Page 456 - Basic Electrical Engineering

Thus at Power factor = 1, both the wattmeter readings will be positive and of equal value.

ii. At 0.5 power factor, i.e., cos ϕ = 0.5 i.e., ϕ = 60°.

Thus, at power factor equal to 0.5, one of the wattmeters will give zero reading.

iii. When the power factor is less than 0.5, i.e., when ϕ > 60°. Let us observe the wattmeter
readings.

When ϕ > 60, W will give positive readings but W will give a negative reading.
2
1
Thus, for power factor less than 0.5, i.e., for ϕ > 60°, one of the wattmeters will give a
negative reading.

iv. When load is purely inductive or capacitive, so power factor will be zero, i.e., ϕ = 90°

W = V I cos (30° − 90°) = V I cos 60°
1
L L
L L

W = V I cos (30° + 90°) = – V I sin 30°
L L
2
L L
Both the wattmeters show equal but opposite readings. Hence, the total power

consumed will be zero.