Energy Meter Definition

An energy meter is a device used to measure the total energy consumed by an electrical load over a given period. It calculates the power consumed by the load in kilowatt-hours (kWh) and is commonly used in both domestic and industrial AC circuits to measure power consumption. These meters are cost-effective and provide accurate readings.

What is a three-phase energy meter?

The three-phase energy meter is used to measure the power consumption of a three-phase supply. It is constructed by connecting two single-phase meters through a shared shaft. The total energy consumed is the combined reading from both meters.

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Construction of the Energy Meter

The construction of a single-phase energy meter involves four main components, which are:

  1. Driving System

  2. Moving System

  3. Braking System

  4. Registering System

Let's look into the details of each part.

  1. Driving System – The driving system is powered by an electromagnet, which acts as a temporary magnet excited by the current flowing through its coil. The electromagnet's core is made from silicon steel laminations. There are two electromagnets: one is the shunt electromagnet (at the top), and the other is the series electromagnet (at the bottom).

    • The series electromagnet is powered by the current flowing through the load.

    • The shunt electromagnet's coil is connected directly to the supply and carries a current proportional to the supply voltage.

    • A copper band is placed at the center limb of the magnet, which helps align the magnetic flux produced by the shunt electromagnet with the supplied voltage.

  2. Moving System – The moving system consists of an aluminum disc mounted on a shaft made from an alloy. This disc is placed in the air gap between the two electromagnets. When the magnetic field changes, eddy currents are induced in the disc. The interaction between these currents and the magnetic flux creates a deflecting torque, which causes the disc to rotate.

    • As the load consumes power, the disc rotates, and after a certain number of rotations, the meter registers the total energy consumed in kilowatt-hours.

  3. Braking System – The braking system consists of a permanent magnet that reduces the disc's rotation. The induced eddy currents in the rotating disc interact with the flux of the permanent magnet, producing a braking torque.

    • This torque opposes the movement of the disc and helps adjust its speed.

    • The permanent magnet is adjustable to modify the braking torque by shifting it to different radial positions.

  4. Registering System – The registering system is responsible for recording the number of rotations of the aluminum disc.

    • The rotations are directly proportional to the energy consumed by the load in kilowatt-hours.

    • As the disc rotates, it drives pointers on dials that indicate the energy usage. The total energy consumption is obtained by multiplying the number of disc rotations by the meter constant.

Working of the Energy Meter

The energy meter works by measuring the rotations of an aluminum disc, which is placed between the two electromagnets. These electromagnets consist of a shunt magnet and a series magnet. The pressure coil of the shunt magnet creates a magnetic field due to the supply voltage, while the current coil of the series magnet creates a magnetic field from the current flowing through the load.

  • The magnetic field created by the pressure coil lags behind the magnetic field of the current coil by 90º.

  • This results in eddy currents being induced in the disc, and the interaction between these currents and the magnetic fields creates a torque that causes the disc to rotate.

  • The speed of the disc's rotation is proportional to the power consumption of the load.

  • The permanent magnet controls the speed of rotation by opposing the disc's movement. The braking torque from the permanent magnet helps balance the rotation according to the power consumed.

  • The cyclometer keeps track of the disc's rotations, recording the total energy consumed.

Theory of the Energy Meter

The pressure coil has a certain number of turns, making it more inductive. The magnetic reluctance is minimal due to the short air gap in the magnetic circuit. The current Ip flowing through the pressure coil creates two fluxes, Φp1 and Φp2.

  • Φp1 mainly passes through the side gap due to the low reluctance, while Φp2 passes through the disc and induces the driving torque.

  • The flux produced by the pressure coil is lagging by 90º and is alternating, which induces eddy currents in the disc.

  • The load current flowing through the current coil generates flux Φs, which also induces eddy currents in the disc. The interaction of these eddy currents with the flux from both coils generates torques in opposite directions.

  • The net torque is the difference between the two opposing torques, causing the disc to rotate, which is proportional to the energy consumed.

Conclusion

The energy meter is a key device that measures power use over time. It has four main parts: the driving system, the moving system, the braking system, and the registering system. These components work together to provide accurate energy measurements.

The use of electromagnetic principles, eddy currents, and braking torque ensures reliable readings. This makes the energy meter vital in both domestic and industrial environments for tracking electricity usage. Its simplicity and accuracy help users manage their energy consumption effectively. With its cost-effectiveness, the energy meter is essential in electrical systems worldwide, allowing control over power systems.