Damping Torque: Methods and Sample Questions

Torque damping is the technique of controlling a moving system to oppose its inherent oscillation.

• When the deflecting force is applied to a moving system, there is deflection, and the system is brought to rest when the deflecting force is balanced by the controlling force. 
• The moving system cannot immediately stop at its steady position, so damping torque must be introduced so that the moving system achieves equilibrium without oscillation.
• Sending an electric current through a magnetic coil generates electromagnetic damping by producing a torque that opposes the coil's natural movement.

Key Terms: Damping Torque, Eddy Current, Instrument, Electromagnetic Damping.

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What is Damping Torque?

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The process of controlling the movement of a system by creating a motion in such a way that the natural oscillation of the system is opposed is known as damping torque. The damping torque is used to bring the moving system into equilibrium and the deflecting force is balanced through it.

The damping torque is proportional to the speed of rotation. The relationship is stated as

Tv = kv d dtθ

Where k is the damping torque constant and d dtθ is the speed of the rotation.

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Conditions Under Damping Torque

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There are three conditions for damping torque that is possible:

1. Under damped condition: When the magnitude of the damping torque is in such a way that the pointer comes to its final steady position quickly without overshooting.
2. Over damped condition: In an over damped instrument, the damping torque is more than the required critical damping. The system moves gradually to its eventual point of stability. It takes time to reach the final position from zero.
3. Critically damped condition: in a critically damped condition, the movement stops at its steady final position without any oscillation. The damping torque is produced in such a way that the moving system comes to a final steady position without any oscillations.

Read More: Eddy Current

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Methods Of Providing Damping Torque

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There are four different methods of providing damping torque:

• Air friction damping
• Fluid friction damping
• Eddy current damping
• Electromagnetic damping

Air Friction Damping

Air friction damping is created when the piston is moved in and out of an air chamber. Compression is caused inside the chamber when the piston is inserted in the chamber and as the piston moves out, a force is experienced. The piston moves in a fixed chamber, which is close to one end. The air chamber is a recessed cast. The aluminium piston should be carefully fitted to prevent serious errors while taking readings. The other end is linked to the pointer.

When the pointer moves toward the left, the piston moves inside the chamber and the air pressure increases, whereas when the pointer moves to the right, the piston moves out and the pressure decreases in the air chamber.

Advantage:

• There is no risk of distortion of fields because there are no permanent magnets.
• It is easy to maintain and is cheap.

Disadvantage:

• Mishandling of the piston may cause errors in readings.
• It is not reliable.

Fluid Friction Damping

Fluid friction damping is caused when there is an oscillation of the disk in and out of the liquid, which is oil. For opposing the motion, the friction between the disc and the fluid is being used. The damping force of fluid is greater than that of air due to the viscosity which is formed. The principle of fluid friction damping is the same as air friction damping.

The fluid friction damping has a vane that is attached to the spindle which is dipped in the oil, and the vane does not require small clearances to give effective damping.

Advantages:

• The oil used has two uses, damping, and heat dissipation.
• The oil is also used as an insulating fluid.
• The vane does not require small clearances to provide better effective clearances, so it is a very suitable method for various instruments.
• There are fewer frictional errors in the fluid friction damping because of the upthrust of oil, and the load on the suspension is reduced.

Disadvantages:

• This can be used only in instruments that are in a vertical position.
• Since there is creeping of oil, the instruments do not remain clean.
• They can be used as laboratory instruments, and it is difficult to apply with portable instruments.

Eddy Current Damping

Eddy current damping is one of the most efficient methods of producing damping torque. It is based on the principle of the production of electromagnetic torque. In this, eddy current and electric fields are together used to create electromagnetic torque which helps in opposing the motion. The eddy current damping is proportional to the strength of the magnetic field and the current which is produced.

Advantages: It is used for moving coil and induction instruments. It can also be used in portable instruments.

Disadvantages: It cannot be used for moving-type instruments.

Electromagnetic Damping

Electromagnetic damping is created by passing the electric current through the magnetic coil in such a way that the torque which is being generated is acting against the natural movement of the coil.

Disadvantage: It distorts the weak electrical fields.

Read More: Electromagnetic Damping

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Things to Remember

• Damping torque is defined as the process of controlling the movement of the system in such a way that the natural oscillation of the system is opposed.
• There are three possible conditions for damping torque: under damped, over damped, and critically damped condition.
• There are four methods of producing damping torque: air friction damping, fluid friction damping, eddy current damping, and electromagnetic damping.
• Air friction damping is created in an air chamber by the movement of the piston and the air pressure which is being caused due to it.
• Fluid friction damping has the same principle as air friction damping, but it is used in oil.
• Eddy current damping is based on the principle of the production of electromagnetic torque.

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