Wattmeter and Energy Meter Basics

Wattmeter and Energy Meter

• A wattmeter is an instrument used to measure electric power given to or developed by an electrical circuit.
• There are two types of wattmeters:
  • Electrodynamometer wattmeter (for both DC and AC power measurement)
  • Induction wattmeter (for AC power measurement only)
• A wattmeter consists of two coils:
  • One coil is connected in series with the circuit (current coil)
  • One coil is connected in parallel with the circuit (voltage coil)

Energy Meter

• An energy meter is a device used to measure and record the consumption of electrical energy.
• It is widely used in residential, commercial, and industrial settings to ensure accurate metering of electricity usage.
• An energy meter has four main parts:
  • Driving System (electromagnet and copper band)
  • Moving System (aluminium disc)
  • Braking System (permanent magnet)
  • Registering System (counting mechanism)
• The working principle of an energy meter is based on Faraday's law of electromagnetic induction.
• The energy meter measures the power consumption of a load by interacting the magnetic fields of a current coil and a voltage coil.

How an Energy Meter Works

• Current Measurement: The energy meter measures the current flowing through it using a current coil.
• Voltage Measurement: The energy meter measures the voltage using a voltage coil.
• Electric Energy Calculation: The energy meter calculates the power consumption by multiplying the instantaneous voltage and current values.
• Power Calculation: The power consumed by the load is calculated by multiplying the voltage and current values.
• Energy Integration: The energy meter integrates the power over time to calculate the total energy consumption.
• Display and Communication: Modern energy meters have digital displays and communication capabilities to transmit data remotely.

Application of Energy Meters

• Single-phase energy meters are used in residential and small commercial settings.
• Three-phase energy meters are used in industrial and commercial settings with high electrical loads.
• Three-phase energy meters are suitable for measuring energy consumption in factories, power requirements in commercial buildings, and consumption of large machinery and equipment.

Types of 3-Phase Energy Meters

• There are two types of 3-phase energy meters:
  • Three phases, three-wire meters (permanent horseshoe magnet)
  • Three phases, four-wire meter (three coils and three aluminum discs)

Benefits of Using a 3-Phase Meter

• The ability to use different phases for different rooms.
• The ability to distribute the load.
• Real-time and recorded power consumption data.
• Protection of valuable equipment from high voltage damages.

Analysis of Three-Phase Systems

• A three-phase system consists of three phase conductors and a neutral conductor.
• The sum of the line currents in a 3-phase system is equal to zero.
• The three-phase system has several advantages, including:
  • Fewer conductors required compared to single-phase systems.
  • Continuous supply to the load.
  • Higher efficiency and minimum losses.
• The three-phase system can be connected in two ways:
  • Star connection (four wires, including a neutral conductor)
  • Delta connection (three wires, no neutral point)### Three-Phase Loads
• A three-phase load can be balanced or unbalanced.
• A balanced load has equal magnitude and phase angle between the three phases.
• In a balanced load, all phase voltages and line voltages are equal in magnitude.

Three-Phase AC Generator

• A three-phase AC generator has three windings, each with two terminals (start and finish).
• Connecting a separate load across each phase winding would require six wires, making the system complicated and costly.

Blondel's Theorem

• Blondel's Theorem is used to determine the type of metering installation needed for a particular service.
• The theorem states that one less element (or CT) is required in a watthour meter than the number of wires in the service to accurately meter the service.
• For a single-phase two-wire service, a watthour meter with one stator (1s meter) is required.
• For a three-phase, three-wire service (delta service), a form 5s meter with two elements is required.

Reactive Power Measurement

• Reactive power (Q) is caused by the phase shift between voltage and current due to inductance and capacitance.
• Reactive power is denoted by Q and expressed in volt-amperes reactive (VAR).
• The formula for reactive power is Q = VI sin Φ.
• Reactive power measurement is essential to determine the nature of the load and measure power factor.

Measurement of Reactive Power in Single-Phase Circuits

• A single-phase varmeter (volt-ampere reactive meter) is used to measure reactive power in single-phase circuits.
• The varmeter has an inductive voltage coil, which causes the current to lag 90° behind the voltage.
• The meter indication is Q = VI sin Φ.

Measurement of Reactive Power in Three-Phase Circuits

• For a three-phase balanced load, the reactive power measured is the same in all phases.
• Only a single wattmeter is required to measure the reactive power.
• The current coil is inserted in one phase, and the voltage coil is connected across the remaining two phases.

Active Power

• Active power is consumed due to the resistive effect of an electrical circuit.
• It is also known as resistive power.
• The active power is always positive, regardless of the instantaneous direction of voltage and current.
• It is the rate of energy consumption by the load.