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Damping torque in instruments is generally not produced
Pneumatically
Electro-magnetically
Electro-statically
By fluid friction
Electro-statically
Damping torque in measuring instruments is designed to oppose the oscillation of the moving system and bring it to rest quickly at its final deflected position. It is produced using air friction (pneumatically), eddy currents (electro-magnetically), or fluid friction. Electro-static forces are extremely weak and are used for deflecting torque in electrostatic instruments, not for producing damping torque.
Damping torque in measuring instruments is designed to oppose the oscillation of the moving system and bring it to rest quickly at its final deflected position. It is produced using air friction (pneumatically), eddy currents (electro-magnetically), or fluid friction. Electro-static forces are extremely weak and are used for deflecting torque in electrostatic instruments, not for producing damping torque.
TdтАЛ=CdтАЛтЛЕdtd╬╕тАЛ тАФ Damping torque proportional to angular velocity
Damping methods rely on energy dissipation to suppress mechanical oscillations. Air friction uses air resistance in a vane/piston assembly, fluid friction uses viscous drag in a liquid, and eddy current damping relies on Electromagnetic Induction where induced currents in a moving conductor create an opposing magnetic force according to Lenz's law. Electrostatic forces operate on charge interaction, which provides insufficient mechanical force density to act as an effective damping mechanism.
Damping torque acts only when the moving system is in motion (d╬╕/dtюАа=0).
It produces no torque when the pointer is stationary at its final position.
Eddy current damping is the most efficient and efficient damping method among all types.
Electrostatic forces are weak forces primary used for voltage deflection in electrostatic voltmeters.
Pneumatic (Air friction) damping is inexpensive and leaves no oil stains.
Eddy current damping is smooth, highly effective, and non-contact.
Fluid friction damping is messy due to oil leakage and affected by temperature changes in oil viscosity.
Air friction damping requires a precise chamber and is less effective than eddy current damping.
Air friction damping: Moving Iron (MI) and Electrodynamometer instruments.
Eddy current damping: Permanent Magnet Moving Coil (PMMC) instruments and Energy meters.
Fluid friction damping: Electrostatic voltmeters.
Option A (Pneumatically): Uses air friction in a chamber with a vane or piston (common in MI instruments).
Option B (Electro-magnetically): Uses eddy currents induced in a metallic former or disc moving in a magnetic field (common in PMMC instruments).
Option D (By fluid friction): Uses viscous force of oil on a dip vane attached to the spindle (common in Electrostatic Voltmeters).
C is correct тАФ Electro-static force is extremely weak and cannot produce sufficient damping torque in measuring instruments.
Always remember the combination: PMMC uses Eddy Current damping, MI uses Air Friction damping, and Electrostatic Voltmeter uses Fluid Friction damping.
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