Page 279 - Electrician - TT (Volume 1)

ELECTRICIAN - CITS

Hence in gravity control instruments, the scales are not uniform but are cramped or crowded at their lower ends.
Comoparison between spring control and gravity control

Spring control Gravity control

Instruments have uniformly or equally Instrument have unequal scale
spaced scale (cramped or crowded at lower end)

Costly It is cheap compared to spring control

Instrument can mounted in any way Mounted only in vertical position

Damping torque
Damping torque is one which acts on the moving system of the instrument only when it is moving and always
opposes its motion. Such damping force is necessary to bring the pointer to rest quickly, otherwise due to inertia
of the moving system, the pointer will oscillate about its final deflected position for quite some time before coming
to rest in the steady position. The degree of damping should be adjusted to a value which is sufficient to enable
the pointer to rise quickly to its deflected position without over-shooting. According to the degree of damping,
damping is of three types as graphically represented below
1 Under damped condition:
The response is oscillatoryie the pointer make oscillation at its final position.
2 Over damped condition:

The pointer rises very slowly from its zero position to final position.
3 Critically damped condition:

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CITS : Power - Electrician & Wireman - Lesson 50-53