Explain moving iron instrument in detail ?

PRINCIPLE :- 

                             When iron pie is placed near the magnet it is attracted toward the magnet. The force of attraction depends on the strength of magnetic field.

           This type  of instrument is a used for measuring voltage or current.

Moving iron instrument are two types :-

 (1) ATTRACTION type MI. 

 (2) REPULSION type MI.

 

1. ATTRACTION type MI :- 

FIG:  Block diagram of Attraction type MI 

The figure shaws contructional detail of an attraction type moving iron instrument . The moving iron is a flate disc of a sector ecentrically mounted.

             The coil is also flate and has the narrow  slote like opening when the current flows through the coil, magnetic field is  produced and the moving iron moves from outside the coil to the stronger field inside the coil. 

The controlling torque is provided by spring.

Damping is provid by air friction with the  help of lite alluminium piston

2 REPULSION type :- 

FIG: block diagram of repulsion type MI 

In the repulsion type there are two vane or plate inside the coil one is fixed and another is movable . This are similarly magnetised when the current through the coil and there is the force of repulsion blue the vans resulting in the movement of moving vane. 

→ Two designs are possible in repulsion type MI.

1) Radical type 

2) Co-exiol type 

1) RADICAL type:- 

                                 In this type the vanes are radical streps of iron this trips are placed within the coil fixed vane to atteched to the coil and movable one is connected to the spindal. 

2) Co-exical type:-

In this type of instrument the fixed and moving vane are section of Co-exical cylender. The controlling torque is provided by spring gravity control can also be used in vertical movement instrument . The damping torque is produced by air , friction , as in attraction type instruments we can used both AC and DC the moving iron insturment are unpolorised instrument that is they are independent of the direction in which the current passes.

→ Derivation of moving iron instrument:- 

     Torque eq ท

   Small increase in curerent small deflection & same mechanical work will be done.

  TD→ Deflecting torque.

Mechanical → Torque x angular 

               deflection.

                       TD x DQ 一(1)

★ Due to change inductance , there is a change in energy strored in the magnetic field. current "I" is increased.

The small changed in current and this inductance is change from L+ dl. 

              

          Initially stored energy =1/2 LI ²

  Change in stored energy 

               = [ 1/2 (L+dl) (L+dI)² ]  -1/2  LI²

                

            = 1/2 (L+dl) (I² + dI² +2IdI)  -1/2 LI²

            

             Negatcing higher order terms.

            

           Finally stored → IdI + 1/2 I² dL 一 (2)

★ To increase the current by dI ,applyed voltage must be increase (V ∝ I) 

    Voltage e = d∅/  dt - d/ dt  (LI) ⇒ IdL/dt + ldI/ dt 

                                                           Total electrical energy

= eIdt 

= put evalue

= I [ IdL /dt + ldI / dt ]

= I²dl + ILdI 一一(3) 

Low of conservation of energy electrical = increase in machanical energy supplied stored + work done energy.

          

           Added all eqท (1) + (2) + (3)

           I²dL + ILdL = IldI + 1/2 I²dL+ IdL

         Id = 1/2 I²dl/d∅

Spring produces controlling torque = Id

            

                   TC ∝ Q 

                 TC = KQ         

              eqท TC  &  Td                          Deflecting torque

                  1/2 I²dl /dQ = KQ                           equal

                       Q ∝ I²                             controlling torque