Page 459 - Basic Electrical Engineering
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The connection diagram for measurement of power in a three-phase Y-
connected load using the two-wattmeter method has been shown in Fig. 4.16
We know that the reading of the two wattmeters will be V I cos (30 −
ph ph
ϕ) and V I cos (30 + ϕ), respectively. For star connection, V = V L
ph ph
ph
and I = I .
L
ph
The total load, P = 30 kW
Power factor, cos ϕ = 0.7 lagging
−1
Phase angle, ϕ = cos (0.7) = 45.57° lagging
So, V I =
L L
=
Reading of wattmeter, W 1 = V I cos (30 − ϕ)
L L
= 24743.6 cos (30 − 45.57°)
= 23.835 kW
Reading of wattmeter, W 2 = V I cos (30 + ϕ)
L L
= 24743.6 cos (30 + 45.57°)
= 6.165 kW
To check, total power, P = W + W = 23.835 + 6.165
1
2
= 30 kW
Example 4.14 A three-phase balanced load connected across a 3ϕ, 400 V ac
supply draws a line current of 10 A. Two wattmeters are used to measure
input power. The ratio of two wattmeter readings is 2:1. Find the readings of
the two wattmeters.
Solution:
