Chroma 66203, User Manual

Page: 73 / 146

Operation
4-47
3 3 2 2 1 1
i v i v i v p
+ + =
3 2 1
W W W p
+ + =
(3-phase 4-wire – using three-wattmeter method)
Based on Kirchhoff’s current law,
0
3 2 1
= + +
i i i
or
) (
2 1 3
i i i
+ − =
. Thus,
2 3 2 1 3 1
2 3 1 3 2 2 1 1
) ( ) (
i v v i v v
i v i v i v i v p
− + − =
− − + =
2 1
W W p
+ =
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
) (
3 1
v v
−
and use the wattmeter on channel 2 to measure the L2 current
2
i
and the
L2 to L3 voltage
) (
3 2
v v
−
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.