Electromechanical energy meters are based on the Ferraris Principle (see note in the margin). The working of these meters is explained.
Working of Electromechanical Meters
The meters have a revolving metallic disc mounted on jewel bearings or magnetic suspension bearings. The display is cyclometer or mechanical counters and accuracy is classically 1% or 2% (class 1.0 or 2.0). They cater to limited tariffs applicable majorly to 1-phase or 3- phase direct connected segment (whole current meters).The electromechanical induction meter operates through counting the revolutions of the disc that rotates at a speed proportional to the power consumed.
The number of revolutions is, therefore, proportional to the energy usage. The metallic disc is acted upon through three magnetic fields, one proportional to the voltage, the other to the current and a third constant field supplied by a permanent magnet. One of the varying fields induces currents in the metallic disc, that are then acted upon through the other varying field to produce a torque.
This results in the torque being proportional to the product of the current and voltage, that is power. As the metallic disc rotates through the permanent magnetic field, eddy currents are again produced that dissipate energy (because the disc has some resistance) and act to slow the rotation. This drag is proportional to the rotation speed. The equilibrium among the applied torque and the drag results within a speed proportional to the power. The rotating disc in this category of meter is, actually, an electric motor of a category known as a reluctance motor or eddy current motor. It consumes a little amount of power, classically around 2 W.