In gravity control – gravity controlled instruments, as shown.
A small adjustable weight is attached to the spindle of the moving system such that the deflecting torque produced by the instrument has to act against the action of gravity.
Thus a controlling torque is obtained.
This weight is called the control weight.
Another adjustable weight is also attached is the moving system for zero adjustment and balancing purpose.
This weight is called Balance weight.
When the control weight is in vertical position as shown
(a), the controlling torque is zero and hence the pointer must read zero.
However, if the deflecting torque lifts the controlling weight from position A to B as shown
(b) such that the spindle rotates by an angle θ, then due to gravity a restoring (or controlling) torque is exterted on the moving system.
The controlling (or restoring) torque, Tc , is given by
Tc = Wl sin θ = k g sin θ where W is the control weight;
l is the distance of the control weight from the axis of rotation of the moving system; and k g is the gravity constant.
Equation shows the controlling torque can be varied quite simply by adjustment of the position of the control weight upon the arm which carries it.
Again, if the deflecting torque is directly proportional to the current,
i.e., Td = kI
We have at the equilibrium position Td = Tc
kI = k g sin θ
I = g k sin θ / k
This relation shows that current I is proportional to sin θ and not θ.
Hence in gravity controlled instruments the scale is not uniform.
It is cramped for the lower readings, instead of being uniformly divided, for the deflecting torque assumed to be directly proportional to the quantity being measured.
Advantanges of Gravity Control
- It is cheap and not affected by temperature variations.
- It does not deteriorate with time.
- It is not subject to fatigue.
Disadvantages of Gravity Control
- Since the controlling torque is proportional to the sine of the angle of deflection, the scale is not uniformly divided but cramped at its lower end.
- It is not suitable for use in portable instruments (in which spring control is always preferred).
- Gravity control instruments must be used in vertical position so that the control weight may operate and also must be leveled otherwise they will give zero error.
- In view of these reasons, gravity control is not used for indicating instruments in general and portable instruments in particular.
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