App-20 IM 760301-01E
• Active Power and Power Factor
In alternating electrical current, not all of the power calculated by the product of
voltage and current UI is consumed. The product UI is called apparent power S. The
unit is VA (volt-ampere). 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 P (or effective power),
and this corresponds to the aforementioned AC power.
S = UI [VA]
P = UIcosφ [W]
cosφ indicates the ratio of the apparent power that becomes the true power, and this
is called power factor λ.
• 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 = UIcosφ is the product of voltage U and current component Icosφ. On the
contrary, the product of voltage U and current component Isinφ is called the reactive
power Q, and the unit is var.
Q = UIsinφ [var]
φ
U
I
Icosφ
Isinφ
Below is the relationship between the apparent power S, active power P, and reactive
power Q.
S
2
= P
2
+ Q
2
Appendix 4 Power Basics (Power, Harmonics, and RLC of AC Circuits)
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