*TOPIC OUTLINE*DAMPING TORQUE*Air Friction Damping*Fluid Friction Damping*Eddy Current Damping*Electromagnetic Damping

K-12EL-09K-12EL-57

Super vision By:Sir Sajid Hussain Qazi

The damping force ensuresthat the pointer comes to rest in its final positionquickly and without undue oscillation.

*DAMPING FORCE

*DAMPING TORQUE

*The moving system of the instrument will tend to move under the action of the deflecting torque. *But the control torque, it will try to occupy a position of rest when the two torques are equal and opposite. *Due to inertia of the moving system, the pointer will not come to rest immediately but oscillate about its final deflected position and takes appreciable time to come to steady state.*To overcome this difficulty a damping torque is to be developed by using a damping device attached to the moving system.

* DAMPING TORQUE

*The damping torque is proportional to the speed of rotation of the moving system, that is

Depending upon the degree of damping introduced in the moving system, the instrument may have any one of the following conditions as depicted in above graph

* DAMPING CONDITIONS:

Under damped condition:If the Degree of the damping is low then pointer

makes oscillation before settling at equilibrium position. Over damped condition:

If the degree of the damping is too high then the pointer takes longer time to reach equilibrium position without making oscillations. Critically damped condition:

If the degree of the damping is just sufficient to enable the pointer to rise quickly to equilibrium point without overshoot.

*The damping torque is produced by the following

methods:

1.Air Friction Damping2.Fluid Friction Damping3.Eddy Current Damping4.Electromagnetic Damping

*1.Air Friction Damping

*. Air Friction damping.  In modern form of air friction damping, an aluminum vane is enclosed in the quadrant shaped recess cast in Bakelite molding or dia casting and air brake chamber is completed by a cover plate.

*2.Fluid Friction Damping

* Similar in action to air friction damping. * Fluid with high viscosity is filled in the chamber instead of air. * Main drawback of this type of damping are Creeping of oil, need to keep the meter vertical, unsuitability in portable type Meter.*Though in this method of damping no care is required as in air friction damping but it is not used due to several disadvantages, such as creeping of oil, the necessity of using the instruments always in vertical position.

*3.Eddy Current Damping

* A thin disc of conducting but non-magnetic material like copper and aluminium mounted on the moving system spindle. * The disc is placed in such a way when ever it rotates it cut the magnetic flux. * Due to this eddy emf is induced in the disc which tries to oppose the movement of the disc. So the eddy current tries to retard the disc.

*4.Electromagnetic Damping

*A thin disc or vane of conducting but non-magnetic material like copper or aluminum is mounted on the spindle carrying the moving system and the pointer.*The disc or vane is positioned so that is edges rotate

between the poles of a permanent magnet.*Damping torque decreases with the movement of the

magnet towards the edge of the disc and becomes zero.*In the induction type instruments by placing a

damping magnet (permanent type) at the opposite side of the disc from the electro-magnet, so that the disc serves for damping as well as for operation purposes

* Refrence: B.L

Thereja&

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