Measurement of Energy Using Single Phase Energy Meter


Aim: To measure electrical energy in a single-phase AC circuit using induction type energy meter.

Apparatus Required

SL NoApparatusSpecificationQty
1.Energy MeterSingle Phase
300V, 10A
2.WattmeterSingle Phase
300V, 5A, UPF
3.Variable Lamp LoadEach lamp
230V, 40W


The meter which is used for measuring the energy utilized by the electric load is known as the energy meter.

Single Phase induction type energy meter is also popularly known as ‘watt-hour meter’.

Construction of Energy Meter

The energy meter has four main parts. They are the

  1. Driving System
  2. Moving System
  3. Braking System
  4. Registering System
    The detail explanation of their parts is written below :

1. Driving System – The electromagnet is the main component of the driving system. It is the temporary magnet which is excited by the current flow through their coil. The core of the electromagnet is made up of silicon steel lamination. The driving system has two electromagnets. The upper one is called the shunt electromagnet, and the lower one is called series electromagnet.

The series electromagnet is excited by the load current flow through the current coil. The coil of the shunt electromagnet is directly connected with the supply and hence carries the current proportional to the shunt voltage. This coil is called the pressure coil.

The center limb of the magnet has the copper band. These bands are adjustable. The main function of the copper band is to align the flux produced by the shunt magnet in such a way that it is exactly perpendicular to the supplied voltage.

2. Moving System – The moving system is the aluminum disc mounted on the shaft of the alloy. The disc is placed in the air gap of the two electromagnets. The eddy current is induced in the disc because of the change of the magnetic field. This eddy current is cut by the magnetic flux. The interaction of the flux and the disc induces the deflecting torque.

When the devices consume power, the aluminum disc starts rotating, and after some number of rotations, the disc displays the unit used by the load. The number of rotations of the disc is counted at a particular interval of time. The disc measured the power consumption in kilowatt-hours.

3. Braking system – The permanent magnet is used for reducing the rotation of the aluminum disc. The aluminum disc induces the eddy current because of their rotation. The eddy current cut the magnetic flux of the permanent magnet and hence produces the braking torque.

This braking torque opposes the movement of the disc, thus reduces their speed. The permanent magnet is adjustable due to which the braking torque is also adjusted by shifting the magnet to the other radial position.

4. Registration (Counting Mechanism) – The main function of the registration or counting mechanism is to record the number of rotations of the aluminum disc. Their rotation is directly proportional to the energy consumed by the loads in the kilowatt-hour.

The rotation of the disc is transmitted to the pointers of the different dial for recording the different readings. The reading in kWh is obtained by multiply the number of rotations of the disc with the meter constant.


  • Connect the circuit as shown in the circuit diagram.
  • Note down the Energy-meter constant ‘K’ rotations/KW-Hr using the same determine the energy consumption recorded in KW-Hr per rotation of the Energy-meter disc.
  • Switch on the supply. With all the lamps On, Note down the time for ‘n’ rotations of the energy meter disc. Also, note down the wattmeter reading.
  • Determine the energy consumption of the lamp during ‘n’ rotations of the disc by both the energy-meter and wattmeter method using appropriate formulas as given.
  • Repeat the above procedure for different number of disc rotations.

Observations and Calculations

Circuit diagram

Tabular Column

No of
rotations ‘n’
Time ‘t’
for ‘n’ rotations
Wattmeter Reading
‘W’ Watts
E=W*t / (36*105)
by the
meter E=n/K


Energy meter constant K = _______ Rotations/KW-Hr

Energy recorded per rotation of the disc, 1/K = ________KW-Hr

Wattmeter Constant, K1= VIcosϕ/ Full scale deflection = ___


  • Energy Consumption as recorded y Energy meter when ______ lamps were on ______ KW-Hr
  • Energy Consumption as calculated using wattmeter when _____ lamps were On _______ KW-Hr

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