Dynamometric-instruments are used for measurement of active, reactive & absolute power in 1 Phase & 3 Phase systems for balanced / unbalanced load conditions. Watt & Var instruments incorporate current coils, which are connected in series with the load & pressure coils of fine wire gauge, which are connected across the load. These instruments work on “Air cored Electro-Dynamometric” principle.
Power Factor meter incorporates two pressure coils one with non-inductive resistance in series & other with capacitance in series & are cross connected. The deflecting torque produced is proportional to the phase angle between current & voltage vectors. The pointer rests anywhere on the scale when meter is not connected in the circuit, as there is no controlling torque.