Single Phase Energy Meter

How Single Phase Energy Meter works?

A single phase energy meter is a device which is connected in line with a single phase electric supply to a electrical powered device for measuring the amount of electrical energy consumed in that device.

An energy meter is called by different names, such as –

  1. Electricity meter.
  2. Electric meter.
  3. Electrical meter.
  4. Kilowatt-hour meter.
  5. Single Phase Energy Meter.
  6. Electromechanical Induction Meter.

Energy meter for residential purposes, is connected directly in-line between source of electric supply and load. For larger loads, current transformers are used, so that the meter can be located somewhere other than in line with the service conductors.

Unit of Energy Meter

The measurement of electrical energy consumed by various domestic and industrial loads is very important from the economic point of view.

Energy meters operate by continuously measuring the instantaneous voltage and currents to give energy measurement. The electrical energy used by the load is basically the power consumed over a period of time. If the time interval is in hours, then the energy measurement is in Watt-hours.

Most of the energy meters measure energy in the unit of Kilowatt-hours (kWh). One kilowatt-hour is equal to the amount of electrical energy used by a load of one kilowatt over a period of one hour.

\therefore 1 \ \text {kWh} = 1000 \ \text {joules / sec} \times 60 \times 60 = 3600000 \ \text {joules} .

Types of Energy Meter

According the working principles, energy meters fall into following two basic categories –

  1. Electromechanical Induction meter.
  2. Electronic or Digital meter.

Electromechanical Induction Meter

Electromechanical induction meter operates through electromagnetic induction by counting the revolutions of a non-magnetic but electrically conducting metal disc which is made to rotate at a speed proportional to the power passing through the meter. The number of revolutions of the disc is proportional to the energy usage.

Construction of Energy Meter

Construction and working procedure of a single phase induction energy meter is shown in figure.


It has following main components –

1) Shunt electromagnet –

It is an electromagnet. Its coil is wound over the middle limb of a three leg silicon steel core. The coil is connected in parallel to the electric supply line. So, it carries the current proportional to the shunt voltage. Hence, it is also called as Shunt electromagnet. Its coil is called Voltage Coil. It produces magnetic flux proportional to the supply voltage.

Voltage coil has large numbers of turns wound on the central limb of a silicon steel core. The central limb also has adjustable copper shading bands. The main function of these bands is to align the flux produced by the magnet in such a way that, it is exactly perpendicular to the supplied voltage.

2) Series electromagnet –

It is a horse shoe type electromagnet. Its coil is wound over both limbs of a silicon steel core. The coil is connected in series with the load. It is excited by the current flowing through the load. Hence, it is also called Current electromagnet. Its coil is termed as Current Coil. It produces magnetic field proportional to the current through the load.

The current coil has very less numbers of turns. The turns are wound on both legs of silicon steel core.

3) Rotor Disc –

Rotor is a light weight Aluminium alloy disc mounted on a spindle. The spindle also carries a pinion which mesh with a gear train arrangement. The spindle connects the rotor disc to the registering mechanism through this train of gears.

The rotor disc is placed in between the air gap of above two electromagnets. Due to current through the electromagnets, eddy current is induced in the rotor disc. This eddy current is cut by the magnetic flux produced by the electromagnets. This interaction between flux and the eddy current in the disc, induces a deflecting torque and disc starts to rotate about its spindle axis.

4) Braking Magnet –

Braking magnet is a horse shoe type of permanent magnet placed at radial position of the rotor disc. It is required to control the speed of rotation of the rotor disc and also to bring the disc to an idle state when the power to the load is disconnected.

While rotating, the aluminium disc induces eddy current which cut the magnetic flux of the braking magnet and hence produces the braking torque. This braking torque opposes the movement of the disc. When electric supply to the load is disconnected, this braking torque results in stopping of disc rotation.

The position of braking magnet is adjustable due for adjusting the braking torque.

5) Cyclometer & Display Mechanism –

Rotor disc drives a registering mechanism through a train of gears mechanism. When the electric power supply to the load is switched on, the aluminium disc starts rotating indicating consumption of energy. The rotation of disc is directly proportional to the energy consumed by the load. Disc rotation is transmitted to the pointers of different dials and cyclometer for recording. After some number of rotations, the disc displays the unit used by the load by a display mechanism.

The gear-turn ratio between the adjacent indicators is in ratio 10 : 1 so that the energy consumed is integrated up to thousands of kilowatt-hour. The number of rotations of the disc is counted at particular intervals of time. The disc measures the power consumption in kilowatt hours.

Working of Energy Meter

The induction energy meter has a Aluminium disc whose rotation determines the power consumption of the load. The disc is placed between the gap of two electromagnets. The coil of shunt electromagnet is connected in parallel to supply voltage and coil of series electromagnet is connected in series with the load current.

The voltage coil produces magnetic field due to supply voltage and the current coil produces magnetic field due to the current through load. Thus, the field induces by the voltage coil is lagging by 90 \degree to the field of current coil. As such, eddy current is induced in the disc. The interaction of this eddy current and the magnetic fields causes a deflecting torque on disc. Thus the disc starts rotating.

The deflecting torque on the disc is proportional to the current and voltage of the coils. The braking magnet opposes the movement of the disc and equalizes it on the power consumption. The cyclometer counts the rotation of the disc.

Advantages of Induction Energy Meter

An induction type of energy meter has following advantages –

  1. This type of energy meter is simple in construction.
  2. These series and shunt type meters are widely used in domestic and industrial applications.

Disadvantages of Induction Energy Meter

An induction type of energy meter has following disadvantages –

  1. Accuracy of an induction energy meter is somewhat less due to creeping and other external fields.
  2. These types of energy meters are prone to tampering, which necessitates an electrical-energy-monitoring system.