exceeded.[3] For example, a (large) UPS system rated to deliver 400,000 volt-amperes at 220 volts can deliver a current of 1818 amperes.

VA ratings are also often used for transformers; maximum output current is then VA rating divided by nominal output voltage.[4] 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.[5]

Cmplxpower
Apparent power is the magnitude of the vector sum (S) of real (P) and reactive (jQ) AC power vectors

See also

References

  1. ^ Ciletti, M. D., Irwin, J. D., Kraus, A. D., Balabanian, N., Bickard, T. A., and Chan, S. P. (1993). Linear circuit analysis. In Electrical Engineering Handbook, edited by R. C. Dorf. Boca Raton: CRC Press. (pp.82–87)
  2. ^ IEEE 100 : the authoritative dictionary of IEEE standards terms.-7th ed. ISBN 0-7381-2601-2, page 23
  3. ^ Watt Ratings Differs From Volt Amp Ratings APC
  4. ^ "Transformers and VA Ratings". 2012-06-27. Retrieved 13 December 2016.
  5. ^ "SI Brochure" (8th ed.).

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