Damping Torque in Indicating Measuring Instruments:
We know deflecting torque, controlling torque and damping torque are present in measuring instruments.When a deflecting force is applied to the moving system, it deflects and it should come to rest at a position where the deflecting force is balanced by the controlling force.
The deflecting torque and controlling forces are produced by systems which have inertia and, therefore, the moving system cannot immediately settle at its final position but overshoots or swings ahead of it. Consider below figure. Suppose O is the equilibrium or final steady position. Because of inertia, the moving system moves to position ‘a’.
Now for any position ‘t’ beyond the equilibrium position, the controlling force is more than the deflecting force and hence the moving system swings back. Due to inertia, it cannot settle at ‘O’ but swings to a position say ‘b’ behind the equilibrium position. At ‘h’, the deflecting force is more than the controlling force and hence the moving system again swings ahead.
The pointer that oscillates about its final steady (equilibrium) position with decreasing amplitude till its kinetic energy (on account of inertia) is dissipated in friction and therefore, it will settle down at its final steady position. If extra forces are not provided to “damp” these oscillations, the moving system will take a considerable time to settle to the final position and hence time consumed in taking readings will be very large. Therefore, damping torque is necessary so that the moving system comes to its equilibrium position rapidly and smoothly without any oscillations.
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Types of damping torque or systems:
The damping torque should be of such a magnitude that the pointer quickly comes to its final steady position, without overshooting. If the instrument is underdamped, the moving system will oscillate about the final steady position with a decreasing amplitude and will take some time before it comes to rest. When the moving system moves rapidly but smoothly to its final steady position, the instrument is said to be critically damped or deadbeat.
If the damping torque is more than what is required for critical damping, the instrument is said to be overdamped. In an overdamped instrument, the moving system moves slowly to its final steady position in a lethargic fashion. The readings are very tedious to take in this case. The below figure illustrates the way an underdamped, an overdamped and a critically damped system moves to its final steady position,θf.
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| Setting curves for different types of damping |
The damping device should be such that it produces a damping torque only while the moving system is in motion. To be effective the damping torque should be proportional to the velocity of the moving system but independent of the operating current. It must not affect the controlling torque or increase the static friction. The methods for producing damping torque are :
(i) Air friction damping
(ii) Fluid friction damping
(iii) Eddy current damping
(iv) Electromagnetic damping
Air Friction Damping:
Two types of air friction damping devices are shown in the below figure. The arrangement of the below figure(a) consists of a light aluminium piston which is attached to the moving system. This piston moves in a fixed air chamber which is closed at one end. The clearance between piston and chamber walls is uniform throughout and is very small. When there are oscillations the piston moves into and out of an air chamber.
When the piston moves into the chamber, the air inside is compressed and the pressure of air thus built up, opposes the motion of the piston and hence the whole of the moving system. When the piston moves out of the air chamber, the pressure in the closed space falls, and the pressure on the open side of the piston is greater than on the other side. Thus there is again an opposition to the motion.
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| Air friction damping |
The arrangement of the above figure(b), consists of an aluminium vane which moves in a quadrant (sector) shaped air chamber. This air chamber is a recess cast in a bakelite moulding or die casting. The chamber is completed by providing a cover plate at the top. The aluminium piston should be carefully fitted so that it does not touch the wall otherwise a serious error will be caused in readings.
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Fluid Friction Damping:
This form of damping is similar to air friction damping. Oil is used in place of air and as the viscosity of the oil is greater, the damping torque is also correspondingly greater. A disc is attached to the moving system [below figure(a)], this disc dips into an oil pot and is completely submerged in oil.
When the moving system moves, the disc moves in oil and a frictional drag is produced. This frictional drag always opposes the motion. In the arrangement shown in the below figure(b), a number of vanes are attached to the spindle. These vanes are submerged in oil and move in a vertical plane. This arrangement gives a greater damping torque.
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| Fluid friction damping |
Eddy Current Damping:
When a conductor moves in a magnetic field an emf is induced in it and if a closed path is provided, a current (known as eddy current) flows. This current interacts with the magnetic field to produce an electromagnetic torque which opposes the motion. This torque is proportional to the strength of the magnetic field and the current produced.
The current is proportional to emf which in turn is proportional to the velocity of the conductor. Thus, if the strength of the magnetic field is constant (if it is produced by a permanent magnet), the torque is proportional to the velocity of the conductor.
Electromagnetic Damping:
The movement of a coil in a magnetic field produces a current in the coil which interacts with the magnetic field to produce a torque. This torque opposes the movement of the coil and slows the response. The magnitude of the current and hence the damping torque is dependent upon the resistance of the circuit to which the instrument is connected. The electromagnetic damping is used in galvanometers.
Comparison of types of damping torques:
Air friction damping provides a very simple and cheap method of damping. But care must be taken to see that the piston is not bent or twisted otherwise it will touch the walls of the air chamber thereby causing serious errors due to solid friction which is thus introduced. This method is used in hot wire and moving iron instruments.
Air friction damping has the advantage, that is, does not require the use of a permanent magnet whose introduction may lead to distortion of the operating field. Therefore, this type of damping is used in moving iron and dynamometer type of instruments where the operating magnetic field is weak and is likely to be get distorted with the introduction of a permanent magnet.
Fluid friction damping has the advantage that the oil which is required for damping, can be used for insulation purposes in some forms of instruments which are submerged in oil. A vane moving in oil instead of air does not require the same small clearances to give effective damping and therefore this method is suitable for instruments. such as electrostatic type where the movement is suspended rather than pivoted. Another advantage of fluid friction damping is that due to the upthrust of oil, the load on bearings or suspension is reduced thus reducing frictional errors.
The disadvantages of fluid friction damping are that it can be used only for instruments which are in vertical position. Also because of the creeping of oil, the instruments cannot be kept clean. Hence this type of damping can be used for laboratory type electrostatic instruments and there are obvious difficulties in the way of its application to portable instruments.
Eddy current damping is the most efficient form of damping. It is very convenient to use in instruments where a metallic disc or a former and a permanent magnet already form part of the operating system. For these reasons this method is used in hot wire, moving coil and induction type instruments. This method cannot be used in instruments where the introduction of a permanent magnet required for producing eddy currents will distort the existing magnetic field as in moving iron or dynamometer type of instruments.
Conclusion:
Now here we have discussed the damping torque or systems in indicating instruments. You can download this article as pdf, ppt.
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