Two Watt Meter Method For Power Measurement


Two Watt Meter Method For Power Measurement

Two watt meter method is used for measuring power in 3 phase 3 - wire star or delta connected systems either with balanced or unbalanced load. Let us discuss two watt meter method for 3 phase power measurement  with star and delta connection in detail.

Measurement of power by two watt meter method in star connection:

In this method we use two watt meters and the current coils of these watt meters are connected in series with two lines and potential coils are connected across these two lines  and load is connected in star as shown in the following figure.

From figure

The instantaneous current flowing through the watt meter W1 is Ir

The voltage across the potential coil of watt meter W1 is 


Instantaneous power measured by the watt meter, W1 is given by,

The instantaneous current flowing through the watt meter W2 is Iy

The voltage across the potential coil of watt meter W2 is 
Instantaneous power measured by the watt meter, W2 is given by
The total 3 phase power will be sum of power measured by watt meters W1 and W2

Adding equations 1 and 2 we get following results.

Where P is the total 3 phase power consumed by load.

Measurement of power by two watt meter method in delta connection:

In this method also we use two watt meters and the current coils of these watt meters are connected in series with two lines and potential coils are connected across these two lines and load is connected in delta as shown in the following figure.

The instantaneous current flowing through the watt meter W1 is,

The voltage across the potential coil of watt meter W1 is Erb 

Instantaneous power measured by the watt meter, W1 is given by,

The instantaneous current flowing through the watt meter W2 is,
The voltage across the potential coil of watt meter W2 is Eyb

Instantaneous power measured by the watt meter, W2 is given by,
The total 3 phase power will be sum of power measured by watt meters W1 and W2

Adding equations 3 and 4 we get following results.

Where P is the total 3 phase power consumed by load.

Note : The power measured by watt meter is average power because of the inertia of their moving system.

In this post we have discussed about measurement of three phase power using two watt meter method.

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