YOKOGAWA WT500 User Manual page 337

IM 760201-01E
Appendix 3 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 = UIcos [W]
f
cos is called the power factor and is expressed as
f
apparent power that becomes true power.
Reactive Power
If current I lags voltage U by
same direction as voltage U, Icos
P, which is equal to UIcos , is the product of voltage U and the current component Icos
f
The product of voltage U and the current component Isin
is expressed as Q. The unit of reactive power is the var.
Q = UIsin [var]
f
U
Isinφ
Icosφ
I
φ
Below is the relationship between the apparent power, active power, and reactive power.
2 2 2
S = P + Q
. It indicates the portion of the
λ
, current I can be broken down into a component in the
f
, and a perpendicular component, Isin
f
f
. Active power
f
is called the reactive power. It
App-17
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App
Index