Induction type instruments are used for a. c. measurements.the induction principle find its widest application as an energy meter. Induction type energy meter used invariably to measure the energy consumed in any a.c. circuits in a prescribed period where supply voltage and frequency are constant. Energy meter is an integrating instrument which measures the total quantity of electrical energy supplied to the circuit in given period.

The basic principle of induction type energy meter is electromagnetic induction. When alternating current flow though two suitably located coils produces rotating magnetic field which is cut by the metallic disc suspended near to the coils thus emf induce in the disc which is circulates eddy current in it. by interaction of rotating magnetic field and eddy currents, torque is developed and cause the disc rotate.

An induction type single phase energy meter, as show in fig has following main parts of the operating mechanism.

  1. Driving system.
  2. Moving system.
  3. Breaking system.
  4. Recording mechanism.  

1.Driving system.
It consists of two electromagnets, called “shunt” magnet and “series” magnet,

Series magnet: it consists of a number of U-shaped laminations of silicon steel together to form a core.A coil of thick wire having a few turns is wounded in both legs of U-shaped magnet. the coil is known is current coil which is connected series with load.produce the magnetic field proportional and in phase with line current I.

Shunt magnet: it consists of number of M-shaped laminations of silicon steel assembled together to form a core.A coil of thin wire having large number of turn in wound on central limb of the magnet. this coil is connected across the load. thus it is excited by current proportional to the supply voltage and known is potential coil.

2.Moving system.
The moving system essentially consists of a light rotating aluminium disk mounted on a vertical spindle or shaft. The shaft that supports the aluminium disk is connected by a gear arrangement to the clock mechanism on the front of the meter to provide information that consumed energy by the load.
The time varying (sinusoidal) fluxes produced by shunt and series magnet induce eddy currents in the aluminium disc.
The interaction between these two magnetic fields and eddy currents set up a driving torque in the disc.

The number of rotations of the disk is therefore proportional to the energy consumed by the load in a certain time interval and is commonly measured in kilowatt-hours (kWh).

3.Breaking system. 
Damping of the disk is provided by a small permanent magnet, located diametrically opposite to the a.c magnets. The disk passes between the magnet gaps. The movement of rotating disc through the magnetic field crossing the air gap sets up eddy currents in the disc that reacts with the magnetic field and exerts a braking torque.
By changing the position of the brake magnet or diverting some of the flux there form, the speed of the rotating disc can be controlled.
4.Recording mechanism.
The function of recording or registering mechanism is to record continuously a number on the dial which is proportional to the revolutions made by the moving  system.the no of revolution on the disc is a measured the electrical energy passing though the meter.

Working of Single phase energy meter:

When the energy meter is connected in circuit, the current coil carries the load current and pressure coil carries the current proportional to the supply voltage. the magnetic field produced by series magnetic in phase with the line current and  magnetic field produced by shunt magnet is in quadrature with the applied voltage.thus, a phase difference exists between the fluxes produce by the two coils. this setup rotating magnetic field which interacts with disc and produce a driving torque and thus, disc starts rotating.the number of revolutions made by the disc depend upon energy passing though the meter. the spindle geared to the recording mechanism so that energy consumed in the circuit is directly registered in kWh.

the speed of the disc is adjusted by adjusting the position of braking magnet. example, if the energy meter registers less energy than the energy actual consumed in the circuit. then the speed of disc has to be increased which is obtained by sifting the braking magnet nearer to the centre of the disc vice -versa.

       Meter Constant, K =       No. of revolutions
                                        Energy supplied in kWh   
By using this relation we can calculate numerical values of problem.


  1. What is the meaning of PFC,fc1 ,and fc2.

    1. PFC means (Power factor Controller Or Power factor Correction)

    2. Field Coil 1,2

  2. Thanks for the Material, it really helped with my exam preparation

  3. Does reversing polarity affects it.

  4. Does reversing polarity affects it.

  5. What are the criteria of rotating disc

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  7. Its very useful for preparation of examination. Thank u

  8. Thanks for the simplicity of explanation of this topic.

  9. Mention errors too sar .., BTW Themk u so much


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