Figs. 26 and 27 show the connection diagram for star-connected and delta-connected loads respectively. As indicated in the figures three wattmeters are connected in each of the three phases of the load whether star or delta connected. The current coil of each wattmeter carries the current of one phase only and the pressure coil measures the phase-voltage of the phase. Hence, each wattmeter measures the power in a single phase. The total power in the load is given by the algebraic sum of the readings of the three wattmeters.
Fig. 26. Star-connected load.
Fig. 27. Delta-connected load.
While using this method following difficulty is met with:
- In case of star-connected load it is not always possible to get at neutral point which is required for connections (Fig. 26).
- In case of delta-connected load, under ordinary conditions it is not generally feasible to break into the phases of the load.
To measure power it is not necessary to use three wattmeters, two wattmeters can be used for the purpose as explained in the Article 9.2.