# Damping Systems – Air friction, Fluid Friction & Eddy Current Damping

- Link: https://studyelectrical.com/2019/11/damping-systems-air-friction-eddy-current-fluid-friction-and-electromagnetic.html
- Published: 2019-11-26T11:31:50+00:00
- Author: Tesla

In this article, you will learn about  four different types of damping systems namely
air friction damping, fluid friction damping, eddy current damping and electromagnetic
damping.

Before going into the details you must understand the need for a damping system 
in a measuring instrument.

In a measuring instrument, the deflecting torque provides some deflection and controlling
torque acts in the opposite direction to that of deflecting torque. So before coming
to the rest, pointer always oscillates due to inertia, about the equilibrium position.
Unless pointer rests, final reading cannot be obtained.

**So to bring the pointer to rest within a short time, the damping system is required**.
The system should provide a damping torque only when the moving system is in motion.

## Underdamped, Overdamped and Critically Damped Systems

The damping torque in a damping system 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 dead beat.

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.

![Underdamped, Overdamped and Critically damped Graphical Representation](https://
studyelectrical.com/wp-content/uploads/2019/11/Damping-Systems.png)

Underdamped, Overdamped and Critically damped systems (Graphical Representation)

The figure illustrates the way an **underdamped, overdamped and critically damped
system** moves to its final steady position. 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 static friction.

The methods for producing damping torque are :

 1. **Air friction damping**
 2. **Fluid friction damping**
 3. **Eddy current damping**
 4. **Electromagnetic damping.**

## 1. Air Friction Damping

There are two types of air friction damping devices.

![air friction damping](https://studyelectrical.com/wp-content/uploads/2019/11/air-
friction-damping.jpg)

Air friction damping

The above figure shows the first method of air friction damping. It consists of 
a light aluminium piston that 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 of the whole of
the moving system. When the piston moves out of the air chamber, pressure in 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.

![Air friction damping with air vane](https://studyelectrical.com/wp-content/uploads/
2019/11/air-friction-damping-with-air-vane.png)

Air friction damping with air vane

The second type of air friction damping is shown in the above figure. The arrangement
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 diecasting.

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.

## 2. 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 force is also correspondingly
greater.

![Fluid friction damping](https://studyelectrical.com/wp-content/uploads/2019/11/
Fluid-friction-damping.png)

Fluid friction damping

A disc is attached to the moving system, 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 above 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.

## 3. Eddy Current Damping

This is also known as electromagnetic 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 that opposes the motion.

![Eddy Current](https://studyelectrical.com/wp-content/uploads/2019/11/Eddy-Current.
png)

Eddy Current

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 
velocity & the conductor.

![Eddy Eddy Current DampingCurrent Damping](https://studyelectrical.com/wp-content/
uploads/2019/11/Eddy-Current-Damping.png)

Eddy Current Damping

![eddy-current-damping](https://studyelectrical.com/wp-content/uploads/2019/11/eddy-
current-damping.jpg)

Eddy current damping

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.

## 4. 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 Methods of Damping

**Air Friction Damping**

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](https://studyelectrical.com/2019/11/moving-iron-instruments-attraction-and-repulsion-type.html)**.

**Fluid Friction Damping**

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 that 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 that are in a 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**

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 eg: [PMMC instrument](https://studyelectrical.com/2019/11/permanent-magnet-moving-coil-pmmc-instrument.html).

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.

There are two common forms of damping devices :

 1. A metal former which carries the working coil of the instrument.
 2. A thin aluminium disc attached to the moving system of the instrument. This disc
    moves in the field of a permanent magnet.

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