Page 187 - Electrician - TT (Volume 1)

ELECTRICIAN - CITS

One watt meter method

This method is applicable only to a balanced load and when neutral is available. This method is generally used
with three phase star connected load. In this method watt meter is connected in any of the phase i.e., current coil
of the watt meter is connected in series to any of the phase and pressure coil connected across that phase and
neutral. The power consumed in the three phase circuit is three times the reading of wattmeter.
Total power = 3 x Wattmeter Reading

If in this system, a neutral is not available, an artificial neutral point can be made by connecting the pressure coil
of wattmeter with two resistances each with a value equal to pressure coil the wattmeter
Two wattmeter method

Considering the above figure in which Two Wattmeter W and W are connected, the instantaneous current
2
1
through the current coil of Wattmeter,
W is given by the equation shown below.
1
W - i R
1
Instantaneous potential difference across the potential coil of Wattmeter, W is given as
1
W = e - e BN
1
RN
Instantaneous power measured by the Wattmeter, W is
1
W = i (e - e ) ... ... ... (1)
1 R RN BN

The instantaneous current through the current coil of Wattmeter, W is given by the equation
2
W = i Y
2
Instantaneous potential difference across the potential coil of Wattmeter, W is given as
2

W = e - e BN
YN
2
Instantaneous power measured by the Wattmeter, W is
2
W = i (e - e ) ... ... ... (2)
2 Y YN BN

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CITS : Power - Electrician & Wireman - Lesson 26-29