What is more damping torque or controlling torque?

Answer 1

Following are the essentials of indicating instruments: 1. Operating torque, 2. Controlling torque, and 3. Damping torque. OPERATING TORQUE Operating torque is produced by making use of any of these effects: magnetic, electromagnetic induction...., it is required to move the moving system of the instrument. CONTROLLING TORQUE Controlling torque opposes the operating torque and increases with the deflection of the moving system. It ensures that the deflection of the pointer is according to the magnitude of electrical quantity being measured. If this torque were not provided, the pointer would continue to move indefinitely and the deflection shall be independent of electrical quantity being measured. It also bring the pointer back to zero when the instrument is removed from the circuit. This torque is either obtained by spring or by gravity. In spring control, one or two phosphor bronze spiral hair-springs are attached to the moving spindle. The other end are attached to the frame. In this case Tc proportional to Angle of deflection. In gravity control, a small weight is attached to the moving system in such a way that it tries to bring the pointer back to the zero position when it is deflected, due to gravity. In this casse Tc proportional to sine of the angle of deflection. DAMPING TORQUE Controlling torque controls the deflection and tries to stop the pointer at its final position where its Td = Tc But due to inertia, the pointer oscillates around its final position before coming to rest. Hence damping torque is provided to avoid this oscillation and bring the pointer quickly to its final position. Thus the damping torque is never greater than the controlling torque. In fact it is the condition of critical damping which is sufficient to enable the pointer to rise quickly to its deflected position without overshooting.