Electrical Engineering ⇒ Topic : Reactive Power with One Wattmeter

Reactive Power with One Wattmeter

If the 3-phase load is balanced, then the reactive power drawn by the load can be determined by using a single wattmeter. For this purpose, the current coil of the wattmeter is connected in one line and the potential coil is connected across the other two lines as shown in Fig. (a). It can be proved that the total reactive power drawn by the load is equal to √3 times the reading of the wattmeter. Fig. (a) shows the connections of a single wattmeter for measuring reactive power in a 3-phase balanced load having lagging p.f. cos Φ. Here current coil of the wattmeter is connected in the R-line and the potential coil is connected across the lines Y and B.

                                                              (a)                                                                                                              (b)

                                                                                                   figure

To obtain V_YB, find the phasor sum of V_YN and V_BN as shown in the phasor diagram in Fig. (b). It is clear from the phasor diagram that the phase angle between V_YB and I_R is (90° - Φ )