MOVING IRON INSTRUMENTS:
Where ruggedness is more important than a high degree of accuracy, Moving Iron
Instruments are used.
There are two types of MI instrument:
- Moving iron attraction type
- Moving iron repulsion type
What is moving iron instrument:
This instrument is one of the most primitive forms of measuring and relay instrument, moving iron
instrument consist of a measuring mechanism consisting of the moving iron system.
instrument consist of a measuring mechanism consisting of the moving iron system.
Moving Iron Attraction Type:
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| Attraction-Type-Instruements Moving-Iron-Instrument |
Working Principle:
The working principle is very easy that a soft iron piece is brought near the magnet gets attracted by
the magnet.
the magnet.
The Moving Iron Attraction Type Instrument is shown in Fig.
Construction and Operation:
- Moving iron attraction type instrument has a movable cast of iron piece and has a standard
- The moving iron mounted on the spindle has a flat design. The spindle is mounted inside the bearings
- The spindle carries a pointer, which moves to a degree.
A coil of insulated copper wire is round wound a hollow bobbin. For a voltmeter, the cross-section of
the wire is fine and it has many numbers of turns.
the wire is fine and it has many numbers of turns.
For Ammeter, the coil is thick and it has few turns.
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| attraction-type-moving-iron-instrument |
- A small piece of iron is pivoted just outside the coil. When the current passes through the coil the moving piece of iron is attracted
- It is drawn into the bobbin and produces a twisting torque.
- Moving iron pivoted by a spindle which carries a pointer
- Moving iron was elliptically shaped so as to obtain a uniform scale in the Picture
- Springs create a controlling torque in which gravity control can be utilized
- Using air friction damping torque is provided. So it can be used for A.C. as well as D.C.
Moving Iron Repulsion Type Instrument:
Principle:
There are two vanes inside the coil, the one is fixed and the other is movable. When the current flows
in the coil, both the vanes are magnetized with like polarities induced on the same side.
in the coil, both the vanes are magnetized with like polarities induced on the same side.
Therefore the driving force between the two vanes is due to driving the poles, which causes the
movement of the moving vane.
movement of the moving vane.
- As shown in the figure, the coil is wound around a hollow bobbin. Inside the bobbin, two pieces of soft iron are placed face to face.
- One piece is fixed to the wall of the bobbin and the other, one is fixed to the spindle and it is allowed to move.
- Shapes of two iron pieces are different according to different makes
- The figure shows a torque shaped fixed vane and a rectangular moving vane.
- These iron vanes are similarly magnetized when current flows through the coil.
- The force of repulsion is produced between the vanes. Hence moving vane deflects. The deflection is shown by a pointer on a scale.
- Controlling torque is produced using spring.
Derivation of Deflecting Torque Equation:
It is based on the principle of conservation of energy
Electrical energy supplied = Change in stored energy + Mechanical work done
Let,
I = Initial current
L = Instrument inductance
Ө= Deflection
If current increased by dI, deflection changes by d Ө doing, and inductance by dL.
To increase the current by dl, there must be an increase in applied voltage.
e = d/dt[L I]
e = L dI/dt + I dL/dt
Electrical energy supplied = e I dt
= L I dI + I² dL
Change in stored energy = ½ [I + dI] ² [L + dL] – ½ L I²
= ½ [I² + 2 I dI + dI² [L + dL ] – ½ L I²
Neglect second and higher order terms
=IL dI + ½ I² dL
According to the principle of conservation of energy
Electric energy supplied = Change in stored energy + Mechanical work done
I² dL + LI dI = IL dI + ½ I² dL + Td d Ө
Td d Ө = ½ I² dL
Advantages:
- Used for A.C & D.C measurement
- Cheapness
- The instrument is robust as moving part don't carry current
Disadvantages:
- Non-uniform scale
- The instrument is subjected to error due to temperature variation.
Application :
Moving iron instrument used for D.C measurement except for inexpensive indicators such as charge-discharge indicators in automobiles.
Used on A.C where 5 to 10% emf is acceptable
Image Credit:
https://circuitglobe.com/moving-iron-or-mi-instrument.html
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