Electrical Engineering ⇒ Topic : Three-Wattmeter Method

Three-Wattmeter Method

In this method, the three wattmeters are connected in such a way that each has its current coil in one line and its potential coil between that line and some **common point x as shown in Fig. (a).It can be shown mathematically that the algebraic sum of the readings of the three wattmeters gives the total power whether the load is balanced or not i.e.

                                           Total power = W₁ + W₂ + W₃                  ..... algebraic sum

Although a Y-connected load has been considered, the above arguments are equally valid for a A-connected load, balanced or unbalanced. A caution if neutral wire is available. Then common point x should be at the neutral wire.

Proof. We shall now prove mathematically that the algebraic sum of the three wattmeter readings is equal to the total power drawn by the Y-load in Fig. (a).

                                                                    figure (a)

It may be noted that a single-phase wattmeter reads the product of the r.m.s. current in its current coil, the r.m.s. voltage across its potential coil and the cosine of the phase angle between these two quantities