Specifications

94
3-phase AC circuit: i) One wattmeter method :
Fig.7.12 - Single wattmether method of power measurement in 3-phase circuit
For balanced 3-phase load i.e. Z
R
= Z
Y
= Z
B
, single wattmeter may be connected into any
one phase as shown in fig. 7.12. This wattmeter will indicate the power in that phase only. Since
the load is balanced, the total power in the 3-phae
circuit will be given by
Total power = 3 x Wattmeter reading
ii) Two-wattmeter method :
This is most the common method for
measuring power in a 3-phase, 3-wire system
since it can be used for both balanced (Z
R
= Z
Y
=
Z
B
) and unbalanced loads (Z
R
≠ Z
Y
≠ Z
B
)connected in either star or delta. The current coils
are connected to any two of the lines, and the
voltage coils are connected to the other line, the
one without a current coil connection, as shown
in Fig.7.13.
Total power = W
1
+ W
2
iii) Three-wattmeter method :
If the installation is 4-wire and the load is
unbalanced, then 3 wattmeters are necessary.
The wattmeter’s connection is shown in Fig. 7.14.
Each wattmeter measures the power in one
phase and the total power will be given by
Total power = W
1
+ W
2
+ W
3
Fig. 7.14. - Three wattmether method of
power measurement in 3-phase circuit
Fig.7.13 - Two wattmether method of power
measurement in 3-phase circuit