These are basically moving coil instruments. In these instruments, the operating field is produced by the fixed coil instead of a permanent magnet. The instruments in which the operating field is produced by the fixed coils are known as electrodynamic or dynamometer type instruments. The dynamometer type wattmeters are very important because they are commonly employed for measuring power in AC circuits.
The construction of this meter is same as that of dynamometer type ammeter and voltmeter. It consists of two identical fixed coils. In the air gap between these fixed coils , moving coil is placed which is attach to the spindle. The fixed coil of the meter is connected in series with the load and known as current coil of wattmeter.
Electro dynamometer Type Wattmeter
To living the current flowing through moving coil a high resistance is connected in series with the moving coil and its circuit is made pure resistive. This coil is known as voltage coil or pressure coil and its terminals are marked as common and voltage. This coil is connected in parallel with the load. Fixed coils are used as current coil because it carries load current therefore can be made more massive also there is no problem of leading the current in or out.
Moving coil is supported by a pivoted spindle and is entirely embraced by the fixed current coils. Spring control is used to produce the controlling torque. Also springs serves the purpose of current carrying leads for moving coil. As the moving coil is constructed with high resistance, current handle by them is very less which is safely carried by the instrument springs. Fixed coil and moving coil are designed for air cored.