Types of energy meters and how they work
An energy meter or watt-hour meter is an electrical instrument that measures the electrical energy used by consumers. Utility companies are one of the power sectors that install these meters in various places such as homes, industries, organizations, commercial buildings, etc. to bill the electricity usage of loads like lights, fans, refrigerators and other home appliances.
The basic unit of power is the watt, measured with a wattmeter. One kilowatt equals one thousand watts. If one kilowatt is used in one hour, one unit of energy is consumed. Therefore, an energy meter measures fast voltage and current, calculates their product and gives the instantaneous power. This power is integrated over a time interval to give the energy used during that time period.
Types of energy meters
Energy meters are divided into two basic categories such as:
Electromechanical induction meter
Electronic energy meter
Taking into account the following factors, electric energy meters are divided into two types:
Display type is analog or digital meter.
Metering point types: secondary transmission, grid, local and primary distribution.
End applications such as commercial, industrial and domestic use
Technology aspects such as single phase, three phase, high tension (HT), low tension (LT) and precision grade materials.
The supply connection can be single phase or three phase, depending on the power source used by the home or commercial facility. In particular, in this article, we will study the working principle of a single-phase inductive energy meter and the working principle of a three-phase electronic energy meter through the following explanations of two basic energy meters.
Single phase inductive energy meter
It is a well-known and most common ancient electric energy meter. It consists of a rotating aluminum disk placed on a spindle between two electromagnets. The rotational speed of the disc is directly proportional to the power, which is integrated through the gear train and counting mechanism. It consists of two silicon steel laminated electromagnets connected in parallel and in series.
A series magnet has a coil with several turns of thick wire connected in series with the circuit, while a parallel magnet has a coil with several turns of thin wire connected to a power source.
A brake magnet is a permanent magnet that exerts a force that opposes normal disc rotation, moving the disc to an equilibrium position and stopping the disc when power is removed.
Magnets connected in series produce a magnetic flux proportional to the current flowing, and magnets connected in parallel produce a magnetic flux proportional to the voltage. Due to inductive properties, these two fluxes lag by 90 degrees. The intersection of these two fields creates eddy currents in the disk using a force proportional to the product of the instantaneous voltage, current, and the phase angle between them. Brake magnets are placed on one side of the brake disc and generate braking torque on the brake disc by using a constant magnetic field provided by the permanent magnets. When the braking and driving torques are equal, the speed of the brake disc becomes stable.
The axis, or vertical spindle, of the aluminum disk is associated with a gear mechanism that registers a number proportional to the number of revolutions of the disk. This gear mechanism sets a series of numbers in a dial and indicates the amount of energy consumed over time.
This type of electric energy meter has a simple structure, but its accuracy is slightly poor due to the influence of external fields such as creep. A primary problem with these types of energy meters is that they are susceptible to tampering, which necessitates the need for an energy monitoring system. These series and split meter meters are widely used in domestic and industrial applications.
Compared to electromechanical induction energy meters, electronic energy meters are accurate, precise and reliable measuring instruments. When connected to a load, they consume less power and start measuring immediately. The following describes the electronic three-phase energy meter and its working principle.
Three-phase electronic energy meter
The meter is capable of performing current, voltage and power measurements in three-phase supply systems. By using these three phase meters, high voltage and current can also be measured using appropriate sensors. One of the types of three-phase energy meters is shown below (as an example), which ensures reliable and accurate energy measurement compared to electromechanical meters.
It uses single-phase power measurement IC AD7755 to collect and process input voltage and current parameters. Sensors such as voltage and current transformers are used to reduce the voltage and current ratings of the power lines to signal levels and provide them to the IC as shown in the figure. These signals are sampled and converted into digital signals, which are multiplied with each other to obtain the instantaneous power. These digital outputs are then converted into frequencies to drive electromechanical counters. The frequency of the output pulses is proportional to the instantaneous power, and (within a given interval) it delivers the energy of a specific number of pulses to the load.
The microcontroller accepts inputs from all three power measurement ICs for three-phase power measurement and acts as the controller of the system by performing all necessary operations such as storing and retrieving data from EEPROM, operating the meter using buttons to view energy consumption. brain, calibrates the phase and clears the readings; and, it also drives the display using a decoder IC.
So far, we have learned about energy meters and how they work. To understand the concept more deeply, the following description about energy meter provides complete circuit details and its connection using a microcontroller.
Energy meter circuit using microcontroller:
The figure below shows the electricity meter circuit implemented using Atmel AVR microcontroller. This circuit continuously monitors and obtains the voltage and current parameters of the mains single-phase power supply. The microcontroller obtains these parameter values from the signal conditioning circuit, which is driven by an operational amplifier IC.
This circuit has two current transformers connected in series with each power 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 into digital values and hence the microcontroller does the necessary calculations to find the energy consumption. The microcontroller is programmed by multiplying and integrating the voltage and current values from the ADC over a specified period of time and driving the counter mechanism accordingly to display the number of units (KW) consumed over a period of time.
In addition to energy measurement, the system also provides earth fault indication in case of any fault or overcurrent that may occur in the neutral or earth conductor and switches on the LED indication appropriately for earth fault detection as well as per unit consumption.
