Test Instruments & Measuring Devices
WATTMETERS
Equation (14-16) is the mathematical representation for calculating power in an AC circuit.
(14-16)
P
VRmsIRmscosq or P
I2R
Three-Phase Wattmeter
Total power in a 3f circuit is the sum of the powers of the separate phases. The total power
could be measured by placing a wattmeter in each phase (Figure 12); however, this method is
not feasible since it is often impossible to break into the phases of a delta load. It also may not
be feasible for the Y load, since the neutral point to which the wattmeters must be connected is
not always accessible.
Normally, only two wattmeters are used in making 3f power measurements (Figure 13).
Figure 12 Wattmeters in Each Phase
In balanced 3f systems, with any power factor, total power is calculated by adding the A and
B phase powers. Equation (14-17) is the mathematical representation for calculating total power
(PT).
(14-17)
PT
WA
WB
where
WA and WB are the power readings in Phase A and Phase B
Rev. 0
Page 21
ES-14
