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A portable instrument is likely to have
Fluid friction damping
Pneumatic damping
Gravitational damping
Eddy-current damping
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.
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.
FdтАЛтИЭB2тЛЕ╧ЙтЛЕ╧Г тАФ Damping force proportional to flux density squared, angular speed, and conductivity
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.
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.
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.
Requires a strong permanent magnetic field.
Cannot be easily used in instruments where stray magnetic fields affect performance unless shielded.
PMMC instruments (Ammeters, Voltmeters)
Induction type energy meters
Hot-wire instruments
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.
D is correct тАФ Eddy-current damping is position-independent, efficient, and leak-free, making it ideal for portable instruments.
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.