MOVING IRON INSTRUMENTS

Moving iron instruments are most common for laboratory or switch-board  and used at power frequency. these instruments can used both ac and dc.

a plate or vane of soft iron or high permeability steel uses the moving element of the system. this iron vane situated in magnetic field produced by stationary coil. this vane is made  by  nickel-iron  alloy   ( nickel 80% & iron 20% ). 

1. these instruments can be used dc as well as ac circuit. 
2. it has non uniform scale


Torque Equation of moving iron instrument: 

Torque equation of moving iron instruments can be draw by energy relation.
                                                                          

where

            Td = Deflection torque

              0 =  deflection and

             L =  coil inductance

Controlling Torque Tc is provided by controlling  spring. Deflection torque is balanced by Controlling torque Tc = K 0 

Where  K = Control Spring constant  Nm/rad.

                                     Tc  =  K 0
              Where             K  =   spring constant 
                               
                     .
Classification of Moving iron instruments:

1. Attraction type
2. Repulsion type       

1. Attraction type:

Show the constructions detail of an attraction type moving iron instrument. 

• the coil  is flat and narrow slot like opening.
• the moving iron is a flat disc and made by nickel iron alloy 
• 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 provide by springs hut gravity control can be used for panel type of instruments which are vertically mounted.
• Damping is provided by air friction with the help of a light aluminium piston (attached to the moving system) which move 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.

2. Repulsion type:

In the repulsion type, There are two vane inside the coil one is fixed and other is movable. when current is flows through fixed coil  then magnetic field is produced. there is a force of repulsion between two vanes resulting in the movement in moving vane.

Two different designs are common used of vanes.

1. Redial Vane Type
2. Co-axial Vane Type

1. Redial Vane Type:

show image where  the vanes are radial  strip iron. Movable is attached to the spindle and fix vane is  placed in coil.

2. Co-axial Vane Type

Show the image where both vanes are section of co-axial cylinders.

Error in Moving iron instruments:  

There are two types error. one is occur in both ac and dc and other is occur only ac

Errors with Both dc and ac.

1. Hysteresis error : this error occurs as the value of  flux density is different of same current for ascending and descending values.the flux density is  higher for descending value there for instruments read higher for descending value current this error can be minimize using small iron parts and other method is used nickel iron alloy 
2. Temperature error: the effect of temperature change on moving iron instruments aries chiefly from the temperature coefficient of spring.for minimize the error the series resistance should be made of material like Manganin which has small temperature coefficient. the value of resistance should large as compare with coil resistance.in order to reduce the self heating.  
3. Stray Magnetic fields: It is a also called demagnetization fields. this is weak at full scale deflection hence it can easily distorted . these error can be minimized using an iron case or iron shied over working parts  

Errors with A.C. only:

1. Frequency error: Change in frequency is also cause of change in reactance of working coil and also change the eddy currents setup in the metal parts of instrument.
2. Reactance of Instruments coil: the change of reactance of the instrument coil is importance in case of voltmeter. where a addition resistance put in series with instrument coil to reduce this effect. 

Advantages and Disadvantages Moving iron instruments:

Advantages: 

1. Can be used both in D.C. as well as in A.C. circuits.
2. Robust and simple in construction.
3. Possess high operating torque.
4. Can withstand overload momentarily.
5. Since the stationary parts and the moving parts of the instrument are simple so they are cheapest.
6. Suitable for low frequency and high power circuits.
7. Capable of giving an accuracy within limits of both precision and industrial grades.

Disadvantages:

1. Scales not uniform.
2. For low voltage range the power consumption is higher.
3. The errors are caused due to hysteresis in the iron of the operating system and due to stray magnetic field.
4. In case of A.C. measurements, change in frequency causes serious error.
5. With the increase in temperature the stiffness of the spring decreases.