SOLUTION

A wattmeter is an instrument with a potential coil and a current coil so arranged that its deflection is proportional to VI cosθ.

Where

V is the voltage (RMS value) applied across the potential coil

I is the current (RMS valued passing through the current coil

θ is the angle between V and I.

• In a three-phase system, total power is the sum of powers in three phases.
• The power is measured by the wattmeter.
• It consists of two coils. (i) current coil, and (ii) voltage coil.
• The current coil is connected in series with the load and it senses current.
• Voltage coil is connected across supply terminals and it senses voltages.

Two Wattmeter Method

• This method is used for balanced as well as unbalanced load.
• The current coils of the two wattmeters are inserted in any two lines and the voltage coil of each wattmeter is joined to a third line.
• The load may be a star or delta connected. The sum of the two wattmeter readings gives three-phase power.

W = W₁ + W₂ = Three-phase