Overview

Reactive power (Q) is a term for the imaginary (non-real) power from inductive loads like motor or capacitive loads (less common). It is generally measured in units of VARs (volt-amps reactive). Sometimes reactive power is reported in units of watts; this isn’t exactly correct, but not all devices or software offer units of VARs. If reactive power is reported in watts, the conversion from watts to VARs is one-to-one. The reactive power is NOT included in the real or active power and energy measurements of the WattNode meters. WattNode meters that report reactive power measure the “fundamental reactive power”, which does not include reactive harmonics.

  • Positive reactive power is caused by inductive loads such as motors and transformers (especially at low loads).
  • Negative reactive power is caused by capacitive loads. This can include lighting ballasts, variable speed drives for motors, computer equipment, and inverters (especially when idle).
Note: some manufacturers use the opposite sign convention and treat negative reactive power as inductive.

See Also

Definitions

…there is no agreement in the scientific community on the concept of reactive power in nonsinusoidal conditions. In fact, in the presence of harmonics in voltages and/or currents, the conventional definition of reactive power is not meaningful any more.” —Antonio Cataliotti, IEEE Transactions On Power Delivery, vol. 23, no. 3, July 2008

There are many competing definitions for reactive power including the following (named after the original authors):

  • Budeanu
  • Fryze
  • Kusters and Moore
  • Shepherd and Zakikhani
  • Sharon / Czarnecki
  • IEEE working group
  • (from Wikipedia article on Volt-Ampere Reactive) 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.
Q = V_{rms}I_{rms}sin(phi)

Reactive Power of Various Loads

  • Motor (no VSD): the reactive power will be positive and will range from roughly the same as the real power for a fully loaded motor to several times the real power for a lightly loaded motor. The power factor of an induction motor varies depending on load:
Motor Load, % Power Factor
0 0.17
25 0.55
50 0.73
75 0.80
100 0.85
  • Motor (with VSD): the reactive power will be small and generally negative. The displacement power factor will generally be 0.9 or higher.
  • Fluorescent Lamps: the power factor of older fixture with magnetic ballasts can range from 0.38 to 0.58. Modern electronic ballasts with power factor correction can exceed 0.98.
  • Gas Discharge Lamps:with magnetic ballasts can range from 0.4 to 0.6 and electronic ballasts with power factor correction can exceed 0.95.
  • Incandescent Lamps: the reactive power is roughly –10% of real power, resulting in a power factor around 0.995. We believe this is due to filament heating and cooling during the AC cycle.
  • Incandescent Lamps with Dimmer: the reactive power varies from near zero to a positive reactive value nearly equal to the real power. The power factor varies from near 1.0 to 0.74.

Keywords: kVAR