The meter which is used for measuring the energy consumed by the electrical load is known as the energy meter. The total power consumed and utilised by the load at a particular interval of time is known as energy. It is used in domestic and industrial AC circuit for measuring the power consumption. The meter is less expensive and accurate. With the ever growing population and rapid urbanization, the demand for electricity has seen a massive surge over the past few decades. Accurate measurement and accounting of energy consumption have become very important for utilities to bill customers fairly as well as enhance power distribution and grid management. This is where energy meters play a important role as the primary instrument for monitoring electricity volume in households and industries.

In this article, we will learn every aspect of energy meters, including their construction, working principles, types, applications, and differences from other meters. The information in this article helps you extensively in your SSC JE Electrical and GATE Electrical preparation journey.

What is Energy Meter?

An energy meter, also known as an electric meter, is an advanced device used to measure the volume of electrical energy (measured in kilowatt-hours or kWh) consumed by a residence, building, or an industrial facility over a period of time (usually monthly). By recording the volume of electricity used at regular intervals, energy meters facilitate utilities to calculate customers' power bills accurately based on their actual consumption.

In essence, energy meters integrate power (measured in kilowatts or kW) consumed over the time of usage to compute the total energy or volume of electricity utilized. For example, if a residence draws power at a rate of 1 kW continuously for 10 hours, the energy consumption would be 1 kW * 10 hrs = 10 kWh. Energy meters are designed to cumulatively tally this consumption over varying time intervals, most commonly monthly.

The construction of a basic energy meter is shown below:

Energy meter has the following main parts:

• Driving System
• Moving System
• Braking System
• Registering System

Fig- Basic Diagram of Induction Type Energy Meter

We will discuss and explain these parts in detail below.

Driving System

The electromagnet is the primary component of the driving system in an energy meter. It is a temporary magnet activated by the current flowing through its coil. The core of the electromagnet is constructed from silicon steel laminations. The driving system includes two electromagnets: the upper one, known as the shunt electromagnet, and the lower one, known as the series electromagnet. The series electromagnet is energized by the load current flowing through the current coil. The coil of the shunt electromagnet is directly connected to the supply, carrying a current proportional to the shunt voltage. This coil is referred to as the pressure coil.

The center limb of the magnet features an adjustable copper band. The primary function of this copper band is to align the flux produced by the shunt magnet so that it is precisely perpendicular to the supplied voltage.

Moving System

The moving system consists of an aluminum disc mounted on an alloy shaft. This disc is positioned in the air gap between the two electromagnets. Due to the changing magnetic field, eddy currents are induced in the disc. These eddy currents interact with the magnetic flux, generating a deflecting torque. As electrical devices consume power, the aluminum disc begins to rotate. After a certain number of rotations, the disc displays the units of energy used by the load. The number of rotations is counted over specific intervals of time, allowing the disc to measure power consumption in kilowatt-hours.

Braking System

The permanent magnet is used to reduce the rotation of the aluminum disc. As the disc rotates, it induces eddy currents. These eddy currents intersect with the magnetic flux of the permanent magnet, producing a braking torque. This braking torque opposes the disc's movement, thereby reducing its speed. The permanent magnet is adjustable, allowing the braking torque to be modified by shifting the magnet to different radial positions.

Registering System

The primary function of the registration or counting mechanism is to record the number of rotations of the aluminum disc. The disc's rotation is directly proportional to the energy consumed by the loads, measured in kilowatt-hours (kWh). The rotation of the disc is transmitted to the pointers on various dials, which record different readings. The energy consumption in kWh is calculated by multiplying the number of disc rotations by the meter constant. 

Working of the Energy Meter

The energy meter contains an aluminum disc whose rotation indicates the power consumption of the load. This disc is positioned in the air gap between the series and shunt electromagnets. The shunt magnet is equipped with a pressure coil, while the series magnet contains a current coil. The pressure coil generates a magnetic field due to the supply voltage, and the current coil generates a magnetic field due to the current flow.

The magnetic field induced by the voltage coil lags 90º behind the magnetic field of the current coil, causing eddy currents to be induced in the disc. The interaction between the eddy currents and the magnetic field produces torque, which drives the disc's rotation. The force acting on the disc is proportional to the current and voltage in the coils. A permanent magnet regulates the disc's rotation by opposing its movement, balancing it with the power consumption. The cyclometer then counts the rotations of the disc.

Fig- Working of Energy Meter

The pressure coil has a large number of turns, making it highly inductive. Due to the small air gap, the reluctance of its magnetic circuit is very low. The current Ip flows through the pressure coil as a result of the supply voltage, and it lags by 90º. The current Ip generates the flux Φp, which is further divided into Φp1 and Φp2. The majority of the flux Φp1 passes through the side gap due to its low reluctance. The flux Φp2 travels through the disc, inducing a driving torque that rotates the aluminum disc.

