A pointer of an instrument once deflected returns to zero position, when the current is removed due

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A pointer of an instrument once deflected returns to zero position, when the current is removed due to

Examoogle · Accepted Answer

Controlling Torque

Explanation: The controlling torque ($T_c$) is responsible for bringing the pointer back to the zero position when the operating current is removed. Without it, the pointer would remain deflected at the position determined by the last deflection torque ($T_d$).

Working Principle: In analog instruments, the deflecting torque ($T_d$) moves the pointer against the controlling torque ($T_c$). When the current is removed, $T_d$ becomes zero, and the $T_c$ (provided by springs or gravity) exerts a restoring force that opposes the displacement, returning the system to its initial state.

Key Formulas: $T_d = T_c$; $T_c = k	heta$

Key Points: Controlling torque ensures the instrument has a defined zero position.; Common methods for producing $T_c$ include spring control and gravity control.; Spring control is preferred as it provides a linear scale.; Damping torque is only active when the pointer is moving to suppress oscillations.

Advantages: Ensures repeatability of measurements.; Enables the pointer to reset automatically after power removal.

Disadvantages: Requires high-quality materials to avoid hysteresis in control springs.; Gravity control necessitates keeping the instrument in a specific vertical orientation.

Applications: Moving coil instruments; Moving iron instruments; Dynamometer-type instruments

Additional Info: Gravity control is limited to vertical mounting positions only.. Option D (Damping Torque) is incorrect because damping only opposes the movement (velocity) to reduce oscillations, not to maintain a zero position.

Answer: C is correct — The controlling torque is the restoring force that acts to return the pointer to its zero position once the deflecting force is withdrawn.

Exam Tip: Always remember: $T_d$ makes the pointer move, $T_c$ brings it back to zero, and $T_{damping}$ stops the vibrations.

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