Measurement of Three Phase power:Three wattmeter method
Wattmeter is a devices in AC circuit used for power measurement. Wattmeter is a devices which consists of two coils called Current coil(CC) and pressure coil (PC).Current coil is a low resistance coil which is inserted in series with line carrying current to be measured and pressure coil is a high resistance coil connected in series with a high resistance between the two points across which potential differences is to be measured. Wattmeter gives reading which is proportional to the product of the current flowing through its current coil,potential difference across its pressure coil and 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.Neutral wire is common to the three phases.each wattmeter reads power in its own phase,and the total power of the load circuit is given by the sum of the readings of the three wattmetres
In case of 3-phase,3-wire star connected circuits 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 star to the three line conductors.(In case of low-voltage circuits three potential coils may be connecetd to form the common star).
In 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 current coils of wattmeters.