2 April 2017

Class C Commutation ( Voltage Commutation or Complementary Commutation or Type C Commutation )


  • The class C commutation is also called as voltage commutation because the SCR is turned off by applying reverse voltage across it.
  • The SCR T1 is main SCR which is connected in series with load. 
  • The commutating capacitor C is connected between anode terminal of SCR T1 and SCR T2. 
  • The SCR T2 is called as complementary SCR and it is connected in parallel with main SCR T1.

class-c-commutation.jpg

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Working 

SCR T1 and SCR T2 OFF

  • The SCR T1 and SCR T2 are in off condition during initial condition.
          SCR T1 – OFF
          SCR T2 – OFF

SCR T1 - ON and SCR T2 - OFF

  • When SCR T1 is turned on by gate pulse, the charging of capacitor is done through (+)Vdc– R – (+) C – C (-) – SCR T1 – Vdc (-). 
  • The charging of the capacitor is done up to Vdc voltage.  As soon as the SCR T1 is turned on, the load current passes through path(+)Vdc – RL – SCR T1 – Vdc(-).
        SCR T1 – ON
        SCR T2 – OFF

SCR T1 - OFF and SCR T2 - ON

  • When SCR T2 is turned on by gate pulse, the SCR T1 gets reverse voltage through capacitor C therefor the SCR T1 is turned off. 
  • The charging of capacitor is done through load resistance by path (+)Vdc – RL – (+) C – C (-) – SCR T2 – Vdc (-). The charging of capacitor is done in the reverse direction.
        SCR T1 – OFF
        SCR T2 – ON

SCR T1 - ON and SCR T2 - OFF

  • The SCR T1 is again turned on by gate pulse. As soon as the SCR T1 is turned on, the SCR T2 gets reverse voltage through capacitor and turned it off.  
  • The capacitor again charge through path (+)Vdc – R – (+) C – C (-) – SCR T1 – Vdc (-).
        SCR T1 – ON
        SCR T2 – OFF
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