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It has 2 coils called current coil and voltage coil. As the voltage applied is always the system voltage(110 or 230 single phase or 220 or 400 3 phase) the excitation is same.But as the current increases the current coil will rotate the aluminium rotor at higher speed so that the kwh reading shown by the counter coupled to the rotor shaft will increase. This was what I learnt in 1960s. Now new technologies(eg:digital)may be used. Contact a reputed manufacturer.

The principle behind the working of energy meter is emi , i.e. electromagnetic induction...
Working is based on the revolution or u can say rotation of the al. disc due to the induced eddy currents in it which react with the flux of the shunt n series magnet to produce effective torque...