Electrical Engineering ⇒ Topic : Dynamometer Wattmeter

Dynamometer Wattmeter

A dynamometer wattmeter is almost universally used for the measurement of d.c. as well as a.c. power. It works on the dynamometer prinicple i.e., mechanical force exists between two current carrying conductors or coils.

Construction. When a dynamometer instrument is used as a wattmeter, the fixed coils are connected in series with the load and carry the load current (I₁) while the moving coil is connected across the load through a series multiplier R and carries a current (I₂) proportional to the load voltage as shown in Fig.(a). The fixed coil (or coils) is called the current coil and the movable coil is known as potential coil. The controlling torque is provided by two spiral springs which also serve the additional purpose of leading current into and out of the moving coil. Air friction damping is provided in such instruments. A pointer is attached to the movable coil.

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Working. When the wattmeter is connected in the circuit to measure power See Fig. (a), the current coil carries the load current and potential coil carries current proportional to the load voltage.Due to currents in the coils, mechanical force exists between them. The result is that movable coil moves the pointer over the scale. The pointer comes to rest at a position where deflecting torque is equal to the controlling torque. Reversal of current reverses currents in both the fixed coils and the movable coil so that the direction of deflecting torque remains unchanged. Hence, such instruments can be used for the measurement of d.c. as well as a.c. power.