Electromechanical Meters

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.