Single phase induction type energy meter

Induction type single phase energy meter:

Single phase induction type energy meter is extensively used to measure energy supplied to a single phase circuit.

Operating principle of Single phase induction type energy meter:

The operation of induction type energy meter depends on the passage of alternating current through two suitably located coils producing rotating magnetic field which interacts with a metallic disc suspended near to the coils and cause the disc to rotate.

The current coil carries the line current and produces field in phase with the line current. The pressure coil is made highly inductive so that the current through it lags behind the supply voltage by 90 degrees. Thus, a phase difference of 90 degrees exists between the fluxes produced by the two coils. This sets up rotating field which interacts with the disc to cause it to rotate.

Construction of Single phase induction type energy meter:

A single phase induction type energy meter generally has:

1. Moving system

2. Operating mechanism

3. Recording mechanism

 

Single phase induction type energy meter diagram

1. Moving system: The moving system consists of a light aluminium disc mounted on a vertical spindle. The spindle is supported by a up-shaped jewelled bearings at the bottom end and has a spring journal bearing at the top end.

There is no pointer and control spring so that the disc makes continuous rotation under the action of deflecting torque.

2. Operating mechanism:  It consists of series magnet, shunt magnet and breaking magnet.

Series magnet:  The series magnet consists of a number of U-shaped laminations assembled together to form a core. A thick wire of few turns is wound on both legs of the U-shaped laminated core. The wound coil is known as current coil and is connected in series with the load so that it carries the load current. The series magnet is placed underside the aluminium disc and produces magnetic field proportional to and in phase with the current.

Shunt Magnet: The shunt magnet consists of a number of M-shaped laminations assembled together to form a core. A fine wire of large turns is wound on the central limb of this magnet. The wound coil is known as pressure coil and is connected across the load so that it carries current proportional to supply voltage. the shunt magnet is placed above the aluminium disc as shown.

In order to obtain deflecting torque, current in the pressure coil must lag  behind the supply voltage by 90 degrees. This necessary phase shift is obtained by placing a copper ring over central limb of shunt magnet. This copper ring acts as a short circuited transformer secondary. As its inductance is high as compared with its resistance, the current circulating in the ring will lag by nearly 90 degrees behind the voltage producing it.

Braking magnet: The speed of aluminium disc is controlled to the required value by the C-shaped permanent braking magnet . The magnet is mounted so that the disc revolves in the air gap between the polar extremities. As the disc rotates, currents are induced in the disc because it cuts the flux produced by the breaking magnet. The direction of the current in the disc is such that it opposes the rotation of the disc. Since the induced currents in the disc are proportional to the speed of the disc, therefore, breaking torque is proportional to the disc speed.

3. Recording mechanism: The number of revolutions of the disc s a measure of the electrical energy passing through the meter and is recorded on dials which are geared to the shaft.

Working: 

When the energy meter is connected in the circuit to measure electrical energy, the current coil carries the load current whereas the pressure coil carries current proportional to the supply voltage. The magnetic field due to current coil is in phase with line current whereas the magnetic field produced due to pressure coil lags approximately 90 degrees behind the supply voltage.

The current coil field produces eddy currents in the disc which reacts with the field due to the pressure coil. Thus, a driving force is created which causes the disc to rotate.

The braking magnet  provides the braking torque on the disc. By altering the position of this magnet, desired speed can be obtained. The spindle is geared to the recording mechanism so that electrical energy consumed in the circuit is directly registered in kWh.