As the name suggests these instruments′ operation
depend on the movement of a soft iron piece. These are of two types- attraction
type and repulsion type.
Attraction type moving
iron instruments are commonly used and therefore, we will focus on it.
Attraction type Moving Iron Instruments
These instruments use soft iron which is a
ferromagnetic material. Ferromagnetic materials magnetize in the direction of a
magnetic field, therefore, if they are brought near one end of a current
carrying coil, poles of opposite polarity are induced in them. This attracts
soft iron into the coil and the force of attraction depends upon the strength of
the coil′s magnetic field.
As shown in above diagram, whatever be the direction
of current through the coil, the iron piece would always be magnetized in such
a way that it is pulled inwards and that is why moving iron instruments can be used
for both ac and dc.
Construction of Attraction type Moving Iron Instruments
A sectional view of the Attraction type Moving Iron
Instrument is shown.
Magnetic field system
:-
The magnetic field is obtained by a
multi-turn rectangular shaped coil. At one end of the coil, a oval-shaped soft
iron disc is pivoted eccentrically so that it can be attracted into the coil
and the pointer attacted to the disc can show reading. Visualize it.
If the instrument is to be used as an ammeter, the
coil has comparatively fewer turns of thick wire and for voltmeter application,
the coil has larger number of turns of thin wire .
Deflecting system
:- The pivoted soft-iron disc experiences torque due to the attractive force by
the coil magnetic field.
Controlling system
:- Gravity control was used previously but now it is replaced by spring control
in relatively modern instruments.
Damping system
:- Air-friction damping is provided by a
light piston moving in an air-chamber.
Attraction type Moving Iron Instruments Theory
Let the axis of the iron disc, when instrument is not
in use i.e current through the coil is zero, subtend an angle of Ï• with a
direction perpendicular to the direction of the field H produced by the coil.
Let current I through the coil produces a deflection θ. The magnetisation of
the soft iron disc is proportional to the component of field H acting along the
axis of the disc
i.e magnetisation of the disc ∝ H
cos [90 - (θ+ϕ)]
or ∝ H
sin (θ+ϕ) ... cos(90- θ)=sin θ
thus, the force F pulling the disc is proportional to H2
sin (θ+ϕ)
or Force ∝ I2
sin (θ+Ï•) ... H∝I for constant permeability
If this force acted at a distance L from the pivot of
the disc, then deflecting torque Td is
Td =
Force×L
cos(θ+ϕ)
Td ∝
I2L
sin (θ+ϕ) cos(θ+ϕ)
As L is constant ,
Td =
K I2 2 sin (θ+ϕ)
cos(θ+ϕ) ... K=a constant
Td =
K I2 sin2(θ+ϕ)
∵ sin2θ= 2 sinθ cosθ
In modern moving iron instruments, spring control is
used, therefore controlling torque is Tc =K1 θ
At steady position of the pointer,
Td = Td
K1
θ
= K I2 sin2(θ+ϕ)
or θ ∝ I2
Showing that moving iron instruments have non-linear scale.
Closely related :-
(1) Permanent Magnet Moving coil Instruments
Closely related :-
(1) Permanent Magnet Moving coil Instruments
