Repulsion Type Moving Iron (MI) Instrument - Construction, Working & Advantages


     In the last article, we have seen about attraction type moving iron instrument. Now let us learn about repulsion type moving iron instruments. Similar to attraction force there will be repulsion force when same magnetic poles are placed near each other. Based upon this repulsion force between two like poles the repulsion type moving iron instruments were developed.

     Repulsion type moving iron instruments are used both for ac and dc measurements. In these instruments, when current flows through the coil, the two vanes i.e., fixed vane and movable vane are magnetized and same polarities are induced in it which results in a force of repulsion between them.


Construction of Repulsion Type Moving Iron Instrument :

     Basically, a repulsion type instrument consists of a fixed field coil and two vanes present inside the coil, out of which one vane is fixed and the other vane is movable. The fixed vane is attached to the coil whereas the movable vane is mounted on the spindle of the instrument. The spindle carries the pointer which moves on a graduated scale. Depending on the design of construction, there are two types of repulsion type MI instruments. They are,
  • Radial vane type, and
  • Co-axial or concentric vane type

Radial Vane Repulsion Type Instrument :

     If the vanes are radial, then it is called radial vane repulsion type instrument as shown below.

Repulsion Type Moving Iron Instrument

     It consists of two iron strips (vanes) are placed radially, in which one is fixed and the other is movable. In this type of instrument, the deflection torque is directly proportional to the actual current in the coil, thus making the scale uniform and readings can be obtained directly. These are the most sensitive type of instruments.

Co-axial Vane Repulsion Type Instrument :

     In co-axial vane type instruments, the vanes are the sections of coaxial cylinders as shown below.

Repulsion Type Moving Iron Instrument

     In this, the two vanes are co-axially placed inside the coil. In which one vane is fixed to the coil frame that remains stationary, while the other is movable which rotates at the central axis inside the stationary vane. But the deflecting torque on the pointer is proportional to the square of the actual current in the coil. Hence the scale cannot be uniform due to concentric vanes. Compared to radial type these instruments are less sensitive.



Working of Repulsion Type Moving Iron Instrument :

     Initially, the current does not pass through the coil. Hence, the two vanes will touch each other and the pointer does not deflect (i.e., it will be at zero position). Whenever current flows through the coil, a magnetic field is set up in it and two vanes are magnetized with the same polarities i.e., north poles are produced at one end in both the vanes and south poles are produced on their other ends. Due to this, a repulsive force exists between two vanes and the movable vane tries to move away from the fixed vane as shown below.

Repulsion Type Moving Iron Instrument

     Hence, the movable vane moves because of the repulsive force, and the pointer which is mounted on a spindle show deflection. The pointer stops deflecting when the controlling torque is equal to the deflecting torque. The amount of repulsion force depends upon the strength of the magnetization field produced by the coil. The magnetic field produced will depend upon the current supplied. In these instruments, controlling torque is provided by the spiral springs and damping torque is provided by air friction.



Advantages of Repulsion Type Moving Iron Instruments :

  • The instruments can measure both ac and dc currents or voltages.
  • In concentric or coaxial type, the deflection up to 250° can be produced.
  • As the coil in the repulsion time instrument occupies more volume, it can allow overloaded currents for a longer duration of time, and hence it provides better cooling which eliminates the problem of overheating.
  • These are suitable for economical production in manufacturing.
  • These are the most widely used instruments.
  • The scale of these instruments is uniform (for radial vane type).
  • The measuring range of the instrument can be extended.
  • Due to stationary coil, the torque to weight ratio will be high thereby decreasing the frictional loss.

Disadvantages of Repulsion Type Moving Iron Instruments :

  • Linearization of the scale is affected in coaxial vane type because of the shape of concentric vanes.
  • Errors are caused because of hysteresis loss and also due to stray magnetic fields.
  • Draws more power.


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