Commutation of thyristor:-

             commutation of thyristor means that turn off(conduction) the thyristor by the natural commutation or forced commutation. commutation are two type 

              1. Natural commutation
              2.Forced commutation

    Natural commutation:-

          In natural commutation, thyristor turns off due to own circuit.no any external circuit is required for turn off. natural commutation also called ac line commutation or Class-F commutation. This commutation is also possible with line commutated inverters, controlled rectifiers, cyclo converters, and AC voltage regulators because the supply is the AC source in all these converters.

       Working of the circuit:-

       For Positive half cycle

                              During the positive half-cycle of the AC supply, anode voltage higher than cathode voltage so that load current flows normally.

        Negative half cycle 

                            During the negative half-cycle, the SCR will turn OFF (because of cathode voltage higher than anode voltage). so that in the negative half-cycle load current value is zero.

               

           Forced commutation are six types  

                  In the case of DC circuits, there is no natural current that will be zero so that turns OFF the SCR. In such type circuits, forward current must be forced to zero with an external circuit (known as forced Commutating Circuit) to commutate the SCR turns off. Hence this type of commutation called Forced Commutation.

                  This commutation circuit consists of components like inductor and capacitor and these electricals elements are called Commutating Components. These commutating components make to apply a reverse voltage across the SCR so that immediately SCR current becomes zero.

                  Depending on the process for achieving zero current across the SCR and this arrangement of the commutating components are Forced Commutation is classified into different six types.

• Class A Commutation: Load commutation
• Class B Commutation: Resonant-pulse commutation
• Class C Commutation: Complementary Commutation
• Class D Commutation: impulse Commutation
• Class E Commutation: External Pulse Commutation
• Class F Commutation: Line commutation

such as classes A, B, C, D, E, and F. This forced commutation is mainly used in chopper and inverter circuits.