4 April 2017

Class D Commutation ( Impulse Commutation or Auxiliary Commutation or Type D Commutation )

  • The class D commutation is also called as impulse commutation. 
  • The SCR T1 is main SCR whereas SCR T2 is auxiliary SCR. 
  • The auxiliary SCR T2 is used to turn off main SCR T1 therefore this method is also called as auxiliary commutation.
Working

Stage 1

  • The SCR T1 and SCR T2 are in off condition during initial condition and voltage across capacitor is zero.
          SCR T1 – OFF
          SCR T2 – OFF

Stage 2

  • The SCR T2 is turned on in order to charge capacitor C. 
  • The charging of capacitor is done through path (+)Vdc – (+) C – C (-) – SCR T2 – RL –Vdc(-). 
  • As the voltage across capacitor increases, the current through SCR T2 decreases. 
  • The SCR T2 is turned off when the voltage across capacitor reaches Vdc and charging current reduces below holding current.
SCR T1 – OFF
SCR T2 – OFF
Voltage across capacitor = Vdc

Stage 3

  • When SCR T1 is turned on, the load current flows through path (+)Vdc – SCR T1 – RL –Vdc(-). 
  • The commutating current at the same time flows through path (+) C – SCR T1 – L – D – C (-). 
  • Once the capacitor discharges, it will start to charge in the reverse direction. ( It means that its upper plate becomes negative and lower plate becomes positive). 
  • The capacitor does not discharge due to diode D after charging in the reverse direction.
SCR T1 – ON
SCR T2 – OFF
Voltage across capacitor =  - Vdc

Stage 4

  • The discharging of capacitor is done through path (+) C – SCR T2 – SCR T1 – C (-) when SCR T2 is turned on. 
  • The SCR T1 is turned off when the discharging current of capacitor increases beyond load current.
SCR T1 – OFF
SCR T2 – ON

Stage 5

  • The capacitor C once again start to charge through path (+)Vdc –  (+) C – C (-) – SCR T2 – RL – Vdc (-). 
  • The whole cycle repeats in this way. The efficiency of Class D commutation is high because the commutation energy transfers in the load. 
  • The application of the Class D Commutation is in the Jones chopper.


class-d-commutation.png
class-d-commutation-waveforms.png






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