29/03/2020

Design of Transformer Interview Question Answer - 3



32
Describe the effect of number of steps on copper loss of the transformer.


Number of steps
  • As the number of steps increases, the length of mean turns of winding also reduces.
  • This will result in decrease in the cost of copper material and copper losses also.


33
Which factors to be considered while selecting number of steps for transformer core?


Advantages of number of steps
  • As the number of steps increases, the diameter of circumscribing circle reduces than the square core of same area.
  • Therefore the length of mean turns of winding reduces.
  • This will result in decrease in the cost of copper material and copper losses also.

Disadvantages of number of steps
  • As the number of steps increases, the shearing and assembly of different size lamination is required.
  • This will increase labour cost and wastage of core material.
  • Therefore there should be compromise while selecting number of steps by considering cost of conductor and labour charges.


34
Describe the effect of air gap in the joint of core section of the transformer?


Air gap at Joints
  • As the length of air gap increases in the joint of core of the transformer, the magnetizing current increases because the air has lower permeability than the iron core of the transformer.


35
How to reduce air gap between joints of the core in the transformer?


The air gap between joints of the core is reduced by making joints as tight as possible and adjacent joints are overlapped.

36
Why the air gap between lamination is less important in small rating transformer?


The length of flux path in the small rating transformer is small therefore the air gap between lamination is less important in it.

37
Describe the effect of thickness of lamination while interleaving lamination in the transformer.


Thickness of lamination
  • It is desirable that number of plates ( two, three or four ) interleaved by considering mechanical strength.
  • As the thickness of interleaving plates increases, the no load current and iron loss of the transformer increases.


38
Why the cross section of the yoke should be made larger than cores in the transformer?


Cross section of yoke
  • The cross section of the yoke should be made 15% higher than the hot rolled core of the transformer.
  • This will reduce flux density in the yoke resulting magnetizing current and iron losses also reduced.


39
What is cross fluxing in the transformer? Describe its importance.


Cross fluxing
  • The core of the transformer is made multi stepped in order to reduce length of mean turns of the winding as well as copper losses.
  • As the yoke does not consist any winding, it is made rectangle or two stepped.
  • The cross section of the yoke is made larger than the core in order to reduce magnetizing current and iron losses. 
  • This will produce cross fluxing between core and yoke and it produces higher iron losses.


40
Why the core and yoke are made of same shape in the transformer?


In order to reduce cross fluxing

41
Whether yoke stepping is done in the transformer? Why?


Shape of yoke
  • The yoke is made rectangular in shape or cruciform in order to make flux distribution between core and yoke made equal and reduce iron loss.


42
Why CRGO core is used as compared to hot rolled core in the transformer?


CRGO core
  • The hot rolled silicon steel allows maximum flux density of 1.45 Weber / meter2 whereas the cold rolled silicon steel allows maximum flux density of 1.8 Weber / meter2.
  • Therefore the weight of core and winding is less in the CRGO core as compared to hot rolled core.


43
Which are the different ways to reduce the height of three phase transformer core?


The height of the three phase transformer core is reduced by
  • Using five limb transformers instead of three limb transformer. 
  • The top and bottom yoke is made 58% whereas two returns limbs are 45% that of main limb in the five limb transformer.
  • The primary and secondary winding wound on central limb and return path is carried by surrounding four yoke section. This arrangement reduces the height of yoke up to four times.
  • Using Circular or square tank


44
What is mitred joint? Describe its importance.


Mitred Joint
  • If the stacking of core and yoke is done at right angle, the magnetic flux lines will turn at right angle to the direction of grain orientation. 
  • This will increase no load current and iron loss. 
  • The no load current and iron loss is reduced by making flux lines to flow along the direction of grain orientation.
  • The mitred joint is one in which the flux lines flows along the direction of grain orientation. 
  • It is made at 450 or 350 / 550.


45
Which type of mitred joints is used in the transformer?


Types of mitred joints
  • 450 – It reduces the waste of core material during cutting process.
  • 350/550  It improves the flow of flux around the core corner.


46
Describe the importance of splitting of the core.


Splitting of core
  • The splitting of the core improves cooling of the core.




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