State True/False for the following statements with reference to commutation of SCRs:

1. Class D commutation is a complementary commutation.

2. Class F commutation is a natural commutation.

Commutation

Commutation refers to the process of turning off a conducting thyristor. There are six major types of commutation methods, classified into two categories:

Category 1: Forced Commutation

• It is used in DC circuits where the current does not naturally reach zero. It requires external components (capacitors, inductors) to force the current to zero.
• Forced commutation is classified into five commutation techniques.
   

Class A (Resonant Commutation)

• Uses an LC circuit to create oscillations that naturally bring the current to zero, turning off the thyristor.
• Completely independent of external circuit components.

Class B (Resonant-Pulse Commutation)

• The thyristor is turned off using an LC resonant circuit discharging when the current reaches zero.

Both Class A and Class B rely on the circuit's natural behavior to turn off the thyristor, making them self-commutation methods.

Class C: Complementary Commutation

• Uses another thyristor (auxiliary SCR) and a capacitor to turn off the main thyristor.
• Used in DC choppers and inverters.

Class D: Auxiliary Commutation

• Similar to Class C but uses an external voltage source to force commutation. Used in high-power inverters and choppers.​

Class E: External Pulse Commutation

• Uses an external commutation pulse from a circuit to turn off the thyristor.
• Used in Cycloconverters and high-power circuits.

Category 2: Class F (Natural Commutation)

• It occurs in AC circuits where the current naturally becomes zero. In AC applications, the supply voltage reverses polarity every half-cycle, automatically bringing the current to zero.
• Example: Line commutation in rectifiers.