Wattmeter - Working Principle - Construction - Types of Wattmeter - Dynamometer Type Wattmeter - Induction Wattmeter - Working Principle - Uses of Wattmeter | FreelyElectrons
Wattmeter
Wattmeter is an electric
instrument which is used to measure the electric power of various electric
circuits. It consists of a current coil and a voltage coil. These
instruments are widely used in transmission and distribution of
electric power and these are also used for protection purposes.
Wattmeter Symbol
Wattmeter Formula
Wattmeters measures power of various
electric circuits.It works on power formula
P = IV CosΘ
The exact wattmeter reading formulas
are given below:
Wattmeter Reading in Single Phase =
Power in load (VIcosφ ) + power in current coil (I2r)
Wattmeter Reading in Single Phase =
VIcosφ + I2r
Three Phase Wattmeter Reading = √3 (VIcosφ
+ I2r)
Types of Wattmeter
Wattmeters are of two types given below:
Dynamometer Type Wattmeter
Induction type Wattmeter
Electro-dynamometer /Dynamometer Type
Wattmeter Working Principle
Dynamometer
Type Wattmeters are moving coil instruments and their
operating field is produced in by another fixed coil. These instruments consist of a fixed coil having two equal
sections which are parallel to each other. These two fixed coils are air-cored to avoid from hysteresis losses. When alternating current passes through the fixed coil, it
produces magnetic effect and moving
coil starts moving and a
pointer is attached to the moving coil with a spring
and it starts moving when moving coil moves.
Advantages of Dynamometer Type
Wattmeter
These instruments are made to give very high accuracy
and these are used as a standard for calibration purposes.
These instruments provide full accuracy on direct
current (DC).
Disadvantages of Dynamometer Type
Wattmeter
These instruments cannot provide full accuracy on
alternating Current (AC).
These instruments cause errors at low power
factor.
Working Principle Induction Type Wattmeter
These instruments are used on both AC and DC
supplies. These wattmeters provide accurate readings only when frequency
and supply voltages are constant. These wattmeters consists of two laminated
electro magnets and one of them is excited by the current with exciting
winding. This winding is connected in series of the circuit. The other
magnet is excited by the current which is proportional to the current. Its
exciting coil is connected in parallel with the circuit. The winding of first
magnet carries line current and the other coil carries voltage and is highly
inductive. This coil is connected to the supply and the flux lags 900
behind the voltage. The torque is produced and is proportional to the
power in the load circuit. The torque acts on the disc and force is produced in
the disc. The disc starts moving and the pointer is attached on it with
a spring and it shows reading on its scale.
Uses of wattmeters
These wattmeters are used in various laboratories,
industries for specific purposes. These wattmeters plays an important role in
measurement of power in distribution and transmission of power.
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