ELECTRODYNAMIC WATTMETER

The electrodynamic wattmeter consists of two coils CC placed parallel to one another to produced a uniform magnetic field.They are connected in series with the load,and are called the current coils.The two fixed coils can be connected in series or parallel to give two current ratings,The current coils carry the full-load current  or a fraction of the full-load current.Thus,the current in the current coils is proportional to the load current.The moving coil VC,in series with a high non-inductive resistance Rs,is connected across the supply Thus,the current flowing in the moving coil is proportional to,and practically in phase with,the supply voltage.The moving coil is also called the voltage coil. Current coils have few turns of thick wire,while voltage coil has large number of turns of thin wire.The moving coil,VC is mounted centrally such that it does not project outside the fixed coils at its maximum deflected position.It is carried on a pivoted spindle and carries the pointer which moves over a calibrated scale.

Two springs S1,S2 are used for providing the corntrolling torque torque and for leading current into and out of the moving coil.Damping is done by air controlling friction.An aluminium vane moving V in a closed sector-shaped chamber B is generally used for this purpose.The control springs and the air damber are shown in fig.
In practice,there are two methods in which a wattmeter,may be connected in a circuit.In the voltage coil VC is connected on the supply side.The current coil CC carries the load current and the voltage across the voltage coil VC is higher than the load voltage because of small voltage drop across the current coil.