A Device Measures The Electrical Power any given electrical AC and DC circuit is called Wattmeter. The Electrical Power is in two form DC and AC. The Unit of Electric Power is Watt so the name of device is  called Wattmeter. The electric current is measures by Ammeter. Ammeter coil (Current coil) is connected in series with electrical circuit and voltmeter use as measure the voltage. Potential coil connected  parallel with the circuit. Because The Electrical power is the product of Voltage and current so Both coil combine use in wattmeter to measure the electrical power.

Such Wattmeters are used to measurenet for electrical power AC as well DC Circuit. That Wattmeter gives the same angle deflection for given power are Three Type

1. Dynamometer type Wattmeter
2. Induction type Wattmeter
3. Electrostatic type Wattmeter

Here only one Wattmeter will discus and explain briefly which is Dynamometer type wattmeter.

Dynamometer

These are basically moving coil instruments. These wattmeter has two coils one is moving coil and other is fixed coil, the operating field is produced by the fix coil instead of permanent magnet. Dynamometer type wattmeter are very important because they are commonly used for measuring power in ac circuits.

Principle: The basic principle of Dynamometer type wattmeter is that when the current carrying fixed coil produce magnetic field and current carrying moving coil placed in magnetic field produce by fixed coil. a forced  exerted on the moving coils sides and moving coil deflection take place. the fixed coil produce field (Fr) and field produce by current carrying moving coil (Fm)tries to come line with fixed coil field, a deflection torque is produce by both field. the deflection torque is exerted on the moving coil system. moving coil deflection takes.

Construction:  The dynamometer type wattmeter instrument consists of two coil  fixed coil and  moving coil and also called current coil and potential coil.

Fixed Coil: The fixed coil is divided into equal two part to give a more uniform field near the centre and to allow passage of instrument shaft. the fixed coils are wound with heavy wire to carrying the main current of the wattmeter. the wire is standard where necessary to reduce eddy current losses in conductors. these conductors are varnished and baked to form a solid assembly. The fixed coil connected in series with load current of circuit  is called current coil. it carry circuit current.

Moving coil: The moving coil is connected parallel with load is called potential coil. the moving coil is mounted on the spindle and it is pivoted between to equal parts of fixed coil. the moving coil is light bur rigid construction. the coil is wound either as a self sustanting coil or else on a non metallic former. A non metallic former can not be used as eddy current would be induced in it by alternating field. To avoid hysteresis effects two fix coil are air cored.

Control: The controling torque is provided  by two control springs.

Damping: Air friction damping is used for  dynamometer type wattmeter instrument and it is provided by a pair of aluminium vans, attached to the spindle at the bottom. these vans move in sector shaped champers.

Scales: The scales are hand drawn, using machine sub dividing equipment. Diagonal lines for fine sub division are usually drawn for main marking on the scale. Most of the high precision instruments have a 300 mm scale with 100,120 or 150 division.

Working:  Show the fig in which fixed coils are series coils connected with load in series and   carries the load current I1 . the moving coil is potential coil. it is connected parallel with load and carries current I2 which is proportional to the load voltage. when supply is applied. Fixed coils produce the field Fm and Moving produce the field Fr. the field Fr tries to line with main field Fm , which produce the deflection torque on the moving coil. Spine mountand on moving coil which produce deflection.

In Case of a.c. power measurement. the current is reversed both coils half cycle. the field produce by both coils also negative. So torque is remain same direction and deflection direction is not change. hence dynamometer type wattmeter can be used in both case a.c. and d.c. supply system.

Deflection Torque:

In Case of d.c. system

V = Voltage across the load
B = Flux Density

Deflection Torque  Td ∝ BI ∝ I1I2 ∝ VI ∝ Power

Hence,  Deflection torque proportional to the power.

In Case of a.c. system

Average Deflection Torque  Td  ∝ VI cosÏ† ∝ Power

Hence,  Deflection torque proportional to  true power in a.c. circuit.

Controlling torque is provided by springs Tc

In steady position of deflection Tc = Td     or   Î¸  ∝  Power   ( Î¸ is angle deflection)