YOKOGAWA WT1800 User Manual page 122

Appendix 2 Power Basics (Power, harmonics, and AC RLC circuits)
Active Power and the Power Factor
In alternating electrical current, not all of the power calculated by the product of voltage and current,
UI, is consumed. The product of U and I is called the apparent power. It is expressed as S. The unit of
apparent power is the volt-ampere (VA). The apparent power is used to express the electrical capacity
of a device that runs on AC electricity.
The true power that a device consumes is called active power (or effective power). It is expressed as P.
This power corresponds to the AC power discussed in the previous section.
S = UI [VA]
P = UIcosΦ [W]
cosΦ is called the power factor and is expressed as λ. It indicates the portion of the apparent power
that becomes true power.
Reactive Power
If current I lags voltage U by Φ, current I can be broken down into a component in the same direction
as voltage U, IcosΦ, and a perpendicular component, IsinΦ. Active power P, which is equal to UIcosΦ,
is the product of voltage U and the current component IcosΦ. The product of voltage U and the current
component IsinΦ is called the reactive power. It is expressed as Q. The unit of reactive power is the
var.
Q = UIsinΦ [var]
U
Isin Φ
Icos Φ
Φ
The relationship between S, the apparent power, P, the active power, and Q, the reactive power is as
follows:
2
S = P
App-16
I
2 2
+ Q
IM WT1801-03EN