Essentials of Indicating Instruments | Deflecting Torque | Controlling Torque | Damping Torque | Freely Electrons
Indicating instruments are
used to indicate the quantity with respect to pointer that is being measured at the time
when it moves on a scale of an instrument. Some instruments consist of pointer
which moves over a calibrated scale. The moving system of the indicating
instruments consists of three torques.
There are three Essentials of Indicating Instruments. These are as follows:
- Deflecting Torque
- Controlling Torque
Spring Control
Gravity Control
- Damping Torque
Air friction Damping
Fluid Friction Damping
Eddy Current Damping
Deflecting Torque
The deflecting torque is produced by utilizing other
effect like magnetic, electrostatic, electrodynamics, thermal
or inductive etc. This deflecting torque causes the moving
system to move from its zero position or when the instrument is
disconnected from the supply.
Controlling Torque
This torque faces the deflecting torque and
increases with the deflection of the moving system. The pointer
moves on its rest position when these two opposing torques are equal. During
the absence of a restoring torque, the pointer doesn’t return to
its zero position when the supply is removed from the instrument.
The controlling or balancing torque in indicating instruments is
obtained by two methods which are described below:
Spring Control
In spring control method of indicating instruments
consists of a control spring(made up of phosphor bronze),Balance
weight,pivot and pointer. The spring is attached to the moving system
of instrument.When pointer moves, the spring is twisted in
the opposite direction. This twist of the spring produces restoring torque
and it is directly proportional to the angle of deflection of the moving
system.When the controlling and the deflecting torque are equal to
each other, the pointer then moves in its rest position.
Gravity Control
In gravity control system, small adjustable weights
are attached to some parts of the moving system. The controlling
torque is proportional to the sine of the angle of deflection.
Tc ∝ I
And at a rest position
Td = Tc
The scales of the gravity controlled instruments
are not uniform. So, the advantages and disadvantages of gravity controlled instruments
are as under:
Disadvantages
- It gives cramped scale.
- The instrument has to be kept in a vertical position
Advantages
- These instruments are cheap.
- These are unaffected by temperature.
Damping Torque
It is a force of an instrument which acts on
the moving system of the instrument when it starts moving and it
opposes its motion. This damping force is necessary for the instrument
to bring the pointer on a rest position because the pointer takes
some time to move from its rest position. If the damping force is increased,
it will make the instrument very slow. The damping force is
produced by air frictions, eddy currents and fluid frictions.
Air Friction Damping
In this type of damping, a light aluminum piston is
attached to the spindle which carries the pointer and the pointer
produces a minor move in it . However, the air on the piston removes the
bent of the pointer to move easily on its final deflected position.
Fluid Friction Damping
Fluid friction damping
consists of discs which are attached to the spindle of the moving
system. The moving system is kept in an oil containing container. The container
is filled with high viscosity oil. When the pointer starts moving, the friction
between the oil and discs starts opposing to the moving of a pointer and then damping
is provided to the pointer.
Eddy Current Damping
In this type of damping, a copper or aluminum disc is
attached to the moving system of the pointer and permanent magnets are
attached to the poles. When the pointer moves the copper or aluminum
disc cuts the magnetic flux between the poles of a permanent magnet and eddy
current is produced with the flow and produces the damping force in
a required direction.
