ENERGY METER: 101 COMPREHENSIVE GUIDE

ENERGY METER

The meter used for the measurement of electrical energy consumed by loads in a definite time period is called an energy meter. The meter’s units are basically graded in Kwh. Energy meters are used to account for all energy transferred via the metering point for the purpose of billing.

The electrical energy is basically the power consumed in a circuit over a time period. Mathematically Energy = Power * Time

=V I cos φ t

• Where, V is Voltage
• I is Current
• t is time
• cos φ is the power factor

For a three phase system, Energy = √3 V I cos φ t.

Electromechanical type energy meters

These meters are based on old electromechanical technology, which are almost obsolete. The basic pronciple of their operation was induced magnetic flux on an aluminium disc by voltage and current coils, which resulted in rotation of the disc via developed torque. The number of rotation of the disc is proportional to the energy consumed at the load.

These type of meters contains:

• Current coil: It is connected in series to the load which produces magnetic field proportional to the flowing current.
• Voltage coil: It is connected in parallel to the load so that the supply voltage produces a proportional magnetic flux in the coil.
• Aluminium Disc: Placed in the fields of the coil, which induces eddy current in the disc, this generates a torque, which rotates the disc.
• Parmanent magnet: A parmanent magnet is placed inside which creates a braking force on the disc so that the disc rotates in proportion to power.
• Register: These are small dials connected to the disc via several gears, which displays the energy consumed in Kwh.

The current and the voltage coils develops flux as the circuit starts consuming power. This fluxes are out of phase and generates eddy current in the aluminium disc. The interraction of the eddy current and fluxes, develops a torque which rotates the disc. The speed of disc rotation has a direct relation to the instantaneous power V I cos φ. However, for energy, the number of disc rotation is counted.

Energy = Number of disc rotation * Meter cosntant, where the meter constant is rotation of disc / Kwh, specified by the meter manufacturer.

Electronic Energy meter

These are also called static type meters which lacks any moving parts. It uses digital technology to sense, measure and record energy consumption.

Sensing: In these meters, voltage is directly measured between phase-neutral for single phase meters or phase-phase for 3 phase meters. Current is measured by a CT or parallel resistor depending upon the rating of the meter.

Signal conditioning: The analogue signals from sensing mechanism are conditioned to remove noise by amplifiers and filters.

Analogue to Digital converter: Here the conditioned signals are converted to digital signals which represents instantaneous voltage and current.

Digital signal processing: Here a microprocessor calculates the power via multiplication of digital voltage and current signals. It also accounts for power factor. This unit can also equate the active energy by multiplying with the recorded time period between each digital signals.

Display: This unit displays the processed output be it instantaneous power, active energy, reactive energy and alternatives.

Difference between Electromechanical energy meter and Electronic energy meter

Applications of Energy meters

• Energy meters used for domestic metering and billing of electricity.
• Meters finds industrial applications for energy auditing, demand tracking, power quality monitoring, etc.
• Meters are used in Grid substations for feeder metering, to detect transmission and distribution loss, for finding out efficiencies in transformers and other energy audit needs. Here the high current and high voltage of the circuit are transformed by the use of current transformers and potential transformers and fed into the electronic meter.

Billing of Electricity

The billing is done by collecting the units measured from energy meter and substracting the previous month recorded energy from the meter. This provides the net units of energy consumed in a month. Now appling the rates/unit as per various utilities, the monthly bill is generated which is basically equal to net units of energy consumed in a month multiplied by rates/unit. Also some utilities charges some extra overhead costs varies utility to utility.

In some states there are unit charges based on various slabs of units consumed. Say for consuming 10units the charge is x while consuming above 100 units charge is x+y. In such case the billing is done according to these prevailing slabs.

FAQ’s

This article is a part of the Metering page, where other articles related to the topic are discussed in details.