Energy meters: working and classification

Energy Meter

What is Energy meter?

The energy meter is nothing but a device or instrument which used to measure the amount of electrical energy consumed by the consumer or the load. This load can be domestic, commercial or industrial. SI unit of electrical power is watt. So, this meter also known as watt-hour (Wh) meter. If an electric load consumed one kilowatt power in one hour, it considered as one kWh consumption or one unit of energy consumed. These meters installed at every place where utility is supplying electrical energy. The energy meter always connected in series with the load. The load will be single-phase or three-phase and according to the type of load, energy meter classified as a single-phase meter or three-phase meter. For single-phase or lower load, energy meter can directly connect with the load but in case of higher load (industrial-3-Φ), step down current transformer used to reduce current.

Basically, these meters are measuring an instantaneous voltage and current, and calculate instantaneous power by multiplying the voltage and current. To find final energy utilization, the instantaneous power integrated over a period of time.

According to the working principle, energy meter classified into three types;

  • Electromagnetic induction type energy meter
  • Electronic energy meter
  • Smart energy meter

Electromagnetic induction type energy meters


This energy meter works on the principle of electromagnetic induction. These meters not used in the present system. This energy meter consists of two electromagnets and one aluminum disc. These two electromagnets are made up of laminated silicon steel. Both electromagnets carry two windings. The upper electromagnet has an M-shaped core and consists of a coil, which known as pressure coil or voltage coil. Upper electromagnet known as a shunt magnet.

The lower electromagnet known as a series magnet (U-shaped) and carries two coils, which known as the current coil. The pressure coil directly connected to the supply. So, current passes through this coil proportional to the shunt voltage. The pressure coil consisting of many turns of thin wire and the current coil consisting of few turns of thick wire. In the lower part of the shunt magnet, the copper band is placed, which provides frictional compensation and the shunt magnet flux.

The Aluminium disc mounted on a spindle between two electromagnets. This disc can easily rotate on the vertical shaft without any friction. The permanent magnet placed near the Aluminium disc. This magnet known as a break magnet.

The registering system used to record the number of rotations of Aluminium disc. Their rotation directly proportional to the electrical energy consumed by the load.

Electromagnetic induction type energy meter
Electromagnetic induction type energy meter

Connection Diagram

For single-phase and three-phase supply, energy meter connected as shown in below figure.

Connection Diagram of single Phase Energy Meter
Connection Diagram of single Phase Energy Meter


Connection Diagram of Three Phase Energy Meter
Connection Diagram of Three Phase Energy Meter


The Aluminium disc placed between the two electromagnets. Voltage coil connected to the supply voltage. The magnetic field induced in the voltage coil proportional to system voltage and the magnetic field induced in the current coil proportional to current flow in the load. Both fields are perpendicular to each other and because of the magnetic fields, the eddy current produced in the Aluminium disc. The torque produced due to the interaction of the eddy current and magnetic field, which creates a force on the disc and disc starts rotating. This torque is directly proportional to the product of the voltage and current.

Torque Equation

torque equation
torque equation


  • The cost of this meter is less compared to the electronic energy meter.
  • Simple in construction and maintenance is easy.
  • High torque to weight ratio.


  • Less accurate
  • Easy to tamper and less secure

Electronic energy meters

Electronics energy meter classified into two parts;

  • Analog energy meter
  • Digital energy meter

Analog energy meters

In analog energy meter, the magnitude of voltage and current obtained from the potential transformer and current transformer respectively. These analog values converted into digital samples with the help of analog to digital converter (ADC). These samples converted into frequency signals by a frequency converter. It used to drive the counter system, which integrates these samples over time and results in the reading of power consumption. Analog energy meter more accurate compared to electromechanical energy meter but less accurate compared to digital energy meter.

Analogue Electronic Energy Meter
Analogue Electronic Energy Meter

Digital energy meters

In digital energy meter, a microprocessor or digital signal processor used. Similar to an analog energy meter, voltage and current transducers connected to an analog to digital converter and it connected with the microprocessor. The voltage and current samples multiplied in the microprocessor. The phase angle between voltage and current measured by a microprocessor which used to calculate reactive power and power factor. To perform these tasks in a microprocessor, it programmed and it is able to calculate parameters like; electrical power consumption, power factor, reactive power, voltage, current, date & time and charges according to the tariff. Digital energy meter has an LCD or LED display, which used to display parameters.

The digital energy meter more accurate compared to analog and electromechanical energy meter and lesser chances of tampering and energy theft in a digital energy meter. But the installation cost of the digital energy meter high compared to analog and electromechanical energy meter. The performance of the digital energy meter is depending on the microprocessor.

Digital Electronic Energy Meter
Digital Electronic Energy Meter

Smart energy meter

The smart energy meter is the next generation of an electric meter. The smart meters are installed in developed countries.

Advantages of the smart energy meter

  • It can detect an outage
  • It can limit the demand
  • Record consumer consumption
  • Remote disconnection and reconnection
  • It can monitor the quality of supply
  • Advanced Communication system

The smart kWh meter has the ability to measure the power in both directions; utility to load and load to the utility. For example, the smart meter is installed at home and a solar rooftop is also installed. Let us assume that, in the present condition, the generation of solar is higher than a load, in this condition power will transfer from load (home) to utility and if generation is less than the demand, in this condition, power will transfer from utility to load. In this case, the meter can measure the power in both directions.

The smart meter has an advanced communication system for data transfer. It can transfer data to the utility like energy consumption, voltage, and current value, power factor, etc. The power theft is not possible in case of a smart meter. The consumer can directly pay money through the meter and not need to visit the consumer for a meter reading. It can connect to your smartphone also. So, you can get details of all parameters from your phone.

The smart meter has only one disadvantage that is the cost. The cost of this meter is very high and to develop and install the smart meter advanced technology required.

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