Two Wattmeter Method can be employed to measure the power in a 3 phase, three-wire star or delta connected the balanced or unbalanced load.
In two wattmeter method, the current coils of the wattmeter are connected with any two lines, say R and Y and the potential coil of each wattmeter is joined on the same line, the third line i.e. B as shown below in figure (A):
The total instantaneous power absorbed by the three loads Z1, Z2 and Z3, is equal to the sum of the powers measured by the two wattmeters, W1 and W2.
Measurement of Power by Two Wattmeter Method in Star Connection
Considering the above figure (A) in which Two Wattmeter W1 and W2 are connected, the instantaneous current through the current coil of Wattmeter, W1 is given by the equation shown below:
Therefore, the total power measured by the two wattmeters W1 and W2 will be obtained by adding the equation (1) and (2).
Measurement of Power by Two Wattmeter Method in Delta Connection
Considering the delta connected circuit shown in the figure below:
Where P is the total power absorbed in the three loads at any instant.
The power measured by the Two Wattmeter at any instant is the instantaneous power absorbed by the three loads connected in three phases. In fact, this power is the average power drawn by the load since the Wattmeter reads the average power because of the inertia of their moving system.