5 April 2017

Class E Commutation ( External Pulse Commutation or Type E commutation )

  • The class E commutation is also called as external pulse commutation because the SCR is turned off by external source. 
  • The SCR is turned off by applying reverse voltage across it.
  • Figure A shows the external pulse commutation by using transistor.
  • The SCR T1 is turned off by applying reverse voltage through transistor Q and voltage source V. 
  • When the SCR T1 is turned off , the load current flows through path (+)Vdc – SCR T1 – RL –Vdc(-).  
  • When a base signal is applied to transistor Q, it turns on and reverse voltage applies to SCR T1 by DC voltage source V. 
  • The base signal must be greater than the maximum load current. 
  • The time for reverse voltage across SCR T1 must be greater than the turn off time of the SCR T1.

external-pulse-commutation.png
  • Figure B shows the circuit for commutation of the SCR T1 by using pulse transformer. 
  • The secondary of the pulse transformer is connected in series with the load. 
  • When the SCR T1 is turned on by gate pulse, the load current is flowing through secondary of pulse transformer and load. 
  • When a pulse is given to primary of the pulse transformer, the voltage across secondary of pulse transformer -Vdc provides reverse voltage across SCR T1 resulting SCR T1 is turned off. 
  • The voltage drop across capacitor is very low because frequency of the pulse transformer is very high. 
  • The voltage drop across capacitor is hardly 1.0 Volt. 
  • The following points to be noted while designing pulse transformer. The efficiency of external pulse commutation is very high due to low energy requires.

No comments:

Post a Comment