In this topic, you study the theory and diagram of Damping Torque or Force in Indicating Instruments.
The deflecting and controlling torques are produced by mechanical systems, which have inertia. The pointer itself also possesses some inertia. Therefore it does not stop immediately at its equilibrium position but oscillates about the position of rest. We can explain the phenomenon by the following illustration.
Fig. 2.9. Damping Torque
See Fig, 2.9. Suppose O is the position of equilibrium of the pointer. where it is supposed to stop immediately. but because of inertia, it moves to position A, but at A the controlling torque will be more than the deflecting torque and as the former acts in reverse direction to the later. it drags the pointer back. Due to inertia it again does not stop at O. but attains a position B. where unfortunately deflecting force is more than the controlling force and due to same reason. the pointer again moves ahead, Thus the pointer will oscillate about its final position O, till its kinetic energy dissipates due to friction and it finally rests at O. These oscillations are not desirable. as it consumes much time in taking the readings. It is therefore necessary to stop (or damp) these unnecessary oscillations. The damping force or torque is provided to do this job.
Note that the amplitude of oscillations will go on decreasing till it reduces to zero. see Fig. 2.9. OA > 0B.
Degree of damping Fig. 2.10.
The degree of damping decides behavior of an instrument. Accordingly the instrument may be.
(a) Under damped: When the pointer oscillates about the final position and takes same time to come to rest.
(b) Overdamped: Where the pointer moves slowly to its final position in a lethargic way. In this case it is very difficult to read the instrument.
Fig. 2.10. Degree Of Damping
(c) Dead beat: Where the pointer moves quickly and stops immediately at its final position of rest. This situation is ideal and called as critical damping.
The damping system should be such that it produces the damping torque only so long the moving pointer is in motion. For effective damping, the torque should be proportional to the velocity of the moving pointer but independent of the operating current. Neither it should affect the controlling torque nor decrease the friction. Below we are describing various methods to produce various torques.
PRODUCTION OF DAMPING TORQUE
The damping torque is required to eliminate unnecessary oscillations of the moving system before coming to rest at the final position. In other words damping torque acts only when the pointer is in motion. When the pointer is in a particular final position, though deflecting and controlling torques exist but dumping torque is zero, because the pointer is in steady position and there is no movement of the moving system. Infect, the damping torque acts like a BRAKE on the moving system of the instrument.
The methods of producing damping torque are following :
(l) Pneumatic or Air friction damping.
(2) Fluid Friction damping.
(3) Eddy currents or electromagnetic damping.
