04/04/2017

Class D Commutation ( Impulse or Auxiliary Commutation )

Class 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.

Power circuit diagram of class D commutation


Class d commutation, working of Class d commutation,  Class d commutation explain, Class d commutation pdf,   Class d commutation circuit diagram, Class d commutation application, Auxiliary commutation

Working of class D commutation

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 – R–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 – R–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. 

Waveform of class D commutation


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Application of class D commutation

The application of the Class D Commutation is in the Jones chopper.



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