Watt-Hour Meter: What is it? (And How Does it Work)

What is a Watt Hour Meter?

A Watt-hour meter is a measuring device that can evaluate and record the electrical power passing through a circuit at a certain time. By implementing a Watt-hour meter, we can know how much electrical energy is used by a home, business, or an electrically powered device. Electrical utilities install watt-hour meters at their consumer’s premises to evaluate their electrical usage (for billing purposes).

The reading is taken in each billing period. Usually, the billing unit is Kilowatt-hour (kWh). This is equal to the total usage of electrical energy by a consumer of one kilowatt during a period of one hour, and it is also equal to 3600000 joules.

The Watt-Hour Meter is often referred to as an energy meter or electric meter or electricity meter, or electrical meter.

Mainly the watt-hour meter comprises a tiny motor and a counter. The motor will operate by diverting the exact fraction of current flowing in the circuit to be measured.

This motor’s running or turning speed is directly proportional to the amount of current flow through the circuit.

Thus, every revolution of the motor’s rotor is analogous to the given quantity of current flow in the circuit. A counter is attached to the rotor to add, and the usage of electrical energy is displayed from the total number of rotor revolutions.

Tampering and Security

Attaching a magnet outside of the old energy meter is the common tampering method seen. The combination of some capacitance and inductive load also results in a reduction in rotor speed.

The most modern meter can store up the previous value with time and date. So the tampering is avoided. Utilities install remote reporting meters to detect tampering.

Types of Watt Hour Meter

Basically, the watt-hour meter is classified into three different types as follows:

  • Electromechanical type induction meter
  • Electronic energy meter
  • Smart energy meters

Electromechanical Type Induction Meter

A non-magnetic and electrically conductive aluminum metal disc is made to revolve in a magnetic field in this type of meter. The rotation is made possible with the power passing through it. The rotation speed is proportional to the power flow through the meter.

Gear trains and counter mechanisms are incorporated to integrate this power. This meter works by counting the total number of revolutions, and it is relative to the usage of energy.

A series magnet is connected in series with the line, which comprises a coil of few turns with thick wire. A shunt magnet is connected in shunt with the supply and comprises a large number of turns with thin wire.

A braking magnet, a permanent magnet, is included for stopping the disc at the time of power failure and placing the disc in position. This is done by applying a force opposite to the rotation of the disc.
watt hour meter
A flux is produced by the series magnet that is directly proportional to the current flow, and the shunt magnet corresponding produces another flux to the voltage. Because of the inductive nature, these two fluxes lag each other by 90o.

An eddy current is developed in the disc, which is the interface of the two fields. A force corresponding produces this current to the product of instantaneous current, voltage, and phase angle.

A break torque is developed on the disc by the braking magnet positioned over one side of the disc. The speed of the disc becomes constant when the following condition is achieved, Braking torque = Driving torque.

The gear arrangement linked with the shaft of the disc is implemented for recording the number of revolutions. This is for single-phase AC measurement. An additional number of coils can be implemented for different phase configurations.

Electronic Energy Meter

The major feature of the electronic meter other than power usage measurement is that it can display the energy usage on a LED or LCD. In some advanced meters, the readings can be transmitted to remote areas.

It can also record the amount of usable energy in on-peak hours and off-peak hours. In addition, this meter can record the parameters of supply and load like voltages, reactive power used, the instantaneous rate of usage demand, power factor, maximum demand, etc.

Smart Energy Meter

In this type of meter, communication in both directions (Utility to the customer and customer to the utility) is possible.

Customer to utility communication includes parameter values, consumption of energy, alarms, etc. The utility to consumer communication includes disconnect/reconnect instructions, automatic meter reading system, upgrading of the software of the meter, etc.

Modems are implemented in this meter to make communications easy. The communication system includes fiber cable, power line communication, wireless, telephone, etc.

Advantages of Different Types of Watt Hour Meter

The main three types of matt hour meters are:

  1. Electromechanical Energy Meters
  2. Electronic Energy Meters
  3. Smart Energy Meter

The advantages of each of these types of watt-hour meters are listed below:

Electromechanical Energy Meter Electronic Energy Meter Smart Energy Meter
Simple construction. Non-linear loads. Reduction in need of visits for reading/taking monthly bills.
Accurate and precise measurement. More robust. Tampering of meters can be avoided.
Reliable. Improvement in quality of electrical distribution. Better accuracy.
The cost is low. Highly accurate High Efficiency 

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