Electrical power is measured with a wattmeter. A wattmeter consists of a current coil connected in series with load, while the other potential coil is connected parallel with load.
Since the sum and subtraction of readings are done to calculate total true power of a three-phase system, methods shown are not used practically in industry. Rather three phase power analyzers are used which are more user-friendly.
Depending on the strength of each magnetic field movement, the pointer gets affected. The true or real power is directly shown in a wattmeter.
In three-phase systems, power can be measured using several methods. For temporary measurements, a single wattmeter can be used.
However, for permanent measurements, a three-phase wattmeter having two elements is used which indicates both balanced and unbalanced loads. For an unbalanced load, two wattmeters must be used as shown in the below.
The total power is calculated by adding the measurement readings given by the two wattmeters. With this method, the power factor can also be obtained.
When using the two-wattmeter method, it is important to note that the reading of one wattmeter should be reversed if the power factor of the system is less than 0.5. In such a case, the leads of one wattmeter may have to be reversed in order to get a positive reading.
In the case of a power factor less than 0.5, the readings must be subtracted instead of being added. The power factor of the three-phase system, using the two wattmeter method (W1 and W2) can be calculated as follows:
Tan φ = (1.732*W1*W2) / (W1 +W2)