Explanation of Single Phase Induction Type Energy Meter

Single Phase Induction Type Energy Meter

Single phase induction type energy meters are extensively used for the measurement of electrical energy in single phase A.C circuits. One can find such meters installed in homes. It is also called as single phase watt-hour meter.

Single Phase Induction Type Energy Meter

Principle: torque is produced to rotate the disc is due to interaction of two fluxes. Since the e.m.f induced in the disc by induction, this instrument operates on A.C only.

Construction: The below fig shows the various parts of a single phase induction type energy meter. It consists of a shunt magnet, a series magnet, a rotating disc, a brake magnet and a counting mechanism.

The shunt magnet consist of a number of M shaped iron laminated core wound with a fine wire of many turns i.e. coils and is connected across the supply. The shunt coil carries a current proportional to supply voltage and is known as Pressure coil.
The flux produced by shunt magnet lags behind the supply voltage by 90^o due to coil inductance. The series magnet consist of U shaped laminated core wound with a few turns of heavy gauge wire and is connected in series with the load and carries the load current. The wound coil is known as current coil.

A thin aluminium disc is mounted on a spindle between the two magnets so that it interacts with the fluxes of both the magnets. Copper shading ring is placed on central limb of the shunt magnet to adjust lagging of shunt coil flux behind the voltage by 90^o. It is also termed as quad band or lag adjuster or P.F compensator. Control springs are fixed to the spindle of the disc which provides controlling torque.

The braking magnet is of C shaped magnet which provides braking torque when the aluminium disc is in motion. The braking magnet rotates, eddy currents re induced in it due to flux produced by braking magnet. The eddy currents opposes the disc rotation (according to Lenz’s law). The number of revolutions of the disc is a measure of the electrical energy consumed by the load and is recorded on dial which are geared to the spindle (i.e. recording mechanism).

Working: When the energy meter is connected in the circuit, the pressure coil carries current proportional to the supply voltage, whereas the series coil carries the load current. The series magnet produces the flux Φ₁, which is in phase with the load current I. similarly the shunt magnet produces the flux Φ₂, which is lagging behind the voltage by 90^o. The two fluxes Φ₁ and Φ₂ induces an emfs (e₁and e₂) in the disc which further produce the eddy currents (i₁and i₂). These eddy currents i₁ and i₂ setup by the respective induced emfs are in phase with the corresponding emfs. The interaction between the fluxes and eddy current produces the driving force which results the disc to rotate.

The breaking torque (controlling torque) is provided by the braking magnet, and by adjusting position of this magnet, desired speed per unit can be obtained.