Wattmeter Types

Wattmeter Introduction:

Wattmeter is used to measure power at any instant( to find the instantaneous value) in the circuit. 

  • Wattmeter gives direct indication of average power in the circuit.
  • In fact wattmeter combines the effect of voltmeter ( pressure coil) and ammeter ( current coil) in one unit and the moving system is acted upon by a force proportional to VI product.
  • Before proceeding further it is highly recommended to read about the Wattmeter construction and operation. Click here to know about How does the wattmeter work?
  • The wattmeter incorporates two coils namely pressure coil(PC)  and current coil(CC) that carry voltage and current of the circuit respectively.
  • The pressure coil produces a flux Φ proportional to the voltage (V).
  • The current coil produces a flux Φ proportional to the current (I).
  • Usually current coil is fixed and pressure coil is free to move.
  • So the pressure coil is made an integral part of the moving system. It moves or deflects in response to the VI product or power.

A wattmeter is suitable for power measurement in DC as well AC systems. It gives the same angle deflection for a given power.

Wattmeter Types:

There are three types of wattmeters. They are

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

The dynamometer type wattmeters are further divided into two classes namely

  1. Suspended-coil torsion wattmeter
  2. Pivoted-coil direct-reading wattmeter
  • Among these types of wattmeter, the suspended-coil torsion wattmeter is used as a standard meter in test laboratories only.
  • The electrostatic wattmeter is suitable for high voltage low power factor circuits like dielectric loss measurement in capacitors.
  • They are used in very small power measurements.
  • The induction type wattmeter and pivoted coil direct reading wattmeters are very commonly used for power measurement. 

In this post we will discuss in brief about the various types of wattmeter.

Dynamometer type

  • It works on the principle of torque production in a current carrying coil lying in a magnetic field created by a fixed set of coils.
  • It is very similar to dynamometer type voltmeter or ammeter, except that the two coils ( fixed and moving coils) carry two separate currents.
  • Dynamometer wattmeter gives accurate reading in DC and slightly erroneous reading in AC due to coil inductance.
  • This meter suffers from low torque/weight ratio and a considerable power is required to make the instrument work.

Induction type:

  • This type of wattmeter works on the principle of torque production by induced currents in a short circuited secondary circuit.
  • So an induction wattmeter can work on AC circuits only.
  • It has a few decisive advantages like sturdy construction and very long circular scale around 300°.

Electrostatic Type:

  • A repulsive force exists on a plate placed in an electric field. This principle is used in this type of wattmeter.
  • As the deflecting torque is very small it is not a commercial wattmeter.
  • Electrostatic type wattmeter is useful in measuring very small powers in dielectric circuits.
  • This type of wattmeter is free from errors due to waveform, frequency, eddy current and low power factors. 

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