How To Calculate Active Power, Reactive Power, and Apparent Power? How Are They Related?
Active power: In an AC circuit, the active power at each instant is different and constantly changing. Generally, the average active power (the average value of the power in a cycle) is used to measure the energy consumption in the circuit.
For a single-phase AC circuit, the calculation formula is P=UIcosΦ
where:
P—active power (watts)
U—effective value of AC voltage (volts)
I—effective value of alternating current (A)
cosΦ—the power factor of the load
The unit of active power is watt (W) or kilowatt (KW), megawatt (MW).
When the load is pure resistance, the voltage and current phase are the same, Φ=0°, cosΦ=1, and the power consumed by the resistor is all active power (P=UI).
When the load is pure inductance or pure capacitance, the phase difference between voltage and current is Φ=90°, cosΦ=0, and active power P=0, so pure inductive or pure capacitive load does not consume active power.
Reactive power: In order to measure the situation of exchanging energy, people use the maximum value of power in the process of energy exchange (that is, the maximum value of instantaneous power) to represent reactive power.
According to the formula, the calculation formula of reactive power Q is: Q=UIsinΦ
In the formula: sinΦ—the sine value of the phase difference between the AC voltage and the current.
The unit of reactive power is var (var) or kilovar (Kvar), megavar (Mvar).
When the load is pure inductance or pure capacitance, Φ=90°, sinΦ=1. So Q=UI, that is, there is only reactive power and no active power is consumed. When the load is pure resistance, Φ=0°, sinΦ=0, so Q=0, that is, only active power is consumed without reactive power.
Apparent power: In a general AC circuit, there are both active and reactive components in the transmitted electrical power. Therefore, the product of the RMS voltage and the RMS current is neither active power nor reactive power, but their combined quantity. This combined quantity is called apparent power, represented by the letter S.
S＝UI
The unit of apparent power is volt-ampere (VA), or kilovolt-ampere (KVA), megavolt-ampere (MVA).
AC power generation equipment is designed and used according to the specified voltage and current, so sometimes it is more convenient to express the capacity of the equipment with apparent power. For example, the capacity of a transformer refers to its apparent power.
The quantitative relationship between apparent power S, active power P, and reactive power Q is exactly equivalent to the three-sided relationship of a right-angled triangle. S is equivalent to the hypotenuse, and P and Q are equivalent to two right-angled sides, which are called power triangles.
Its conversion formula is as follows:
S²＝Q²＋P²
cosΦ=P/S It can be seen that the power factor cosΦ can be defined as the ratio of the active power consumed by the load to its apparent power, which represents the proportion of the active power consumed by the load in the apparent power.
