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Examoogle • User • info@examoogle.com • EE-2024-8821
Chapter 1 of 12 • Page 1 of 248🔒 Protected PDF • Watermarked
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A pointer of an instrument once deflected returns to zero position, when the current is removed due to
Action of gravity
Mass of the pointer
Controlling Torque
Damping Torques
Controlling Torque
Quick Summary: 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 ($T_d$).
The controlling torque (Tc) 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 (Td).
Td=Tc
Tc=kθ
In analog instruments, the deflecting torque (Td) moves the pointer against the controlling torque (Tc). When the current is removed, Td becomes zero, and the Tc (provided by springs or gravity) exerts a restoring force that opposes the displacement, returning the system to its initial state.
Controlling torque ensures the instrument has a defined zero position.
Common methods for producing Tc 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.
Ensures repeatability of measurements.
Enables the pointer to reset automatically after power removal.
Requires high-quality materials to avoid hysteresis in control springs.
Gravity control necessitates keeping the instrument in a specific vertical orientation.
Moving coil instruments
Moving iron instruments
Dynamometer-type instruments
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.
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torque is the restoring force that acts to return the pointer to its zero position once the deflecting force is withdrawn.
Always remember: Td makes the pointer move, Tc brings it back to zero, and Tdamping stops the vibrations.