App-29
IM 253710-01E
Appendix
App
Appendix 7 Power Basics (Power/Harmonics/Three Constants Related to the AC Circuit)
Active power and the power factor
Not all of the power represented by the product of the voltage and current, UI, is
dissipated. The product, UI, is called the apparent power (S). The unit is VA (volt-
ampere). Apparent power is used to express the electrical capacity of the devices
that run under AC power.
Of the apparent power, the true power that is dissipated by the device is called the
active power or effective power (P). This power is the same as the AC power
described earlier.
S = UI [VA]
P = UIcos
φ
[W]
cos
φ
represents the ratio of the true power with respect to the apparent power and
this ratio is called the power factor (
λ
).
Reactive power
When the current I lags the voltage U by
φ
, the component of the power in the same
direction as the voltage U is expressed as Icos
φ
and the component of the power
perpendicular to the voltage U is expressed as Isin
φ
. The active power P=UIcos
φ
is
the product of the voltage U and the current component Icos
φ
. Conversely, the
product of the voltage U and the current component Isin
φ
is called reactive power (Q)
and its unit is var.
Q = UIsin
φ
[var]
φ
U
I
Icos
φ
Isin
φ
Apparent power S, active power P, and reactive power Q are related in the following
way.
S
2
= P
2
+ Q
2
