App-17
IM 760201-01E
Appendix
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App
Index
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
f
[W]
cos
f
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
f
, current I can be broken down into a component in the
same direction as voltage U, Icos
f
, and a perpendicular component, Isin
f
. Active power
P, which is equal to UIcos
f
, is the product of voltage U and the current component Icos
f
.
The product of voltage U and the current component Isin
f
is called the reactive power. It
is expressed as Q. The unit of reactive power is the var.
Q = UIsin
f
[var]
φ
U
I
Icos
φ
Isin
φ
Below is the relationship between the apparent power, active power, and reactive power.
S
2
= P
2
+ Q
2
Appendix 3 Power Basics (Power, harmonics, and AC RLC circuits)
