# power measurement

Power is divided into active power (P), apparent power (S) and reactive power (Q). Accordingly, power measurement technology also distinguishes between the measurement of active power and apparent power. In the simplest case, power can be determined from the multiplication of voltage and current. Generally, all power measurements are performed with wattmeters.

Determining power from current and voltage: for direct current, the electrical power is calculated from the product of voltage (U) and current (I); for alternating currents, the power can be calculated from the rms values. The simplest method of power measurement is the U-I measurement with separate determination of current and voltage, followed by multiplication of both values.

Active power measurement:In AC networks, a phase shift between current and voltage often occurs. The power triangle shows the relationships. When determining the power with a simple U-I measurement, one does not obtain the active power, but the apparent power with the unit volt- ampere( VA), since the measuring instruments cannot measure the phase shift. If the phase shift angle 'phi' is taken into account, the active power is obtained in watts (W). According to the laws of angular functions, the cosine `phi` is the ratio of active power to apparent power. This ratio is called the power factor. The active power can be determined with electrodynamic measuring instruments.

Active power is measured by forming the product of the voltage, the current in the corresponding branch and cosine 'phi'. Electrodynamic meters display this product. When measuring, the terminal designation of current and voltage path must be observed, which is printed on the housing of most bench instruments. Today, digital meters equipped with a processor are increasingly being used to measure active power. These devices also display the power factor cosine 'phi'.

Reactive power measurement: As already explained, the reactive power can be calculated using the angular functions as follows: `Sin phi` = reactive power (Q) / apparent power (S).

If the cosine is shifted by 90 degrees, the sine is obtained. If the reactive power is to be measured with an active power meter, the phase must therefore be shifted by 90 degrees. For this purpose, an inductive or capacitive resistor is used in the voltage path.