Types of Energy Meters and Their Working Principles

Energy Meter or Watt-Hour Meter is an electrical instrument that measures the amount of electrical energy used by the consumers. Utilities are one of the electrical departments, which install these instruments at every place like homes, industries, organizations, commercial buildings to charge for the electricity consumption by loads such as lights, fans, refrigerator, and other home appliances.

The basic unit of power is watts and it is measured by using a watt meter. One thousand watts make one kilowatt. If one uses one kilowatt in one-hour duration, one unit of energy gets consumed. So energy meters measure the rapid voltage and currents, calculate their product and give instantaneous power. This power is integrated over a time interval, which gives the energy utilized over that time period.

Types of Energy Meters

The energy meters are classified into two basic categories, such as:

• Electromechanical Type Induction Meter
• Electronic Energy Meter

Energy meters are classified into two types by taking the following factors into considerations:

• Types of displays analog or digital electric meter.
• Types of metering points: secondary transmission, grid, local and primary distribution.
• End applications like commercial, industrial and domestic purpose
• Technical aspects like single phases, three phases, High Tension (HT), Low Tension (LT) and accuracy class materials.

The electricity supply connection may be either single phase or three phase depending on the supply utilized by the domestic or commercial installations. Particularly in this article, we are going to study about the working principles of single-phase electromechanical induction type energy meter and also about three-phase electronic energy meter from the explanation of two basic energy meters as described below.

Single Phase Electromechanical Induction Energy Meter

It is a well-known and most common type of age-old energy meter. It comprises a rotating aluminum disc placed on a spindle between two electromagnets. The rotation speed of the disc is proportional to the power, and this power is integrated by the use of gear trains and counter mechanism. It is made of two silicon steel laminated electromagnets: shunt and series magnets.

Series magnet carries a coil which is of a few turns of thickness wire connected in series with the line; whereas the shunt magnet carries a coil with numerous turns of thin wire connected across the supply.

Braking magnet is a kind of permanent magnet that applies the force opposite to the normal disc rotation to move that disc a balanced position and to stop the disc while power gets off.

Series magnet produces a flux which is proportional to the flowing current, and shunt magnet produces a flux proportional to the voltage. These two fluxes lag at 90 degrees due to inductive nature. The interface of these two fields produces eddy current in the disk, utilizing a force, which is proportional to the product of instantaneous voltage, current and the phase angle between them. A braking magnet is placed over one side of the disc, which produces a braking torque on the disc by a constant field provided by using a permanent magnet. Whenever the braking and driving torques become equal, the speed of the disc becomes steady.

A Shaft or vertical spindle of the aluminum disc is associated with the gear arrangement that records a number proportional to the revolutions of the disc. This gear arrangement sets the number in a series of dials and indicates energy consumed over time.

This type of energy meter is simple in construction and the accuracy is somewhat less due to creeping and other external fields. A foremost problem with these types of energy meters is their proneness to tampering, which necessitates an electrical-energy-monitoring system. These series and shunt type meters are widely used in domestic and industrial applications.

Electronic energy meters are accurate, precise and reliable type of measuring instruments when compared to electromechanical induction type meters. When connected to loads, they consume less power and start measuring instantaneous. So, electronic type of three-phase energy meter is explained below with its working principle.

3-Phase Electronic Energy Meter

This meter is able to perform current, voltage and power measurements in three phase supply systems. By using these three phase meters, it is also possible to measure high voltages and currents by using appropriate transducers. One of the types of three-phase energy meters is shown below (given as an example) that ensures reliable and accurate energy measurement compared to the electromechanical meters.

It uses AD7755, a single-phase energy measurement IC to acquire and process the input voltage and current parameters. The voltage and currents of the power line are rated down to signal level using transducers like voltage and current transformers and given to that IC as shown in the figure. These signals are sampled and converted into digital, multiplied by one another to get the instantaneous power. Later these digital outputs are converted to the frequency to drive an electromechanical counter. The frequency rate of the output pulse is proportional to the instantaneous power, and (in a given interval) it gives energy transfers to the load for a particular number of pulses.

The microcontroller accepts the inputs from all the three energy measurement ICs for three-phase energy measurement and serves as the brain of the system by performing all the necessary operations like storing and retrieving data from EEPROM, operating the meter using buttons to view energy consumption, calibrating phases and clearing readings; and, it also drives the display using decoder IC.

Till now we have read about the energy meters and their working principles. For a deeper understanding of this concept, the following description about the energy meter gives complete circuit details and its connections using a microcontroller.

Energy Meter Circuit Using Microcontroller:

The below figure shows the watt-hour meter circuit implemented by using Atmel AVR microcontroller. This circuit continuously monitors and acquires voltage and current parameters of the mains single phase supply. Microcontroller gets these parameter values from a signal conditioning circuit, which is driven by OP-AMP ICs .

 This circuit has two current transformers connected in series with each supply line: phase and neutral. The current values from these transformers are sent to the respective ADC of the microcontroller, and then the ADC converts these values to digital values, and thus the microcontroller does necessarily calculations to find the energy consumption. The Microcontroller is programmed in such a way that the voltage and current values from the ADC are multiplied and integrated over a specified time period, and then correspondingly drive the counter mechanism that displays the number of units consumed (KWs) over a time period.

In addition to the energy measurement, this system also provides earth fault indication in case of any fault or overcurrent that may occur in neutral or earth line and appropriately turns the Light Emitting Diodes indication for earth fault detection as well as for every unit consumption.

This article is about watt-hour meter circuit and its working principles. This is also known as energy meter- which is used in developing electrical and electronic projects kits by different technologies. For any help regarding the concepts like tampering of energy meter and energy meter billing using wireless technology, or comment in the section given below.

Photo Credits:

• Single Phase Induction Energy meter by engineering
• 3-Phase Electronic Electricity Meter by analog
• Electricity Meter Circuit Using Microcontroller by next