Page 305 - Electrician - TT (Volume 1)

ELECTRICIAN - CITS

Two element 3-phase energy meters: This energy meter works on the principle of measurement of power by
the two wattmeter method. Two elements of a current coil and two elements of a potential coil are used in this
energy meter. These assemblies can be arranged on the different sectors in a horizontal position (Fig 1) with a
single aluminium disc which rotates between the poles of a single braking magnet.

The two elements can also have individual driving discs on a common spindle. In this case they will have individual
braking magnets (Fig 2). The second type usually preferred by the manufacturers due to the construction simplicity.

Fig 1 & 2

In both the cases the driving torque produced by individual elements are summed up. The recording mechanism
which is attached to the train of gears i.e., cyclometer or counter type dial shows the sum of the energies that has
passed through the elements. The two element energy meter is only suitable for a 3-phase 3-wire system but can
be used for both balanced and unbalanced loads.
3-element 3-phase energy meter: This works on the same principle as that of the 3 wattmeter method of power
measurement with a 3-phase load. Here 3 units, each with a current coil and a potential coil, are used. The
potential coils of the 3 elements are connected in star to the supply lines with their common point connected to
the neutral line of power supply.
The current coils are connected in series to the individual lines. As is the case with the two element energy meter,
these three elements can be arranged in the different sectors of a common single aluminium disc which serves
as a rotating part connected to driving dial (Fig 3).
The three elements can also have a common spindle with three individual discs and braking magnets (Fig 4).
Here also the 2nd type is usually preferred by manufacturers due to the easiness in construction. The driving
torque produced by the three individual elements are summed up and the recording mechanism shows the sum
of energies that has passed through the individual elements. This energy meter is suitable for the 3-phase 4-wire
system.
Application of 3-phase energy meter: A two element 3-phase energy meter is used with three phase loads in
which a neutral is not used such as for an industry or irrigation pumpset motors etc. having three phase loads
only or with an 11kV 3-phase 3-wire supply to an industry.
A 3-phase 4-wire element energy meter is used with three phase load in which balanced or unbalanced loads are
connected with individual phases and neutral such as for a large domestic consumer or for an industry having
lighting loads also.

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