Wattmeter | Definition | Symbol | Formula | Types | Uses

Definition of  Wattmeter:

The wattmeter is an electric instrument that is used to measure the electric power of various electric circuits. It consists of the current coil and voltage coil. These instruments are widely used for transmission and distribution of electric power and these are also used for protection purposes.

Wattmeter Symbol:

 Wattmeter Symbol

Wattmeter Formula:

The wattmeter measure power of the various electric circuit. It works on a using power formula.


The exact wattmeter reading formulas are given below:

  • Wattmeter reading in single-phase = Power load (VICOSφ) + Power in the current coil ((I2r)
  • Wattmeter reading in single-phase  = VICOSφ + I2r
  • Three-phase wattmeter reading = √3 (VIcosφ + I2r)

Type of  Wattmeter:

There are two types of Wattmeter given below:
  • Dynamometer Type Wattmeter
  • Induction type Wattmeter

1. Dynamo type Wattmeter working principle:

Dynamo type wattmeter is a moving coil instrument and their operating field is produced in by other fixed coil. These instruments consist of a fixed coil having two equal sections which are parallel to each other. These two fixed coils are forced to avoid hysteresis losses. When an alternating current passes through the fixed coil, it produces a magnetic field and moving coil starts moving and the pointer is attached to the moving coil with a spring and it starts moving when moving coils move.

Advantages of Dynamometer type wattmeter:
  • These instruments provide full accuracy on direct current.
  • These instruments are made to give very high accuracy when these are used 

2. Induction type Wattmeter working principle:
These instruments are used on both of AC as well as DC supplies. These wattmeters provide accurate readings only when the frequency and supply voltage are constant. These wattmeters consist of two laminated electromagnets 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 promotional to the current. Its exciting coil is parallel connected in the circuit. The winding of the 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 is 90 degrees behind the voltage. The torque is produced and is proportional to the power in the load circuit. The torque acts on the disc and the force is produced in the disc. The disc started moving and the pointer is attached on it with a spring and it shows reading on its scale.

Advantages of induction type wattmeter:
  • They are inexpensive as compared to moving iron type instrument. 
  • They rating their accuracy over a wide range of temperatures as well as loads.
  • Long scale, freedom from the effect of start field.
  • They have high torque is to weight ratio as compared to other instruments or devices.
Disadvantages of induction type Wattmeter:
  • Change in temperature cause variation in the resistance of the moving element, affect the eddy current therein and so the operating torque. The error due to this is in part offset by a balancing effect due to a change in temperature of the windings.
  • Change in frequency from that of the calibration value causes variations in both reactance of the voltage coil circuit. Which is highly inductive and also in the amount of compensation from the phase compensating circuit. Within the limit of frequency met within the practice on the main.

Uses of Wattmeter:

The wattmeter is used in various like:
  • Laboratories
  • Industries 
  • Measurements of power in distribution and transmission power.

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