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)
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