In the last article, we have seen about single-phase induction type energy meter that can be used to measure the energy consumed in the single-phase circuit. In order to measure the energy consumption of a three-phase circuit three-phase energy meters are used. The three-phase energy meter can be built by two single-phase energy meters, by adding the readings of two meters gives total energy consumption in the 3-phase circuit.
Construction of Three Phase Energy Meter :
The construction of a three-phase induction type energy meter appears as an assembly of two single-phase induction type energy meters in one case, having a common spindle and registering mechanism. The schematic diagram of the meter is shown in the below figure.
The essential parts of the 3-phase energy meter are,- Driving system
- Moving system
- Braking system
- Registering system.
Driving System :
The combination of a shunt and series magnet is called an element. So, it consists of two elements. The connections for the winding of these electromagnets are shown in the figure. The shunt magnet windings are provided such that, during no-load conditions, the torque developed by both the shunt magnets is opposite in nature.
In order to make the resultant driving torque equal to zero, a magnetic shunt is provided in the meter. Its position is adjusted until the discs stop 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.
Moving System :
The moving system consists of two aluminum discs (one for each element) which are mounted over a single spindle. The torque developed by each disc will be added up and as a result, the total torque will be proportional to the 3-phase power consumed by the load.
Braking System :
This system provides the necessary braking action on the discs. An individual permanent magnet is provided for each disc. There is a provision for adjusting the position of each brake magnet in order to vary the braking torque.
Registering System :
This system is attached to the moving system through the pinion and a 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 considered period of time which is nothing but energy.
Working of Three Phase Energy Meter :
The principle of working of a 3-phase energy meter is similar to the single-phase energy meter. When the load is connected to the meter both the pressure coil and the current coil of the two elements mounted on the shunt and series magnet produces magnetic flux. This flux when links with the discs cause an eddy current to flow in it.
Interaction of eddy currents with the flux imposed by the two coils causes the production of torque on discs. Since two discs are attached to one spindle, the torque exerted on the two discs added mechanically. Hence, rotation of the shaft gives the 3-phase energy consumed.
