Classification of Moving Iron Instruments | Working principle of moving iron instrument and its 2 types

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moving iron instrument

moving iron instrument

his instrument is one of the most primitive forms of measuring and relay instruments. Moving iron type instruments are of mainly two types. Attraction type and repulsion type instrument.
Whenever a piece of iron is placed nearer to a magnet it would be attracted by the magnet. The force of this attraction depends upon the strength of the said magnetic field. If the magnet is electromagnet then the magnetic field strength can easily be increased or decreased by increasing or decreasing current through its coil.
Accordingly, the attraction force acting on the piece of iron would also be increased and decreased. Depending upon this simple phenomenon attraction type moving iron instrument was developed.

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A moving iron instrument is an electrical measuring instrument that uses the interaction of a moving iron element with a magnetic field to measure an electrical quantity such as current or voltage. The moving iron element is typically a small iron vane or pointer that is attached to a coil of wire. The coil is positioned in a magnetic field, which is created by a permanent magnet or an electromagnet.

Working Principle Of Moving Iron Instrument And Its 2 Types

The working principle of a moving iron instrument is based on the interaction of the magnetic field with the current flowing through the coil. When a current flows through the coil, it creates its own magnetic field that interacts with the external magnetic field. This interaction causes the coil and the attached iron element to move. The amount of movement is proportional to the current flowing through the coil.

The movement of the iron element is then used to indicate the value of the current or voltage being measured. The movement can be directly read from a scale or can be amplified by gears or levers to move a pointer on a scale.

There are two types of moving iron instruments, one is attraction type and the other is repulsion type. In the attraction type, the moving iron element is attracted towards the pole of the magnet when current flows through the coil, and in the repulsion type, the moving iron element is repelled away from the pole of the magnet.

It should be noted that moving iron instruments have a limited range of measurement and are typically used for measuring low to medium values of current and voltage.

Moving iron instruments are of two types

(i) Attraction type.

(ii) Repulsion type.



 Attraction type.

 

  •     The coil is flat and has a narrow slot-like opening.
  •     The moving iron is a flat disc or a sector eccentrically mounted.
  •     When the current flows through the coil, a magnetic field is produced and the moving iron moves from the weaker field outside the coil to the stronger field inside it, or in other words the moving iron is attracted in.
  •     The controlling torque is provided by springs but gravity control can be used for panel types of instruments that are vertically mounted.
  •     Damping is provided by air friction with the help of a light aluminum piston (attached to the moving system) which moves in a fixed chamber closed at one end as shown in Fig. or with the help of a vane (attached to the moving system) which moves in a fixed sector shaped chamber a shown.


Repulsion Type

      In the repulsion type, there are two vanes inside the coil one fixed and the other movable. These are similarly magnetized when the current flows through the coil and there is a force of repulsion between the two vanes resulting in the movement of the moving vane. Two different designs are in common use

(I) Radial Vane Type

  •     In this type, the vanes are radial strips of iron.
  •     The strips are placed within the coil as shown in Fig.
  •     The fixed vane is attached to the coil and the movable one to the spindle of the instrument.

        (a)    Radial vane type.   (b) Co-axial vane type

(ii) Co-axial Vane Type

  •     In this type of instrument, the fixed and moving vanes are sections of coaxial cylinders as shown in Fig.
  •     The controlling torque is provided by springs. Gravity control can also be used in vertically mounted instruments.
  •     The damping torque is produced by air friction as in attraction-type instruments.
  •     The operating magnetic field in moving iron instruments is very weak and therefore eddy current damping is not used in them as the introduction of a permanent magnet required for eddy current damping would destroy the operating magnetic field.
  •     It is clear that whatever may be the direction of the current in the coil of the instrument, the iron vanes are so magnetized that there is always a force of attraction in the attraction type and repulsion in the repulsion type of instruments.
  •     Thus moving iron instruments are unpolarised instruments i.e., they are independent of the direction in which the current passes.
  •      Therefore, these instruments can be used on both ac. and D.C.

Torque Equation of Moving Iron Instrument:

         Considering the energy relations when there is a small increment in current supplied to the instrument may derive an expression for the torque-moving iron instrument. When this happens there will be a small deflection dฮธ a mechanical work will be done. Let Td be the deflecting torque.

Mechanical work done = Td. dฮธ

Alongside there will be a change in the energy stored in the magnetic field owing to a change in inductance.
Suppose the initial current is I, the instrument inductance L, and the deflection ฮธ. If the current is increased by di then the deflection changes by dฮธ and the inductance by dL. In order to affect an increment in the current there must be an increase in the applied voltage given by

Comparison between Attraction and Repulsion Types of Instruments

โ€ข    In general it may be said that attraction-type instruments possess the same advantages, and are subject to the limitations, described for the repulsion type.

โ€ข    An attraction-type instrument will usually have a lower inductance than the corresponding repulsion-type instrument, and voltmeters will therefore be accurate over a wider range of frequencies and there is a greater possibility of using shunts with ammeters.

โ€ข    On the other hand, repulsion instruments are more suitable for economical production in manufacture, and a nearly uniform scale is more easily obtained; they are, therefore, much more common than the attraction type.

Errors in Moving Iron Instruments

There are two types of errors, which occur in moving iron instruments โ€” errors that occur with both a.c. and D.C. and the other which occur only with ac. only.

Errors with both D.C. and A.C

i)    Hysteresis Error

ii)    Temperature error

iii)    Stray magnetic field

Errors with only A.C

 Frequency errors

Advantages & Disadvantages

1) Universal use
(2) Fewer Friction Errors
(3) Cheapness
(4) Robustness
(5) Accuracy
(6) Scale
(7) Errors
(8) Waveform errors.

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