The A.C. Watt Meter

A watt meter is an instrument used to measure directly the power consumed in a circuit. A dynamometer type watt meter consists of two fixed coils CC which are called the current coils. These are connected in series with the load in AC circuit. It has a movable coils V called the voltage coil. The coils V with a high non-inductive resistance R is connected parallel to the load.

The same current i flows through the load and current coils. The p.d V across the load is proportional to the current in the voltage coil i₂.

Torque experienced by the moving coil τ ∞ i x i₂
    Τ ∞ iV        [. : V ∞ i2]
And τ ∞ deflection θ.
 . :     Vi ∞ θ
Power ∞ θ.
Vi gives the instantaneous power which is true power. Thus the power is measured.


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