Question

In measuring instruments, The Damping torque is used to:

• A. Increase Sensitivity
• B. Reduce Oscillations
• C. Increase Deflection
• D. Measure Current

Hint:

Damping must be "critical" or "slightly underdamped" to ensure the pointer reaches the final reading quickly without creeping or oscillating excessively.
Eddy current damping is the most efficient and is used where strong permanent magnetic fields are present.

Answer 1

The Correct Option is B

Step 1: Understanding the Question:
The question asks for the primary function of "Damping torque" in analogue indicating instruments.
Without proper damping, the pointer of an instrument would take a very long time to settle, making quick readings impossible.

Step 2: Key Formula or Approach:
When an instrument is energized, deflecting and controlling forces act in opposite directions on the moving system.
Because the moving system has mechanical inertia, the pointer will overshoot and undergo sustained oscillations about its final equilibrium position.
To damp out these oscillations quickly and bring the pointer to rest smoothly, a damping torque (\( T_d \)) is introduced that acts only when the system is in motion.

Step 3: Detailed Explanation:
Let us analyze the operational mechanics and types of damping:

• Function: Damping torque opposes the velocity of the pointer. It is zero when the pointer is stationary and maximum when the pointer is moving fast:
\[ T_{\text{damping}} \propto \frac{d\theta}{dt} \] This quickly absorbs the kinetic energy of the moving coil assembly, reducing oscillations without affecting the final static deflection value.

• Types of Damping:
- Air Friction Damping: Uses a light aluminum piston moving inside a closed air chamber. Commonly used in Moving Iron instruments.
- Fluid Friction Damping: Uses a vane submerged in high-viscosity oil. Excellent performance but limited to vertical instruments.
- Eddy Current Damping: Uses electromagnetic induction where a conductive disc rotates through a permanent magnetic field. It is the most effective and is used in PMMC instruments.

Step 4: Final Answer:
The damping torque in measuring instruments is used specifically to reduce oscillations of the pointer.