Page 278 - Electrician - TT (Volume 1)

ELECTRICIAN - CITS

Indicating intrument
Essentials of indicating instruments
As defined above, indicating instruments are those which indicate the value of quantity that is being measured
at the time at which it is measured. Indicating instruments consist essentially of a pointer which moves over a
calibrated scale and which is attached to a moving system pivoted in jewelled bearings. In an indicating instrument,
it is essential that the moving system is acted upon by three torque for satisfactory working. The moving system
is subjected to the following 3 torques:

1 Deflecting torque or operating torque
2 Controlling torque or restoring torque
3 Damping torque
Deflecting torque
The deflecting or operating torque (Td) is produced by utilizing one or other effects, e.g., magnetic, electrostatic,
electrodynamic, thermal or inductive. The method of torque production depends on the type of instrument. The
deflecting torque causes the moving system (and hence the pointer attached to it) to move from its zero position.
Controlling torque

Controlling torque or restoring or balancing torque is the torque which controls the movement of the pointer on a
particular scale. Controlling torque is acted opposite to the deflection torque. Important of controlling torque are;
The magnitude of the moving system would be somewhat indefinite under the influence of deflecting torque,
unless the controlling torque existed to oppose the deflecting torque.
It increases with increase in deflection of moving system.
Without controlling torque the pointer will swing at its maximum position & will not return to zero after removing
the source.
It is obtained by two methods which are discussed below
i Spring control
In the spring control method, a hair spring usually of phosphor bronze, attached to the moving system is used.
Two springs are arranged, one at the bottom and other at the top of the pointer, and are twisted in the opposite
direction. With the deflection of the pointer, one of the spring is twisted and other unwind. This twist in the spring
produces restoring torque which is directly proportional to the angle of deflection of the moving system. The
pointer comes to a position of rest (or equilibrium) when the deflecting torque (T ) and the controlling torque (T )
c
d
are equal.
T α I
d
T α Ɵ
c
At final position, T = T d
c
Ɵ α I
Since deflection Ɵ is directly proportional to current I, the spring-controlled instrument have a uniform or equally-
spaced scales over the whole of their range.
ii Gravity controll
Gravity control is obtained by attaching a small adjustable weight to some part 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. The degree of control is
adjusted by screwing the weight up or down the carrying system
As shown in the figure, the controlling or restoring torque is proportional to the sine of the angle of deflection, i.e
T α sin Ɵ
c
T α I
d
At final position, T = T d
c
sin Ɵ α I

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