Introduction And Measuring of Active Power And Reactive Power

What is active power?

Active power is the power to make the machine working, in another word, the electric power to transfer electric energy to other mode energy, such as mechanical energy, optical energy, heat energy, etc.

For example, 5.5kW motor can transfer 5.5kW electric energy to mechanical energy so as to make water pump to pump water or make thresher threshing.

What is reactive power?

Reactive power is a little abstract, it is mainly used in the exchange between electric field and magnetic field in the electric circuit, also establishing and maintaining magnetic field in the electrical equipment. It doesn’t make any power externally, but transform to other mode energy. Electrical equipment with electromagnetic coils needs to consume reactive power to establish magnetic field.

For example, 40W fluorescent lamp needs not only over 40W active power to make light (ballast also needs to consume parts of active power), but also almost 80Var reactive power to make the ballast’s coil establish alternating magnetic field.

Relationship between active power and reactive power

The rotating magnetic field needed by the motor running is established and maintained by reactive power. It can make motor rotate, and make the machine running so that mechanical energy is transformed by electrical energy.

Transformer working also needs reactive power to make primary coil generate magnetic field, secondary coil induces a voltage.

In summary, all electrical equipment with electromagnetic coil need to establish magnetic field to work, whereas the consuming energy to establish and maintain magnetic field comes from the reactive power.

Without reactive power, motor can’t rotate, transformer can’t work, electric reactor and relay can’t work either. If the magnetic field in the electrical equipment can’t be established, the equipment can’t work either.

Thus besides active power, the power system also needs to supply reactive power to make electrical equipment run.

SA2200 Power quality analyzer has the active power and reactive power measuring function, which will help users to calculate and reduce the power consumption. Please kindly check the measuring data as below:

Output signal: frequency 50Hz, voltage 230V, current 500A, current phase lagging voltage phase 30°(inductive load)

Figure1. the output signal shown in SA2200 Scope mode and Unbalance mode.

Then press [Power&Energy] to measure as Figure 2 show.

Figure2.

Thus we know the measured active power is 99.63kW, fundamental reactive power is 57.51kvar, which can be a reference for the reactive power compensation capacitor selection.

Note:

P(kW): active power

S(kVA): apparent power, the product of Vrms and Irms

Q1(kvar): fundamental wave’s reactive power

PF: power factor, ratio of active power and apparent power

cosФ: displacement power factor, cosine value of the angle of fundamental current and fundamental voltage

tanФ: ration of reactive power and active power

Urms: rms voltage

Irms: rms current

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