What is the difference between the watt hour meter and Var hour meter in terms of internal connection and work theory ie When the meter records watt-hour an when the meter records var hours?

Answer 1

Only watts (power) can produce work (energy) because of the phase relationship of the voltage and current in an alternating current, electric power system. With watts, the portion of the current and voltage that are in-phase with each other can do work. Vars, on the other hand are volt-amps reactive, which are out-of-phase with each other. Since Vars cannot do work, their existence in the electric power system is parasitic. The reactive current which flows in a power line, for instance uses a portion of line capacity, and also causes heating (I^2 * R power loss) due to the inherent resistance of the conductors.

So, in the construction of a simple electromechanical meter, measuring Watt-hours or Var-hours, the respective current and voltage coils used to drive the rotating disk are situated to produce torque. For a Var Hr meter, the coils are rotated 90 degrees relative to each other to counter the lagging or leading phase shift and thus produce torque.