Induction type instruments are used only for a.c measurements. These instruments can be used either as ammeter, voltmeter or wattmeter. However, the induction principle finds its widest application as an energy meter. Induction type single-phase energy meter is used invariably to measure the energy consumption in any a.c circuit in a prescribed period where supply voltage and frequency are constant. Energy meter is an integrating instrument which measures the total quantity of electrical energy supplied to the circuit in a given period.
Principle:
The basic principle of induction type energy meter is electromagnetic induction.
When alternating current flows through two suitably located coils (current coil and potential coil) produces rotating magnetic field which is cut by the metallic disc suspended near to the coils, Thus an e.m.f is induced in the disc which circulates eddy currents in it. By the interaction of rotating magnetic field and eddy currents, torque is developed and causes the disc to rotate.
The driving system of the meter consists of two electromagnets, a. Series magnet
b. shunt magnet
a. Series magnet:
It consists of a number of U- shaped laminations of silicon steel together to form a core. A core of thick wire having a few turns is wound on both the legs of U-shaped magnet as shown in fig. This coil is connected in series with load. Thus it is excited by the circuit current I and is known as current coil. This magnet is placed below the aluminium disc and produces the magnetic field φse proportional to and in phase with the line current I.
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b. Shunt Magnet
It consist of a number of M-shaped lam ina t io ns of s il i c o n steel assembled together to form a core. A coil of thin wire having large number o f turns is wounded on the central limb of the magnet as shown m above: Fig. The coil is connected across the load. Thus it is excited by the current proportional to the supply voltage and is known as, potential or pressure coil. This magnet is placed above the aluminium disc.
In order to obtain deflecting torque, current in the pressure coil must lag behind the supply voltage by 90°. For this the copper shading band (Short circuiting copper ring) is provided on the central limb of the shunt magnet. The phase difference of 90° is obtained by adjusting the position of this shading band. The shading band acts as Short circuited transformer secondary. Since its resistance Is negligibly small as compared to its inductance, therefore current circulating in the shading band lags behind the supply voltage nearly by 90°. Thus the shunt magnet produced a field φsh proportional to applied voltage. This field is in phase with the current flowing through the pressure coil Ip but is in quadrature with the applied voltage.
Moving System:
It consists of a light aluminium disc mounted on a vertical spindle. The aluminium disc is positioned in the air gap between series and shunt magnets, The spindle is supported by a cup shaped jewelled bearing at the bottom end and has a spring jewel bearing at the top end. Since there is no control spring the disc makes continous rotation under the action of deflecting torque.
Braking system:
A permanent magnet positioned near the edge of the aluminium disc as showin the Fig.
forms the braking system. When the aluminium disc moves in the field of the braking magnet, flux is cat and currents are induced in the disc. The direction of induced current is such that it opposes the rotation (lenz's law). Thus braking torque is produced. Since the induced current is proportional to the speed of the disc (N) therefore braking torque (T ) is proportional to the disc speed (ie) T. x N.
The position of braking magnet is adjustable and therefore, braking torque can be adjusted by shifting the magnet to different radial positions. If the braking magnet is moved towards the centre of the disc, flux cut the disc is less which reduces the induced current and thus the braking torque is reduced. Hence by the inward movement of the magnet, braking torque decreases but the speed of the disc increases and vice-versa.
Recording mechanism
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The function of recording or registering mechanism is to record continuosly a number on the dial which is proportional to the revolutions made by the moving system. The number of revolutions of the disc is a measure of the electrical energy passing through the meter.
Working:
When the energy meter is connected in the circuit, the current coil carries the load current and pressure coil carries the current proportional to the supply voltage. The magnetic field produced by the series magnet (series coil) is in phase with the line current and magnetic field produced by the shunt magnet (pressure coil) is in quadrature with the applied voltage (since the coil is highly inductive). Thus a phase difference exist between the fluxes produced by the two coils. This set up a rotating field which interacts with the disc and produces a driving torque and thus, disc starts rotating. The number of revolutions made by the disc depends upon the energy passing through the meter. The spindle is geared to the recording mechanism so that electrical energy consumed in the circuit is directly registered in kWh.
The speed of the disc is adjusted by adjusting the position of the braking magnet. For example, if the energy meter registers less energy than the energy actually consumed in the circuit. Then, the speed of the disc has to be increased which is obtained by shifting the braking magnet nearer to the centre of the disc and vice-versa.
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