The meter used for measuring the consumption of electrical energy or electric power of a three phase electric supply to a device, is known as three phase energy meter.
Construction of a three phase energy meter is similar to the construction of a single phase induction energy meter.
A three phase energy meter works on the same principle as that of a single phase electromechanical induction meter. It operates through electromagnetic induction by counting the revolutions of a non-magnetic but electrically conducting metal disc which is made to rotate at a speed proportional to the power passing through the meter.
It is made by connecting two single phase energy meter elements through a common spindle. The combination of a shunt electromagnet with a series electromagnet is called an element. The total energy consumed is the sum of the reading of both the elements. The torque of both the elements is added mechanically, and the total rotation of the disc is proportional to the three phase energy consumption.
In a three phase meter, the driving torque produced by both the elements is equal but opposite in direction. Hence, they do not allow the disc to rotate. If the torque becomes unequal and the disc rotates then the position of braking magnet is adjusted.
The torque can be also be adjusted by varying the turns in current coils of both the elements in the series and their voltage coils in parallel. For balancing, the meter is tested in full load condition.
Three phase electromechanical induction meters are of two types depending upon their connections –
- Three wire, three phase electromechanical induction meter – This type of energy meter is suitable for delta connected installations.
- Four wire, three phase electromechanical induction meter – This type of energy meter is suitable for star connected installations.
This type of energy meter is used for measuring the consumption of electrical energy for an installation having three wire three phase system i.e. a system having delta connection of phases.
A three wire, three phase induction energy meter is basically an assembly of two single-phase induction type energy meters in one case, having a common spindle and registering mechanism. Both the elements has its individual rotor discs, braking magnets, copper shading bands and the compensator for getting the correct reading.
The schematic diagram for construction and working of a three wire, three phase electromechanical induction energy meter is shown in the figure.
A three wire, three phase induction energy meter has following main components –
1) Shunt electromagnet –
A three phase meter has two sets of shunt electromagnet. The connections for the winding of voltage coils are connected in parallel to the supply voltage. The coils carry the current proportional to the shunt voltage between two phases of supply line.
The voltage coil winding is so arranged that, during no load conditions, the torque developed by both the shunt magnets is opposite in nature.
2) Series electromagnet –
A three phase meter has two sets of series electromagnet. The connections for the winding of current coils are connected in series to each of phase supply to the load. It carries the current equal to the current flowing through the load.
In order to make the resultant driving torque on rotor disc equal to zero, a magnetic shunt is provided in the meter. Its position is adjusted until the discs stops rotating at no-load. The necessary driving torque is obtained due to the interaction between the shunt and series magnetic fields on the disc in each element.
3) Rotor Discs –
Meter has two numbers of light weight Aluminium alloy disc mounted on a common spindle. The torque developed by each disc is added up and as a result, the total torque is proportional to the three phase power consumed by the load.
The spindle also carries a pinion which mesh with a gear train mechanism which connects the disc to the registering mechanism and display.
4) Braking Magnets –
Each rotating disc is provided with its individual braking magnet. It is a horse shoe type of permanent magnet placed at radial position of the rotor disc. It is required to control the speed of rotation of the rotor disc and also to bring the disc to an idle state when the power to the load is disconnected.
There is a provision for adjusting the position of each brake magnet in order to vary the braking torque.
5) Cyclometer & Display Mechanism –
This system is attached to the spindle of rotor disc through a pinion and gear train. It continuously counts or registers the number of revolutions made by the discs. It means it integrates the power consumed by the three phase load over a period of time.
The rotation of rotor disc is directly proportional to the three phase energy consumed by the load. Disc rotation is transmitted to the pointers of different dials and cyclometer for recording. After some number of rotations, the disc displays the unit used by the load by a display mechanism.