Wattmeter is an instrument which is used to measure electric power in the circuit. Electric power is the product of current and voltage, but in case of AC circuit power factor (cos Φ) is multiplied by an electric power. In this article, we will discuss about the construction and working of wattmeter.

Construction of Wattmeter

A wattmeter basically consists of two coils, one coil is called the current coil and the other one is called pressure coil. As from the definition of wattmeter, it measures electric power. So current coil (CC) measures current and pressure coil (PC) measures voltage. The current coil (cc) has less number of turns, which carries the current in the load and has very low impedance. The pressure coil (PC) has more number of turns having high impedance which is connected across the load. The load voltage is immersed across the pressure coil.

wattmeter
Wattmeter fig 1

From above figure, M and L represent the current coil. Common and V represents pressure coil. The terminal M denotes the main side, L denotes the load side, commonly denotes the common point between current coil(CC) and pressure coil (PC) and V denotes the second terminal of pressure coil. A wattmeter is a four terminal device.

Working

When current flows through the current coil (cc) and pressure coil (PC), they set up magnetic fields in space. The electromagnetic torque is produced by the interaction of magnetic fields set up in current coil and pressure coil. In both coils, one coil is fixed coil and another coil is moveable. Under the influence of the torque, the moveable coil moves on a scale. The produce torque in the coil is directly proportional to the current flowing through the coil. Large current flows in a coil produce large torque and vice-versa. The torque, which moves the pointer is called deflecting torque. This torque is controlled by controlling torque. Controlling torque is provided by a spiral spring. The wattmeter deflection is proportional to the average power (VI Cos Φ) delivered to the circuit.

Wattmeter downscale reading
Wattmeter fig 2

Sometimes, wattmeter gives backward deflection or downscale reading. This is due to the improper connection of the current coil and pressure coil.