Damping Torque - Air Friction, Fluid Friction & Eddy Current Damping

What is Damping Torque in Measuring Instruments ?

In presence of deflection and controlling systems, the pointer will attain a steady position where the torque produced by these two systems balance each other. But, before attaining the steady position, the pointer will oscillate about its final steady position, even though deflecting and controlling torques cancel each other.

The moment of inertia associated with the moving pointer will oppose the action of the pointer to come to rest suddenly. So, the pointer continues to move beyond the final suddenly.

At some point, where the controlling torques overcome the moment of inertia and deflecting torque, it makes the pointer move towards its final position. Even now, the moment of inertia of the pointer makes it move below its final position.

At some point, where the deflecting torque overcomes the moment of inertia and controlling torque, it makes the pointer move towards its final steady position. In this way, the pointer oscillates.

Meanwhile, the energy associated with the moment of inertia will be dissipated in the friction of the moving system and so the magnitude of oscillations goes on reducing. Finally, the pointer comes to rest at its steady position.

Even though, the pointer finally settles down, so that the reading can be observed, the time taken to observe each reading will be quite long. In order to obtain the reading immediately, the oscillations are to be damped i.e., suppressed.

This feature is provided by the damping system. This system produces the damping torque (i.e., the torque which damps the oscillations) that is proportional to the magnitude of oscillations. Therefore, the pointer comes to its equilibrium position immediately without any oscillations.

Damping Torque

The damping force or torque produced should be in such a way that that pointer should settle at a final steady position without any oscillations or overshooting. If the pointer reaches the final steady position quickly and smoothly then the system is said to be critically damped.

In under-damped systems, there will presents of oscillations but with decreased amplitude. Here there will be a delay for the pointer to settle at the final steady position. In the case overdamped system (damping torque more than critical damping) the oscillations are damped but the movement of the pointer will be slow, thereby decreasing the response of the system.

Methods of Providing Damping Torque :

The various methods used for providing damping torque are,
  • Air friction damping
  • Fluid friction damping
  • Eddy current damping

Air Friction Damping :

The figure below shows the arrangement of the air friction damping. It consists of a piston placed in an air chamber closed at one end. The other end of the piston is connected to the pointer. The damping torque is produced due to air pressure created in the air chamber by the piston movement.

Damping Torque & Methods of Providing Damping Torque

Whenever the pointer is deflected towards the right, the piston moves out of the air chamber and the pressure falls. Due to this, the pressure on the open end is more than the closed end hence, opposes the motion of the pointer and moves it towards the left.

Whenever the pointer is deflected towards the left or swings towards the left, the piston moves into the chamber, which increases the air pressure. The increased air pressure opposes the motion of the pointer and moves it towards the right. Hence, in this way, it opposes the swinging of the pointer and reduces the oscillation.

Advantages :

  • This method is very simple and very cheap.
  • It does not require a permanent magnet and hence no problems of field distortion.
  • It can be easily maintained.

Disadvantages :

  • Pistion should be carefully handled.
  • Due to mishandling of piston, it may get twisted and will cause errors in the readings.
  • It is not reliable.

Fluid Friction Damping :

The principle of operation of fluid friction is the same as that of air friction damping as shown below. The only difference is that the piston is placed in a fluid (oil) of high viscosity. The damping force is produced by the frictional drag due to the movement of the piston in the oil.

Damping Torque & Methods of Providing Damping Torque

The oil used in fluid friction damping must have the following properties,
  • Its viscosity should not vary due to variation in the temperature.
  • It should not evaporate easily.
  • It should not have corrosive action upon metals.
  • It should possess good insulating properties.

Advantages :

  • It provides effective damping when compared to air friction type.
  • The oil used serves two purposes, one is for damping, and the other is for heat dissipation.
  • It is the best-suited method for electrostatic type instruments.
  • Errors due to friction are reduced to a great extent.

Disadvantages :

  • It can be used only for vertical position instruments.
  • It can't be used in portable type instruments.
  • Possibilities of leakage of oil in the other parts of the instrument.

Eddy Current Damping :

This method is based on the production of electromagnetic torque. We know that whenever a conductor cuts the magnetic flux or whenever there is a rate of change of magnetic flux an emf is induced according to Faraday's law of electromagnetic induction. Suppose if the conductor forms a closed path, the current will circulate through the conductor known as eddy current.

These eddy currents will produce an electromagnetic torque due to magnetic field in such a way that it opposes the motion. The torque produced on the conductor will be proportional to the velocity of conductor. Hence, the electromagnetic torque provides the necessary damping force.

Damping Torque & Methods of Providing Damping Torque

Advantages :

  • It is the most efficient form of damping compared to other methods.
  • It is especially used for moving coil and induction-type instruments.
  • It can be used in portable instruments as well.

Disadvantages :

  • It can't be used for moving iron or dynamo-type instruments.

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