A portable instrument is likely to have

Question

Examoogle · Accepted Answer

The correct answer is Option D: Eddy-current damping. Portable instruments require a damping system that is light, robust, high-performing, and independent of position. Eddy-current damping provides a strong damping force without relying on liquids or air chambers, making it ideal for portable instruments.

Step-by-Step Solution & Concepts:
Portable instruments require a damping system that is light, robust, high-performing, and independent of position. Eddy-current damping provides a strong damping force without relying on liquids or air chambers, making it ideal for portable instruments.

Working Principle: When a metallic disc or former attached to the moving system rotates in a permanent magnetic field, eddy currents are induced in it. According to Lenz's law, these induced currents create an opposing magnetic field that exerts a drag force, quickly bringing the pointer to rest without mechanical contact or fluid friction.

Key Formulas: $F_d \propto B^2 \cdot \omega \cdot \sigma$ — Damping force proportional to flux density squared, angular speed, and conductivity

Key Points: Eddy-current damping is the most efficient and robust damping method.; It requires a permanent magnet, which is already present in Permanent Magnet Moving Coil (PMMC) instruments.; It operates without any mechanical friction or fluid leaks, ensuring long-term reliability in portable equipment.

Advantages: Most effective damping system with fast response and no overshoots.; No risk of liquid leakage or contamination (unlike fluid friction damping).; Independent of orientation and operational positioning.

Disadvantages: Requires a strong permanent magnetic field.; Cannot be easily used in instruments where stray magnetic fields affect performance unless shielded.

Applications: PMMC instruments (Ammeters, Voltmeters); Induction type energy meters; Hot-wire instruments

Additional Info: Fluid friction damping is unsuitable for portable instruments due to the danger of fluid spilling or changing viscosity.. Gravitational damping (or control) requires the instrument to strictly remain in a vertical position, making it unfit for portable use.. Pneumatic (air friction) damping is mainly used in Moving Iron (MI) and Dynamo-type instruments where strong stray magnetic fields must be avoided.

Answer: D is correct — Eddy-current damping is position-independent, efficient, and leak-free, making it ideal for portable instruments.

Exam Tip: For GATE/ESE exams, remember: Portable instruments ALWAYS use spring control (never gravity control) and prefer eddy-current or air-friction damping over fluid damping.

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Option A: Fluid friction damping

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Option B: Pneumatic damping

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Option C: Gravitational damping

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