The flux Φp is proportional to the applied voltage and lags by an angle of 90º. This alternating flux induces an eddy current Iep in the disc. The load current passing through the current coil induces the flux Φs. This flux generates the eddy current Ies in the disc. The eddy current Ies interacts with the flux Φp, and the eddy current Iep interacts with Φs to produce additional torques. These torques are opposite in direction, and the net torque is the difference between them.

Phasor Diagram and Theory of Energy Meter

The operation principle can be further explained using phasor diagrams showing current and voltage relationships. When voltage is applied to potential coil, it produces a flux φp which lags the voltage V by an angle Δ.

Fig - Phasor Daigram of Energy Meter

Let

• V is applied voltage
• I is load current
• ∅ is the phase angle of load current
• Ip is pressure angle of load
• Δ is the phase angle between supply voltage and pressure coil flux
• f frequency
• Z is impedance of eddy current
• ∝ is the phase angle of eddy current paths
• Eep is eddy current induced by flux
• Iep is eddy current due to flux
• Eev is eddy current due to flux
• Ies is eddy current due to flux

The net driving torque of the dis is expressed as

where

• K1 is constant
• Φ1 and Φ2 are the phase angle between the fluxes. For energy meter, we take Φp and Φs.
• β is the phase angle between fluxes Φp and Φp = (Δ – Φ), so

Driving Torque Td,

But Φp is proportional to v, and Φp ∝ I

If f, Z and α are constants,

\(T_{d}= K_{3}VI\sin \left ( \Delta -\Theta \right )\)

Assume N is steady speed, braking torque Tb will be:

\(T_{B}= K_{4}N\)

At steady state, the driving torque is equal to the braking torque.

\(K_{4}N = K_{3}VI\sin \left ( \Delta -\Theta \right )\)

\(N = KVI\sin \left ( \Delta -\Theta \right )\)

If Δ = 90º,

Then Speed wil be, 

\(N = KV\sin\left ( 90^{\circ}-\phi \right )=KVI\cos \phi\)
\(= K* Power \)

The speed of the rotation is directly proportional to the power.

Total no of Revoition = \(\int Ndt= K\int VI\sin \left ( \Delta -\Phi \right )\)

If Δ = 90º, total number of revolutions:

 \(= K\int VI\cos \phi dt\) 

For the large power measurement we use three phase energy meter.

Creeping in Energy Meter

In some meters, a phenomenon known as "creeping" may occur even when no current is flowing through the current coil. This slight continuous rotation is due to over-compensation of friction in the meter. Can caused due to:

• Stray Magnetic Fields: External magnetic fields affecting the meter.
• Excessive Friction: In the registering mechanism causing the disc to rotate slowly.
• Overvoltage: Causing the voltage coil to produce a magnetic field even without current flow.

To prevent creeping, small holes are drilled in the disk which stop its motion under the poles' edge, limiting rotation to a maximum of half a revolution. Some meters also use an attached iron piece achieving a similar effect.

Types of Energy Meter

We will learn about energy meter 3 phase, energy meter single phase etc. here!

• Single Phase Energy Meter: A single-phase energy meter is used in residential and small commercial applications where the electrical supply is single-phase. It measures the energy consumption in a single-phase circuit.
• Three Phase Energy Meter: A three-phase energy meter is used in industrial and large commercial applications where the electrical supply is three-phase. It measures the energy consumption in a three-phase circuit.
• Digital Energy Meter: A digital energy meter uses digital technology to measure and record energy consumption. It provides more accurate readings and additional features such as data logging and remote monitoring.
• Smart Energy Meter: A smart energy meter is an advanced type of digital meter that provides real-time data on energy consumption, supports remote monitoring, and can communicate with the utility for automated billing and energy management.

Applications of Energy meter

Energy meters are used in various applications, including:

• Residential: Measuring household energy consumption for billing purposes.
• Commercial: Monitoring energy usage in businesses for efficient energy management.
• Industrial: Recording energy consumption in factories and large installations.
• Utilities: Managing and forecasting load for efficient energy distribution.

Difference between Energy Meter and Wattmeter

While energy meters and wattmeters both measure power parameters, they have some distinct differences:

Conclusion

Energy meters are important instrumentation in electricity supply and distribution networks. By precisely monitoring energy volumes consumed at consumer premises, utilities can implement fair and transparent billing practices. Advanced smart meters further empower both utilities and customers through features like remote data collection, outage notifications, and consumer energy dashboards for efficiency. A thorough understanding of energy meters is essential for electrical engineers working with power distribution and management systems.

This article concludes all the information related to Energy meter, which helps to propel your preparation for various AE/JE examinations. To boost up your preparation, you should test yourself through a series of Mock Tests for Electrical Engineering Exams. You can check the syllabus for the AE/JE exam. You can visit the Testbook app to keep yourself updated with all the exam-oriented information related to the upcoming examinations, including GATE Electrical, SSC JE, ESE, RRB JE, and state AE/JE Electrical exam.