4 March 2020

Commutation of SCR - Interview Question Answer


1
What is meaning of commutation of SCR?


Commutation
  • The process by which the SCR is turned off from turn on condition is called as commutation.


2
Describe the different methods of commutation of the SCR.


Commutation of the SCR
  • Forced commutation : Class A , Class B , Class C , Class D and Class E
  • Natural commutation : Class F


 3
Describe the main condition necessary to turn off the SCR.


The force commutation circuit turn off time must be greater than the SCR turn off time.

4
Explain : Line commutation
           OR
Explain : Natural commutation
           OR
Explain : AC commutation


Natural ( Line or AC ) commutation
  • The SCR is turned off automatically during negative cycle of the alternating supply is called as natural commutation. 
  • It is not possible when the SCR is operated on the DC supply.


4
Explain the term : Forced commutation


Forced commutation
  • When the SCR is operated on the DC supply, it is not turned off naturally. The SCR is turned off by some external circuit is called as forced commutation. 
  • This external circuit is called as commutation circuit.


5
Explain the term : Current commutation, Voltage commutation


Current commutation
  • It is a commutation process in which the SCR is turned off from turn on condition by reducing the anode current below the holding current for sufficient time.      

Voltage commutation
  • It is a commutation process in which the SCR is turned off from turn on condition by applying reverse voltage across SCR terminal for sufficient time. 
  • The voltage and current commutation is a type of forced commutation.


6
Which commutation method is known as self commutation?


Class A commutation and class B commutation

7
Which commutation method is known as resonant commutation? Why?


Resonant commutation
  • The Class A commutation is known as resonant commutation because the commutation process in this method is done by series RLC circuit.


8
Give reason : Class A and Class B commutation is known as current commutation.


Current commutation
  • The commutation in the class A and class B is done by reducing the anode current below the holding current for sufficient time therefore it is called as current commutation.


9
Explain the term : Self commutation


Self commutation
  • The SCR turned off from turned on condition after some time automatically and again it on after some time. 
  • This process is called as the self commutation. 
  • However the time interval between turn off and turn on depends upon circuit parameters.


10
Write the application of class A commutation


Series inverter, Self commutated inverter

11
Why the class A commutation is used for high frequency applications?


Class A commutation
  • The weight of the commutation component L and C becomes bulky particular for low frequency therefore the class A commutation is used only for high frequency applications.
  • The resonance angular frequency

              ω0 = 1 / √ ( LC )

12
Which commutation method is also known as complementary commutation?


Class C

13
Which commutation method is also known as voltage commutation?


Class C

14
Write the application of Class C commutation.


Mcmurray Bedford inverter

15
Why the Class C commutation is called as complementary commutation?


Complementary commutation
  • An auxialiary thyristor is connected across with the main thyristor in the Class C commutation. 
  • This auxiliary thyristor turn off the main thyristor and vice versa therefore this method is also called as complementary commutation.


16
Which type of force commutation requires minimum energy?


External pulse commutation or Class E commutation



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