12/03/2020

Transformer Interview Question Answer - 1


1
At what principle the transformer operates?


Principle of transformer
  • The transformer works on the principle of mutual induction between two or more than two coils which are linked by common magnetic flux.


2
What is main function of the core in the transformer?


Function of transformer core
  • To reduce reluctance of the magnetic path
  • To support the winding of the transformer


3
Why laminated core is used in the transformer?


The laminated core is used in the transformer in order to reduce eddy current loss.

4
Describe the difference between core type and shell type transformer.


Core type transformer
  • The core is surrounded by the windings.
  • Two windings are placed on the separate limbs but actual they are interleaved to reduce leakage flux.
  • Flux consists of single path.
  • Generally ‘L’ type laminations are used.
  • Coil may be circular/oval/rectangular are used in small transformers because they provide better mechanical strength.
  • Low voltage, high current application
  • It provides better cooling for windings than shell type construction.

Shell type transformer
  • The windings are surrounded by the core.
  • Two windings are placed on the central limbs therefore the width of the central limbs is twice that of outer limbs.
  • Flux divides into two paths.
  • Generally ‘E’ and ‘I’ type laminations are used.
  • Coil may be from wound or multilayer disc type.
  • High voltage, low current application
  • It provides better cooling for cores rather than windings


5
Why core stepped is advantageous over square core in the transformer?


Stepped core      
  • The small rectangular cores are used in the small rating transformer but for large rating transformer it is wasteful to use rectangle coils and therefore the cylindrical coils are used.
  • Due to two stepped or cruciform cores, the diameter of the circumscribing circle becomes smaller than that of square core of the same area. 
  • This will result in length of mean turn of copper is reduced and the consequent copper loss is also reduced.


6
Which type of winding is used in the shell type transformer?


Multi layer disc type or sandwich coil type

7
Define: Stacking factor


Stacking factor
  • It is defined as the net cross sectional area of the core to that gross cross sectional area of the core.

      Stacking factor = Net area of the core / Gross area of the core
  • The net area of the core is always less than the gross area of the core due to lamination and insulation of the core therefore the value of the stacking factor always less than unity.


8
Why transformer winding must be rigidly braced with core?


Winding braced with core
  • When a current is passing through the winding, force of attraction is produced at the center of the core due to opposite direction of the current. 
  • If the winding is not braced rigidly, there is possibility of movement of winding and insulation damage.


9
Which are the losses occurs in the ideal transformer?


Ideal transformer
  • There are no losses occurs in the ideal transformer because the windings do not consists of ohmic resistance.


10
Why the direction of the flux and magnetizing current is same?


Direction of flux and magnetizing current
  • When a rated supply voltage is given to the primary winding, the magnetizing current flows through the winding.
  • This current produces flux which is proportional to the current. Therefore the direction of flux and magnetizing current is same.


11
Describe the meaning of ‘ back emf ’ in the transformer.


Back emf in the transformer
  • When supply voltage is given to the transformer primary winding, self induced emf is produced in it.
  • This emf is known as back emf of the transformer primary winding.


12
Explain ‘ the volts / turns ratio is same for the transformer’.


EMF equation of the Transformer 
E  = 4.44fN1Фm
Primary induced emf E1  
4.44fN1Фm
Secondary induced emf E2 
4.44fN2Фm
Therefore
E1 / N1 = E2 / N2 = 4.44f Фm= constant
  • We can say that the volt per turn is constant for the transformer winding.               


13
What is direction of magnetizing current in the ideal transformer when the secondary winding is open circuited?


The winding coil is purely inductive in the ideal transformer therefore the magnetizing current always 900 lagging with respect to supply voltage. 
           
14
What is meaning of the self induced emf and mutual induced emf in the transformer?


Self induced emf and Mutual induced emf
  • The induced emf in the transformer primary winding is called as self induced emf whereas the induced emf in the secondary winding is called as mutual induced emf.


15
Explain the term : Voltage transformation ratio and turns ratio


Voltage transformation ratio
  • It is the ratio of the secondary voltage to the primary voltage.

             K = V2 / V1
Turns ratio 
  • It is defined as the ratio of the primary turns to the secondary turns.


16
Give reason : The no load current Io is not 900 with respect to supply voltage


The no load current Io is not 900 with respect to supply voltage due to inductance of the winding.

17
Describe the function of magnetizing current and working iron loss component in the transformer.


Function of magnetizing current
  • The function of the magnetizing current is to magnetize the core of the transformer.

Function of iron loss component
  • The function of the working component of the no load current is to supply no load losses in the transformer.


18
Why the no load current is very small percentage ( 1 to 2% ) to that of full load current in the transformer?


No load current
  • The core of the transformer is made of silicon steel having low reluctance path therefore the no load current is only small percentage of full load current.


19
Why magnetizing current is not represented by vector quantity?


The permeability of the core in the transformer varies with the exciting current therefore the waveform of the magnetizing current is not purely sinusoidal resulting it is not presented by vector quantity.

20
Why copper loss is neglected during no load test in the transformer?


No load test
  • As the no load current is very small, the no load copper loss is negligible therefore the copper loss is neglected during no load test in the transformer.




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