Page 302 - Electrician - TT (Volume 1)

ELECTRICIAN - CITS

Energy meter (analog)

Objectives: At the end of this lesson you shall be able to:
• describe the construction and working principle of single phase energy meters
• state and explain creeping error in energy meter.

Necessity of energy meter: The electrical energy supplied by the Electricity board should be billed, based on
the actual amount of energy consumed. We need a device to measure the energy supplied to a consumer.
Electrical energy is measured in kilowatt hours in practice. The meter used for this is an energy meter.
Principle of a single phase induction type energy meter: The operation of this meter depends on the induction
principle. Two alternating magnetic fields produced by two coils induce current in a disc and produce a torque
to rotate it (disc). One coil (potential coil) carries current proportional to the voltage of the supply and the other
(current coil) carries the load current. (Fig 1) Torque is proportional to the power as in wattmeter.
The watt-hour meter must take both power and time into consideration. The instantaneous speed is proportional
to the power passing through it.

The total number of revolutions in a given time is proportional to the total energy that passes through the meter
during that period of time.
Parts and functions of an energy meter: The parts of the induction type single phase energy meter are (Fig 1).

Iron core: It is specially shaped to direct the magnetic flux in the desired path. It directs the magnetic lines of
force, reduces leakage flux and also reduces magnetic reluctance.
Potential coil (voltage coil): The potential coil is connected across the load and is wound with many turns of fine
wire. It induces eddy current in the aluminium disc.
Current coil: The current coils, connected in series with load, are wound with a few turns of thick wire, since they
must carry the full load current.
Disc: The disc is the rotating element in the meter, and is mounted on a vertical spindle which has a worm gear
at one end. The disc is made of aluminium and is positioned in the air gap between the potential and current coil
magnets.
Spindle: The spindle ends have hardened steel pivots. The pivot is supported by a jewel bearing. There is a
worm gear at one end of the spindle. As the gear turns the dials, they indicate the amount of energy passing
through the meter.
Permanent magnet/brake magnet: The permanent magnet restrains the aluminium disc from racing at a high
speed. It produces an opposing torque that acts against the turning torque of the aluminium disc.
Functioning of energy meters: The rotation of the aluminium disc (Fig 2) is accomplished by an electromagnet,
which consists of a potential coil and current coils. The potential coil is connected across the load. It induces an
eddy current in the aluminium disc. The eddy current produces a magnetic field which reacts with the magnetic
field produced by the current coils to produce a driving torque on the disc.
The speed of rotation of the aluminium disc is proportional to the product of the amperes (in the current coils) and
the volts (across the potential coil). The total electrical energy that is consumed by the load is proportional to the
number of revolutions made by the disc during a given period of time.
A small copper ring(shading ring) or coil (shading coil) is placed in the air gap under the potential coil, to produce
a forward torque, large enough to counteract any friction produced by the rotating aluminium disc.
This counter torque is produced when the aluminium disc rotates in the magnetic field established by the permanent
magnet. The eddy currents, in turn, produce a magnetic field that reacts with the field of the permanent magnet,
causing a restraining action that is proportional to the speed of the disc
Creeping error and adjustment: In some meters the disc rotates continuously even when there is no current
flow through the current coil i.e. when only the pressure coil is energised. This is called creeping. The major
cause for creeping is over-compensation for friction. The other causes for creeping are excessive voltage across
the pressure coil, vibrations and stray magnetic fields.
In order to prevent creeping, two diameterically opposite holes are drilled in the disc (Fig 3). The disc will come
to rest with one of the holes under the edge of a pole of the potential coil magnet, the rotation being thus limited
to a maximum of half a revolution.

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CITS : Power - Electrician & Wireman - Lesson 50-53 CITS : Power - Electrician & Wireman - Lesson 50-53