Wattmeter is an instrument which is used to measure electric power in the circuit. It measure electrical power in watt. Electric power is the product of current and voltage but in case of ac circuits power factor (cos Φ) is multiplied by an electric power. In this article, we will discuss about basic 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 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. Circuit diagram of wattmeter is given below.
From above figure, M and L represents current coil. Common and V represents pressure coil. The terminal M denotes the main side, L denotes the load side, common 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 Principle of wattmter
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 an other coil is moveable. Under the influence of the torque, the moveable coil moves on a scale. The produce torque in 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 spiral spring. The deflection is proportional to the average power (VI Cos Φ) delivered to the circuit.
Sometimes, meter gives backward deflection or downscale reading. This is due to improper connection of the current coil and pressure coil of wattmeter.