Introduction to Single Phase Induction Type Energy Meter

Single phase induction type energy meter is the most common form of a.c. meters met with in every day domestic installations. These meters measure the electric energy in kilo-watt hours (KWH). The measurement of energy is the same process as measurement of power except that the instrument is not merely indicates the power or rate of supply of energy but must take into account also the length of time for which the rate of energy is continued.

Principle

The basic principle of induction type energy meter is electromagnetic induction. When an alternating current flow though two suitably located coils produces a rotating magnetic field which is cut by the metallic disc suspended near to the coils, thus emf induced in the disc which is circulates eddy current in it. by the interaction of rotating magnetic field and eddy currents, torque is developed and cause the disc rotates.

Constructions

The construction of this meter is more or less similar to an induction type wattmeter.  The main alterations are the provision of only one pressure coil upon the central limb of the shunt magnet and only one copper shading band upon this limb.  A single phase phase induction type energy meter has the following main parts of the operating mechanism;

  1. Driving system
  2. Braking system
  3. Moving system
  4. Recording mechanism

1.Driving System:

The driving system of the meter consists of two electromagnets. The core of these electromagnets is made up of laminated silicon steel. The coil of one of the electromagnets is excited by the load current. This coil is called current coil.  The coil of second electromagnet is connected across the supply and this coil carries the current proportional to the supply voltage. This coil is called pressure coil. The two electromagnets are known as series and shunt magnets respectively.

In order to obtain deflecting torque, current in the pressure coil must lag behind the supply voltage by 900. For this purpose, copper shading bands are provided on the central limb. The position of these bands is to bring the flux produced by the shunt magnet exactly in quadrature with the applied voltage.

  1. Moving System

This consists of aluminum disc mounted on a light alloy shaft. This disc is positioned in the air gap between series and shunt magnets. The upper bearing of the rotor is steel pin located in a hole in the bearing cap fixed to the top of the shaft. Since there is no control spring, the disc makes continuous rotation under the action of deflecting torque. The unique design for the suspension of the disc is used in the floating-shaft energy meter.

  1. Braking System

A permanent magnet positioned near the edge of the aluminium disc forms the braking system. The aluminium disc moves in the field of this magnet and thus provides a braking torque. By adjusting the position of the braking magnet speed of the disc can be controlled. If the braking magnet is moves toward the centre of the disc, flux cut by the disc is less which reduces the induced current and thus 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.

4.Registering or Counting Mechanism

 The function of registering mechanism is to record continuously a number which is proportional to the revolutions made by the moving system. The numbers of revolutions made by disc is a measure the electrical energy passing through the meter.single phase induction type energy meterWorking

When the single phase induction type energy meter is connected in the circuit, the current coil carries the load current and the pressure coil carries the current proportional to the supply voltage. The pressure coil winding is highly inductive as it has a large number of turns. The agnetic field produces by pressure coil is quadrature with the applied voltage. The magnetic field produced by the series magnet is in phase with the line current. Thus, a phase difference exists between the fluxes produced by the two coils. This sets up a rotating field which interacts with the disc and produces driving torque and, thus, disc starts rotating. The number of revolution 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.

Theory

When induction type energy meter is connected in the circuit to measure energy, the pressure coil carries current proportional to the supply voltage and the series magnet carries the load current. Therefore, expression for the driving torque is the same as for induction wattmeter.

Driving torque, Td  α power

Power α n

Power * t α nt

Energy α N

N = (nt) is the total number of revolutions in time t.

Meter Constant

Meter constant, K = No. of revolutions/kWh

Hence the number of revolutions made by the disc for 1 kWh of energy consumption is called meter constant.

Single phase induction type energy meter is extensively used for the measurement of electrical energy in a.c. circuits.