A three-wattmeter method is used to measure power in a three-phase, four-wire system.
A wattmeter is a device in an AC circuit used for power measurement. A wattmeter is a device that consists of two coils called the current coil (CC) and the pressure coil (PC). A current coil is a low-resistance coil that is inserted in series with a line carrying the current to be measured, and a pressure coil is a high-resistance coil connected in series with a high resistance between the two points across which potential differences to be measured. The wattmeter gives a reading that is proportional to the product of the current flowing through its current coil, the potential difference across its pressure coil, and the cosine of the phase angle between voltage and current.
This method is used for the measurement of power in 3-phase,4-wire load circuits. The neutral wire is common to the three phases. Each wattmeter reads power in its phase, and the total power of the load circuit is given by the sum of the readings of the three wattmeters
In the case of 3-phase, 3-wire star-connected circuits, the difficulty is experienced in getting neutral. For the 3-phase, 3-wire circuits to employ the measurement of power, an “artificial star” can be formed by connecting three equal high resistances in the star to the three line conductors. (In the case of low-voltage circuits, three potential coils may be connected to form the common star.)
In the case of delta-connected circuits, the difficulty in adopting the above methods of power measurement is due to the fact that the phase coils are required to be broken for inserting the current coils of wattmeters.
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