- Energy Meter Definition: An energy meter, also known as a watt-hour meter, is a device that measures electrical power consumption.
- Main Components: The energy meter consists of four main parts: driving system, moving system, braking system, and registering system.
- Working Principle: The meter works by rotating an aluminum disk using magnetic fields from voltage and current coils, with the rotation proportional to power consumption.
- Braking System: A permanent magnet generates eddy currents that create a braking torque to control the disk’s speed.
- Energy Display: The energy consumed is displayed on a meter window using dials that show the rotations of the aluminum disk.
Energy meters measure electrical energy, not instantaneous power alone. A meter that records energy in watt-hours is also called a watt-hour meter. This page describes the construction and operation of a single-phase induction-type electromechanical meter for AC active energy.
The operating mechanism has four systems. Each system converts, controls or records the disc motion:
- Driving system
- Moving system
- Braking system
- Registering system
Driving System
The driving system uses two laminated magnetic cores. The series magnet carries a few turns of thick wire, and its current coil is connected in series with the load. Its flux therefore follows the load current.
The shunt magnet carries many turns of fine wire. Its voltage coil is connected across the supply and is designed with a high inductance-to-resistance ratio. Adjustable copper shading bands set the required phase displacement of the shunt flux and provide compensation during calibration.
Moving System
A thin aluminium disc sits in the air gaps of the series and shunt magnets and turns on a vertical spindle. The alternating flux from each magnet induces eddy currents in the disc. Interaction between those currents and the two magnetic fields produces driving torque. Under the intended phase relationship, the average driving torque is proportional to active power.
Braking System
A permanent brake magnet spans the edge of the rotating disc. Motion through its field induces additional eddy currents in the aluminium. Their interaction with the magnet’s field creates a torque opposite to rotation. This braking torque increases with speed, so the disc settles where braking torque balances driving torque. The magnet position is adjusted during calibration.
Registering System
The registering system counts disc revolutions. A worm or pinion on the spindle drives a gear train connected to the dials. Each revolution represents a fixed amount of energy, so the accumulated count can be displayed directly in kilowatt-hours.
Working Principle of Energy Meter
Single-phase induction-type energy meters use two linked mechanisms:
- Driving and braking an aluminium disc at a speed proportional to active power.
- Counting disc revolutions to register the energy transferred over time.
Rotation of an Aluminum Disk
The two coils create alternating fluxes with the required phase relationship. One magnetic field is set by the supply voltage, while the other is set by the load current. The voltage-related flux is adjusted to lag the supply voltage by approximately 90o. Each flux induces an eddy current in the disc, and the cross-interactions produce driving torque proportional to active power.
The permanent magnet supplies opposing torque proportional to disc speed. At steady state, the balance of the two torques makes rotational speed proportional to active power. If the load consumes no active power, there is no sustained driving torque.
Arrangement of Counting and Displaying the Energy Consumed
The spindle gear drives a register made from a train of gears and numbered dials. The gear ratio converts disc revolutions into a cumulative energy reading. The meter constant states the watt-hours represented by one revolution, while the dials present the total in kilowatt-hours. During calibration, the copper shading ring is adjusted so the shunt-magnet flux has the required phase relationship to the supply voltage. This adjustment supports correct registration across the specified load and power-factor conditions.
