Moving Iron Instruments

        M.I instruments are mainly used for the measurement of alternating currents and voltages, though it can also be used for d.c measurements.

       The general principle of a M.I instrument can be explained under;

           Let a plate or vane of soft iron or of high permeability steel forms the moving element of the system. The iron vane is situated so as, it can move in a magnetic field produced by a stationary coil. The coil is excited by the current or voltage under measurement. When the coil is excited, it becomes an electromagnet and the iron vane moves in such a way so as to increase the flux of the electromagnet. Thus, the vane tries to occupy a position of minimum reluctance. Thus, the force produced is always in such a direction so as to increase the inductance of the coil.

       There are two types of Moving- iron instruments.

                      i. Attraction type:-

             In this type of instrument, a single soft iron vane (moving iron) is mounted on the spindle, and is attracted towards the coil when operating current flows through it.

Deflecting torque equation

          The force F, pulling the soft -iron piece towards the coil is directly proportional to;

a) Field strength H, produced by the coil.

b)pole strength ‘m’ developed in the iron piece.

F α mH

Since, m α H,

F α H²

Instantaneous deflecting torque  α H²

Also, the field strength H = μi

If the permeability(μ) of the iron is assumed constant,

Then,   H α i

Where, i® instantaneous coil current, Ampere

Instantaneous deflecting torque α i²

Average deflecting torque, T_d α mean of i² over a cycle.

Since the instrument is spring controlled,

                                             T_c α θ

In the steady position of deflection, T_d = T_c

                                             θ α mean of i² over a cycle

                                                α I²

                                                

   Since the deflection is proportional to the square of coil current, the scale of such instruments is non-uniform (being crowded in the beginning and spread out near the finishing end of the scale). 

                    ii.Repulsion type:-

                 In this two soft iron vanes are used; one fixed and attached the stationary coil, while the other is movable (moving iron), and mounted on the spindle of the instrument. When operating current flows through the coil, the two vanes are magnetised, developing similar polarity at the same ends. Consequently, repulsion takes place between the vanes and the movable vane causes the pointer to move over the scale.

    Two types

                                         i.radial vane type: - vanes are radial strips of iron.

                                       ii.co-axial vane type:-vanes are sections of coaxial cylinders.

Deflecting torque:-

        The deflecting torque results due to repulsion between the similarly charged soft- iron pieces or vanes. If the two pieces develop pole strength of m₁ and m₂ respectively, then;

                       Instantaneous deflecting torque α m₁m₂ α H²

If the permeability of iron is assumed constant, then;  H α i,  where, i is the coil current

                          Instantaneous deflecting torque α i²

 Average deflecting torque, T_d α mean of i² over a cycle.

  Since the instrument is spring controlled, T_c α θ

 In the steady position of deflection, T_d = T_c

                                                          θ α mean of i² over a cycle.

                                                             α  I²

                                                             

     Thus, the deflection is proportional to the square of the coil current.  The scale of the instrument is non- uniform; being crowded in the beginning and spread out near the finish end of the scale. However, the non- linearity of the scale can be corrected to some extent by the accurate shaping and positioning of the iron vanes in relation to the operating coil.

………………………………………………………………………………………………….. Reference:-

(i)                 A.K Sawhney, Electrical and Electronic Instrumentation and Measurements, page no:- 315-317

(ii)               V.K Mehta, Rohit Mehta, Basic Electrical Engineering, page no:- 790-792

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