Induction-type watthour meters are basically induction motors with the following essential parts:
The rotor, which consists of an aluminum disk mounted on a shaft that is free to rotate;
The stator, which consists of voltage coil and a current coil wound on laminated iron cores;
A braking magnet, which generates a torque that opposes disk rotation; and
A revolution counter.
The voltage and current coils produce fluxes that induce eddy currents in the aluminum disk. With proper space and phase displacement the interaction between these fluxes and the eddy currents will generate a rotational torque on the disk.
The space displacement is achieved by a suitable arrangement of the coils and laminations. A 90° phase displacement is realized in part by the fact that the voltage coil is highly inductive.
The remaining phase shift is obtained by a compensating coil and resistor that is magnetically coupled to the voltage coil. Induction watthour meters are frequency dependent and may have accuracies approaching 0.1%.
Power for their operation is derived from the circuit in which they are connected. In the induction watthour meter, as the disk rotates, the flux lines generated by the permanent magnet are cut.
A voltage is generated in the disk which results in eddy current flow. The eddy currents react with the permanent magnet flux to produce a retarding torque that is proportional to the speed of the disk.
Considering these relationships, if
driving torque µ power (watts)
retarding torque µ disk speed
For steady-state conditions:
driving torque = retarding torque
Therefore: power (watts) µ disk speed and energy µ number of disk revolutions
Each disk revolution of a watthour meter represents a finite amount of energy in watthours, as defined by the value given on a meter nameplace as the disk constant, Kh.
The register on a watthour meter totals the number of revolutions the disk makes through a mechanical gear train. A register generally shows kilowatthours (kWh), with the smallest division on the units dial being 1 kWh.