*Air Friction Damping
*Fluid Friction Damping
*Eddy Current Damping
*Electromagnetic Damping
Super vision By:
Sir Sajid Hussain Qazi
The damping force ensures
that the pointer comes to rest in
its final position
quickly and without undue
*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.
overcome this difficulty a damping torque is to be
developed by using a damping device attached to the
moving system.
*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
 Under damped condition:
If the Degree of the damping is low then pointer
makes oscillation before settling at equilibrium
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 Damping
2.Fluid Friction Damping
3.Eddy Current Damping
4.Electromagnetic 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.
* 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
*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.
* 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
* 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
B.L Thereja

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