Operation
4-47
332211
ivivivp ++=
321
WWWp ++= (3-phase 4-wire – using three-wattmeter method)
Based on Kirchhoff’s current law,
0
321
iii or )(
213
iii
. Thus,
232131
23132211
)()( ivvivv
ivivivivp
−+−=
−−+=
21
WWp +=
From the formula above, it can use two wattmeters to measure the power of 3 phases that is
to use the wattmeter on channel 1 to measure the L1 current
1
i
and the L1 to L3
voltage
)(
31
vv − and use the wattmeter on channel 2 to measure the L2 current
2
i and the
L2 to L3 voltage
)(
32
vv − as Figure 4-71 shown below. Using the two-wattmeter method
can measure the 3-phase power easily; however, the fault is that it cannot measure the
power of each phase directly but only applied in the measurement of 3-phase 3-wire power
system.
Figure 4-71 Two-Wattmeter Method Diagram
4.7.4 Selecting 3P4W Wiring Mode, Three-Wattmeter
Method
The total power of 3-phase 3-wire power system can be measured using the two-wattmeter;
however, if it is desired to get the power and voltage of each phase, it needs to connect the
“-“ of each voltage meter together as a false neutral for measurement as Figure 4-72 shows.
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