Page 186 - Electrician - TT (Volume 1)

ELECTRICIAN - CITS

Measurement of three phase power

Power measurement in 3 phase system depends upon whether the load connected across it is balanced or
unbalanced and whether it is in star or delta.Various methods are used for the measurement of power in three
phase circuits. On the basis of number of wattmeter used, there are three methods
1 Three watt meters method
2 Two watt meters method
3 Single watt meter method.

Three watt meter method

This method of power measurement can be used to measure the power in a three phase circuit for both balanced
and unbalanced load. It is generally employed to measure power in a 3 phase, 4 wire system. The connections
for measuring power of a star connected load by three wattmeter method is shown below.
The pressure coil of all the Three watt meters namely W , W and W are connected to a common terminal known
2
3
1
as the neutral point. The product of the phase current and phase voltage represents as phase power and is
recorded by individual wattmeter.
The total power in a Three wattmeter method of power measurement is given by the algebraic sum of the readings
of Three watt meters. i.e.
Total Power P= W +W +W 2
1
2
However, this method can also be employed in a 3 phase, 3 wire delta connected load, where power consumed
by each load is required to be determined separately.

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CITS : Power - Electrician & Wireman - Lesson 26-29