Electrical Engineering ⇒ Topic : Damping Torque

Damping Torque If the moving system is acted upon by deflecting and controlling torques alone, then pointer, due to inertia, will oscillate about its final deflected position for quite some time before coming to rest. This is often undesirable because it makes difficult to obtain quick and accurate readings. In order to avoid these oscillations of 1 the pointer and to bring it quickly to its final deflected position, a damping torque is provided c in the indicating instruments. This damping torque acts only when the pointer is in motion and always opposes the motion. The position of the pointer when stationary is, therefore, not *affected by damping.

The degree of damping decides the behaviour of the moving system. If the instrument is under-damped, the pointer will oscillate about the final position for some time before coming to rest. On the other hand, if the instrument is over-damped, the pointer will become slow and lethargic. However, if the degree of damping is adjusted to such a value that the pointer comes up to the correct reading quickly without passing beyond it or oscillating about it, the instrument is said to be dead-beat or critically damped. Fig. (a) shows graph for under-damping, over damping and critical damping (dead-beat). The damping torque in indicating instruments can be nroyided by 01 air-friction Oil fluid friction and OM eddy currents

                                                                figure (a)

DAMPING TORQUE

When the instrument is connected to electrical circuit to indicate the value of measured quantity, there is a deflecting torque as well as controlling torque.Controlling torque balances the deflecting torque and the pointer comes to the steady state position after oscillations. To get the final reading, time will be consumed due to oscillations. To damp out the oscillations, damping is provided to the instrument.Figure (a) shows the effect of different types of damping. The methods of producing damping torque are: (i) Air friction damping, (ii) Fluid friction damping, and (iii) Eddy current damping.

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                                                                           figure (a)