Electrical Transformer related questions with answers #1|Stepped core|electrical transformer tappings|stacking factor

1.why cores are stepped in three phase transformer ?

Ans.

      

         

     

                     (a) Two stepped core

 

 

             

  

                   (b) Three stepped core

 

           The cores are stepped in three phase transformer because when stepped cores are used the diameter of the circumscribing circle is minimum for a given area of the core.

      This helps in reducing the length of mean turn of the winding. Hence ,it is possible to reduced requirement of copper so cost of copper will reduced and also copper loss will reduced.

 

2. Why circular coils are preferred in transformer winding over rectangular coils ?

Ans.

        During the occurring of short circuit and over loads, the excessive leakage flux will produced, it’s develop severe mechanical stresses on the coils. But in case of circular coils these forces are radial and hence there is no tendency for the coil to change it’s shape.

        But in case of rectangular coils these forces are perpendicular to the conductors and hence tends to deform the coil in circular form.

 

3.Why tappings are usually provided on high voltage side ?

Ans.

       Following reasons to provide tappings on the high voltage (HV) side:

(1) HV windings has more number of turns than LV windings therefore it is easy to make more no. of tappings and it's easy to provide tappings. 

(2)  LV winding is placed in the inside nearer to the core while the HV winding is placed on the outside. Therefore, practically it is simple to provide tappings on the high (HV) voltage winding.

 

 

(3)  Due to high current on LV side than HV side we should provide tappings on HV side because of less current so it gives less sparking on the tap changer contacts.

 

(4)  No. of turns in HV winding is more therefore a better voltage regulation is obtained, But in case of LV winding it is may not be possible.

 

 

4. Why HV coils are provided outside ?

Ans.

       HV coils are provided on outside because of the following reasons:

(1)  As compared with HV winding the amount of insulation required to isolate LV winding from core is less.Hence overall cost of insulation and therefore transformer manufacturing cost is reduces.

(2)  Normally tappings are provided on HV side so the number of turns calculated above need to be increased according to the percentage of tappings required.

(3) If we provide HV winding nearer the core the iron losses increases hence it is provided outside.

 

5. What is stacking factor ?

Ans.

       The ratio of net core section (A_i) to the gross core section (A_gi) is called stacking factor K_s.

 

K_s=Net core section/Gross core section

    =Ai/Agi