Wattmeter is an instrument which is used to measure electric power. A wattmeter consists of two coils, one coil is called the current coil (CC), and other coil is called potential or pressure coil (PC). The coil with less number of turns is the current coil, which carries the current in the load and has very low impedance. The coil with more number of turns is called potential coil, which is connected across the load and has high impedance. The load voltage impressed across the potential coil.
The terminal M denotes the main side, L denotes load side, common denotes the common point of current coil and pressure coil, and V denotes the second terminals of the potential coil, usually selected as per the range of the load voltage in the circuit.
The wattmeter has four terminals shown in figure, two for current coil and two for potential coil. When the current flows through the potential coil (PC) and current coil (CC), they setup, magnetic field in space. An electromagnetic torque is produced by the interaction of the two magnetic fields. Under the influence of the torque, one of the coil moves on a calibrated scale against the action of a spring. The instantaneous torque produced by the electromagnetic action is proportional to the product of instantaneous values of the currents in the two coils. The wattmeter deflection is therefore, proportional to the average power (VIcosα) delivered to the circuit.
Sometimes, a wattmeter connected in the circuit to measure power gives downscale reading or backward deflection. This is due to improper connection of the current coil and pressure coil.
Types of wattmeter