Class F Commutation
Figure 48 shows the class F commutation and it is voltage and current waveforms are depicted in Fig. 49. This type of commutation is also known as natural commutation or ac line commutation. This type of commutation only occurs when the input voltage is ac. When the thyristor based converter circuit is energised by an ac source voltage and the gate signal is applied to thyristor T1 during positive half-cycle of supply voltage, thyristor T1 becomes turn ON and current flows thyristor and load. At the end of each positive half-cycle of supply voltage, current passes through its natural zero and then ac source applies a reverse voltage across thyristor automatically. Consequently, thyristor T1 will be turned OFF. This commutation technique is called natural commutation as no external circuit is required to turn OFF the thyristor. Usually, this commutation method is commonly used in controlled rectifiers, line commutated inverters, ac voltage controllers and cyclo-converters.