Measurement Of Power In Three Phase Circuit

Measurement of Power in Three phase circuit: Real power in balanced (or unbalanced ) 3 phase circuits can be measured by using two wattmeters connected in any two lines of a three-phase three wire system. In Fig.4(a) the current coils are connected in lines A and C, with the voltage coil reference connections in line B. The current °owing through the current coil of the wattmeter connected in line A is I_A and that in line C is I_C. Similarly, the voltage applied to the voltage coils is V _ab and V _cb (and not V _bc) respectively. Their readings will be:

From Fig.3 (a or b) angle between Vab and I_A is (30 θ) and between V_cb and Ic is (30-θ). Therefore the wattmeter readings will be: