Moving Iron Instruments

MOVING IRON INSTRUMENTS

The Moving Iron instruments are of two types one is attracted type and another is repulsive type. Among these two types there is a common basic working principle. These instruments exist with soft iron which is connected to the pointer with the help of the spindle. The below figure shows the topic which we are discussing in this session.

For every instrument the mechanism will learn by three ways

I) Principle

ii) Construction

iii) Working

1. Attracted type:

    
  This is one of the type of moving iron instruments, which works on electromagnetism phenomenon.

Principle:

 These instruments works on the principle of electromagnetism that when a soft iron is placed in the magnetic field of a coil, then the iron is attracted towards coil.

*The supply current set up a magnetic field which attracts iron piece, this iron piece is attached to the pointer which shows readings.

Construction:

The construction of attracted type instruments consists of a cylindrical coil or solenoid, this cylindrical coil is provided with spindle. This spindle is attached to the pointer which deflects easily over the scale. The produced different torques is controlled by spring control (Controlling torque) and by Air friction (Damping torque).

Working:

If the supply is fed to the instrument the current produces the magnetic field in a coil based on Ampere`s law. Due to the attraction power of magnets the soft iron piece attracts towards the coil. On continuing this process a twisting force (Torque) is produced which moves the pointer attached to the soft iron.

*The movement of pointer is controlled by controlling torque with the help of spring & damping torque is minimized with air friction.

*If the direction of current in the coil reversed, the magnetic field also reverses. In this case also the magnetism produced in the soft iron. Hence the direction of deflecting torque remains unchanged.

*These Instruments are applicable for both DC and AC.

2. Repulsion type:

     
   This is one of the type of moving iron instruments, which works on electromagnetism phenomenon.

Principle:

 These Instruments works on the principle of repulsion between two similar iron pieces. This also works on the phenomenon of electromagnetism.

Construction:

 The construction of Repulsion type instrument consists of cylindrical coil or solenoid. Inside the coil there are two iron pieces one is fixed with the coil and another is movable attached to pointer.

*The pointer moves over the scale on the phenomenon of deflecting torque. The different types of torque`s are controlled by using spring control and air friction.

Working:

  
When the Instrument is fed with the supply the coil energized and produces magnetic field. The magnetic field magnetizes both the iron pieces in the same direction i.e. both iron pieces are having same polarity (such as North-North or South-South). Hence there exists a repulsive force between fixed and movable magnets. Due to repulsive force the pointer moves over the scale.

Advantages:

1. These are robust, cheap in cost and simple in construction

2. High operating torque

3. Used for DC and AC circuits

4. It gives result accurate.

Disadvantages:

1. The scale is Non-uniform

2. Less sensitive

3. High power consumption

4. In AC measurement errors are occurred due to frequency   fluctuations. 

       

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