
Three-phase Systems and Circuits 379
We will consider a power factor of unity, 0.5, less than 0.5 and 0 and study the effect on the
wattmeter readings.
1. At unity power factor, when cos f = 1, that is, f = 0
WVI
WVI
1
2
0
0
Ph Ph
Ph Ph
cos
cos
Thus, at power factor = 1, both the wattmeter readings will be positive and of equal value.
2. At 0.5 power factor, cos f = 0.5, that is, f = 60°
WVI
VI
WVI
1
2
0
330
3306
°
+°=
Ph Ph
Ph Ph
Ph Ph
cos( )
cos
cos( )
Thus, at power factor equal to 0.5, one of the wattmeters will give zero reading.
3. When the power factor is less than 0.5, that is, f > 60. Let us observe the wattmeter readings.
WVI
WV ...