Types of Transformer and Comparison between Core and Shell type,E.M.F Equation of a Transformer

As-Salaam-Alaikum, Welcome to my blogger "World of Knowledge".Here we discuss some important topics of a Transformer.

Points to be discuss in this Article are given below :

• Types of Transformer
• Comparison between Core and Shell Type
• E.M.F Equation of a Transformer

Types of Transformer:

There are three main types of transformer.

1. Core Type Transformer
2. Shell Type Transformer
3. Berry Type Transformer

1. Core Type Transformer:

It has a single magnetic circuit.The core is rectangular with two limbs.The winding encircles the core.The core used are of cylindrical type and core is made up of large number of thin lamination.As we that that,the coils are wound in helical layers with type of insulated from each other by paper or mica.Both coils are placed on both limb.The low voltage coil is placed inside near the core and the high voltage coil surrounds the low voltage coil.The winding are uniformly distributed over the two limbs,the natural cooling is more effective.The coil can easily remove by removing the laminations of the top yoke for maintenance.

Figure 1

2. Shell Type Transformer:

It has a double magnetic circuit. The core has three limbs.Both the winding are placed on the central limb.The core encircle both part of winding.The coil used are generally multilayer disc type or sandwich coils.As we that that each high voltage coils is in between two low voltage coils and low voltage coils are nearest to top and bottom of yoke.

Figure 2

3. Berry Type Transformer:

This has distributed magnetic circuit.The number of independent magnetic circuits are more than 2.Its core construction is like spokes of a wheel. Otherwise it is symmetrical to that of shell type. 

Figure 3

These type of transformer are generally kept in tightly fitted sheet metal tanks. The tanks are constructed of specific high quality steel plate cut formed and welded into the rigid structures. All the joints are painted with a solution of light blue chalk which turns dark in the presence of oil, disclosing even the minutest leaks. The tanks are filled with the special insulating oil. The entire transformer assembly is immersed in the oil. The oil served two functions.

• keep the coils cool by circulation
• Provides the transformers an additional insulation

Comparison of Core and Shell Type:

Core Type The winding encircle the core. It has single magnetic circuit. The core has two limbs. The cylindrical coils are used. Perferred for low voltage transformer.

Shell Type The core encircle most part of the winding. It has double magnetic circuit The core has three limbs. The multilayer disc or sandwich type coil are used. Perferred for high voltage transformer.

Figure 4

EMF Equation of a Transformer:

When a sinusoidal voltage is applied to the primary winding of a transformer, alternating flux ϕ_m sets up in the iron core of the transformer. This sinusoidal flux links with both primary and secondary winding. The function of flux is a sine function.The rate of change of flux with respect to time is derived mathematically.

Derivation:

                                           ϕ_m = maximum value of flux in Weber

                       f   = supply frequency in Hz

·                           N1= number of turns in the primary winding

·                           N2 = number of turns in the secondary winding

                            ϕ = flux

                            E1 = R.M.S value of primary induced e.m.f

                            E2 = R.M.S value of secondary induced e.m.f 

      

Figure 5

    From the Faraday law of electromagnetic  induction,

                
                Average e.m.f per turn = average rate of change of flux 

                
                Average e.m.f per turn =  d ϕ / dt

                  
                  d ϕ / dt = Change in flux / Time required for change in flux

      Consider the 1/4th cycle of the flux .Complete cycle gets completed in 1/f seconds hence for 1/4th time,the changing in flux is from 0 to ϕ_m

                                                   d ϕ / dt =  ϕ_m - 0 / (1/4f)                                             

                                                              
                                                         = 4 f  ϕ_m Wb/sec

                           Average e.m.f per turn = 4 f ϕ_{m   volts}                                         

   
      _{As flux is sinusoidal, the induced e.m.f in each turn of both windings is also sinusoidal in nature.For sinusoidal quality,}

                  _{Form Factor = R.M.S value / Average value = 1.11}

                             R.M.S value = 1.11 *  Average value                                   

                   R.M.S valued of induced e.m.f per turn = 1.11 *4 f  ϕ_m  =  4.44 f  ϕ_m   

                   
     There are N1 number of primary turns hence R.M.S value of induced emf of primary denoted as E1 is,

                   E1 = N1 * 4.44 f ϕ_{m  volts}                                                                 
      
     There are N2 number of primary turns hence R.M.S value of induced emf of primary denoted as E2 is, 

                          E2 = N2 * 4.44 f ϕ_{m  volts}                                                               

      

    The expression E1 and E2 are called e.m.f equation of transformer.

                        

                              E1 = 4.44 f ϕ_{m N1 volts}    
                                                                   

                           E2 = 4.44 f ϕ_{m N2 volts}