THE WORKING AND DESIGN OF ENERGY METERS

About 90% of electricity consumers use Commercial distribution companies as their primary source, (private or government), while the payment system and rates vary per country, the system via which electricity is measured remains constant.

^{Digital Energy meter Wikimedia commons}

Last time we spoke about Electrical measuring instruments and some of its constructional/operational mechanism. We majorly dwelt on the measurement of voltage and current instantaneously which we classified as Indicating instruments. Today we will look at another type of Electrical measuring instruments known as the Integration Instruments.

An integration instrument is an indicating instrument plus its additional ability to summate the previously measured value over a given period of time. i.e. to measure the current taken over a 3- minute interval, I must design a system such that each instantaneously measured current value would be cumulated and summed.

The major applications of integrating instruments is the energy meters.

But what is energy, energy is of course the ability to do work, electrically speaking, it goes with the formulae

E=VIT where V – Voltage; I- current and T- Time taken.

VI is the power expended by the circuit, this means that an energy meter is simply a wattmeter that has the ability to cumulate its readings over a period of time.

WATTMETERS

A wattmeter measures electrical power but HOW? It is an indicating instrument which works both as a voltmeter and ammeter together. There are different types of wattmeter types e.g. the Induction type or the dynamometer type wattmeter.

But do we need a special wattmeter when we can just have a voltmeter and an ammeter then multiply their readings together when connected to the circuit? YES, there was an intentional mistake in the earlier formulae of Power.

It is true that Power = V*I, but when an instrument is connected to measure power, it measures the real component of the power which is given by VIcosΦ with Φ being the phase angle, therefore, using the previous method would only work where cosΦ=1 which in most cases is only applicable in dc circuits.

The induction type wattmeter works on the principle of induction while the dynamometer wattmeter works like the dynamometer however designed to measure both current and voltage thereby generating a deflection torque that would indicate the value of the power to be measured, as watt-meters are indicating instruments, they also have control torque and damping torque.

WATT-HOUR METER CONSTRUCTION

The Watthour meter is generally accepted as the most important electrical instrument economically. By virtue of construction, the Watthour meter is basically a Wattmeter having both control and damping torque removed, though it has a moving system, it does not have a pointer, its moving system is however connected to a recording mechanism/system.

Since it has no pointer, and there is no control mechanism to bring it to rest, the moving system keeps rotating and thereby acts as a motor. But to limit its speed a braking mechanism which would produce a braking torque must be factored into its design.

Consequently, the conversion sequence between Watt-meters and Watthour meters
Deflection torque ------------ Driving Torque
Control Torque ------------ Braking Torque

No need for damping torque as oscillation is not of importance in Energy meters.

The recording mechanism of the instrument works such that it measures the revolution per second of the rotating motor and by means of dials and gears indicating the value of power measured over a given time.

TO GET A DEEPER UNDERSTANDING OF HOW ENERGY METERS WORK, LET’S LOOK AT HOW WATT-METERS WORK

Like voltmeters and ammeters, the wattmeter also varies in design and construction, Now for the Induction type Wattmeter, it works by the principle of induction. It consists of 2 electromagnets, one to measure current the other to measure current proportional to the voltage the first e-magnet is known as the shunt magnet having a current coil around it while the other is known as the series magnet having a current carrying coil around it.

The shunt magnet is designed such that the flux produced by it would lag the supply voltage by 90 degrees. This statement simply means that the shunt magnet would be highly inductive in nature while the series magnet is designed such that its connection is in series with the supply thereby carrying the load current.

Its made to be non-inductive such that it doesn’t determine the lag or lead but the lag or lead of this coil would be largely determined by the load.

Having 2 electromagnets, an aluminum disc material will be placed between them such that it cuts the magnetic flux around them. By the principle of induction, eddy current is induced in the aluminum disc causing a deflection torque which is retarded by a controlling torque produced by spiral springs. So we have the measured power of the induction the instrument to be proportional to the deflection torque.

Technically speaking, by design, the shunt magnet lags the supply voltage by 90 thus produce a flux Φ_v but for the series magnet, it is in phase with the current and produces Φ_c.

INDUCTION WATTHOUR METER

Understanding the working of wattmeters itself as discussed previously will bring us to a more vivid understanding of your energy meter, we will go into more specific details of how that your out-door prepaid or postpaid energy meter works now. In old times, most outdoor energy meters work on the principle of induction.

While constructing it, we remove its control mechanism (hairspring) and replace it with a braking magnet, the aluminum disc acts as the motor in this case and the speed is retarded and brought to steady state by a braking magnet clipped on the aluminum disc, to induce a current that would oppose the flux of the disc which will eventually produce an equal and opposite torque called the braking torque.

When the braking torque is equal to the driving torque, the aluminum is said to be in the steady rotation and the speed can hence be measured at this point. Eventually, the steady state speed would be recorded in dials. But for digital displays, an ADC (analog to digital converter) chip is used to give a digital display.

Let’s talk DIGITAL METERING SYSTEM

Electromechanical energy meters (Induction energy meters) have been used a lot in recent times especially in Nigeria. However, we are coming to the realization of an even better metering system which is the digital system, the induction meters though rugged is still quite inaccurate especially when compared to the digital meters.

By design, the digital meter doesn’t need any of those torques to take its readings, it rather makes use of Voltage and current sensors (transducers) to measure the individual values of voltages and current coming into the load from the supply. Of course, these sensors shouldn’t receive a full dose of the supply voltage and current but should rather receive a fraction of it. Therefore a potential divider and a step-down element is used to step down the voltage received by the digital meter.

The output given by these sensors is transferred for processing. A signal conditioner does this phase of the work, it also contains a digital multiplexer which acts as a switching device for the 2 signals into a PIC (Peripheral interface controller) the PIC is used as a means of controlling the Integrated circuit.

The PIC is the heart of our meter because it is a Programmable Integrated circuit, (Usually programmed in the C language) it is programmed in such a way as to analyze the signal it receives and uses it to calculate the consumed power over a given time. Thereby feeding its result to an LCD display screen.

Digital meters are widely used in domestically and industrially today. i ask myself, who still uses analog ?.

REFERENCES

How digital energy meters Work

How an electric meter works

watthour meter

How the wattmeter works

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