Two Wattmeter Method of Power Measurement

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):

TWO-WATTMETER-METHOD-OF-POWER-MEASUREMENT-FIG-1

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.

Contents:

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:

two-wattmeter-method-eq1

The instantaneous potential difference across the potential coil of Wattmeter, W1 is given as:

two-wattmeter-method-eq2

Instantaneous power measured by the Wattmeter, W1 is

two-wattmeter-method-eq3

The instantaneous current through the current coil of Wattmeter, W2 is given by the equation:

two-wattmeter-method-eq4

The instantaneous potential difference across the potential coil of Wattmeter, W2 is given as:

two-wattmeter-method-eq5

Instantaneous power measured by the Wattmeter, W2 is:

two-wattmeter-method-eq6

Therefore, the total power measured by the two wattmeters W1 and W2 will be obtained by adding the equation (1) and (2).

two-wattmeter-method-eq7Where, P – the total power absorbed in the three loads at any instant.

Measurement of Power by Two Wattmeter Method in Delta Connection

Considering the delta connected circuit shown in the figure below:

TWO-WATTMETER-METHOD-OF-POWER-MESUREMENT-FIG-2The instantaneous current through the coil of the wattmeter, W1 is given by the equation:

two-wattmeter-method-eq8

Instantaneous power measured by the Wattmeter, W1 will be:

two-wattmeter-method-eq9

Therefore, the instantaneous power measured by the wattmeter, W1 will be given as:

two-wattmeter-method-eq10

The instantaneous current through the current coil of the Wattmeter, W2 is given as:

two-wattmeter-method-eq11

The instantaneous potential difference across the potential coil of wattmeter, W2 :

two-wattmeter-method-eq12

Therefore, the instantaneous power measured by Wattmeter, W2 will be:

two-wattmeter-method-eq13

Hence, to obtain the total power measured by the two wattmeter the two equations, i.e. equation (3) and (4) has to be added.

two-wattmeter-method-eq14Where 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.

19 thoughts on “Two Wattmeter Method of Power Measurement”

    1. The motor can run in reverse mode by reversing the line connections of either the main winding or the starting winding.

  1. Very Informative. Please add more information regarding different types of connections made with watt meter in three phase systems…..

  2. will there be any difference in result if we use 2wattmeter method vs 3 wattmeter method on same 11kV circuit?with same load?

  3. While measuring power in a 3 phase load by 2 wattmeter methodbthe reading of 2 wattmeter are equal nd opposite why

  4. Does the two wattmeter method of power measurement applicable for a 3 phase, 4 phase unbalanced load.if not how to measure the total power absorbed by three unequal load

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