Class B Commutation ( Self or Type B Commutation )

• The commutating components L and C are connected in parallel with SCR T1 in the class B commutation. 
• When SCR T1 is turned off, the charging of capacitor is done through path(+)V_dc – (+)C –C(-) – L – R_L –V_dc(-). 
• When the SCR T1 is turned on, the load current flows through path (+)V_dc – SCR T1 –R_L – V_dc(-). 
• As soon as SCR T1 is turned on, the discharging of capacitor is done through path (+)C – SCR T1 – L–C(-).
• Therefore the load current as well as capacitor discharging current flows through SCR T1. 
• Once capacitor discharges, it will again charge with opposite polarity ( upper plate negative and lower plate positive ).
• Therefore the commutating current I_C flows in the opposite direction to that of load current I_L. 
• When the capacitor current I_C is greater than the load current, the SCR T1 is turned off.
• As the SCR T1 is turned off, the charging of capacitor again done through path (+)V_dc – (+)C – C(-) – L – R_L – V_dc(-). 
• When the voltage across capacitor becomes V_dc, the SCR T1 is again switched on. 
• The SCR T1 turned on and turned off automatically in this way. 
• The switching time of SCR T1 depends upon commutating components L and C.

  

CLASS B COMMUTATION DRAWING.PDF

Application

• The application of class B chopper is in the DC chopper. 
• The SCR T1 is turned on during T_ON time and turned off during T_OFF time automatically.
• The SCR automatically turned on and turned off in the class A and class B commutation therefore it is called as self commutation.

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