Two Wattmeter Method
The circuit diagram for two wattmeter
method of measurement of three phase real power is as shown in the figure 34.7.
The current coil of the watt meters W1 and W2 are
inserted respectively in R and Y phases. The potential coils of the two watt meters
are joined together to phase B, the third phase. Thus, the voltage applied to
the voltage coil of the meter, W1 is VRB= VR-VB,
while the voltage applied to the voltage coil of the meter, W2 is VYB=VY-VB,
where, VR, VB and VC are the phase voltage
values of lines R, Y and B respectively, as illustrated by the phasor diagram
of figure 3.8. Thus, the reading of the two watt meters can be obtained based
on the phasor diagram of figure 4.8, as follows:
W1
= IR VRB
= IL VL cos (30 - Ø) (3.9)
W2
= IY VYB
= IL VL cos (30 + Ø) (3.10)
Hence, W1+W2
= √3 VL IL cos Ø = P3ph (3.11)
And W1-W2
= VL IL sin Ø (3.12)
So that
then,
Tan Ø = √3 [W1-W2]/ [W1+W2]
Where Ø is the lagging PF angle of the load. It is to be noted that the equations (3.11) and (3.12) get exchanged if the load is considered to be of leading PF.
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