Voltage Commutated Chopper Circuit:

This Voltage Commutated Chopper Circuit comprises an auxiliary thyristor Th₂, a diode D, inductor L and capacitor C as shown in Fig. 11.32 wherein the total chopper circuitry has been outlined with a dotted box.

To start the Voltage Commutated Chopper Circuit, capacitor C is initially charged by firing the thyristor Th₂; charging is now from the source via C, Th₂ and load with the charging thyristor turning off as the charging current decays to zero.

Voltage and current waveforms of a voltage Commutated Chopper are shown in Fig. 11.33. The main thyristor Th₁ is triggered at t₀. Current flows in two paths—load current i_a through Th₁ and commutation current i_C through C, Th₁, L and D. The commutation current i_C is oscillatory of frequency determined by the LC series circuit. At the end of the first pulse of i_C, C is charged negatively and the diode D blocks-off further flow of current.

To turn-off the main thyristor Th₁, the auxiliary thyristor Th₂ is triggered at t₂. This causes the fully-charged capacitor C to reverse-bias Th₁ and causes it to turn off. The capacitor C itself gets charged positively from the source through Th₂ and load; Th₂ itself turns off at t₃, the end of the charging process when the charging current decays to zero, i.e. it is load-commutated. The load current now free­wheels through D_FW. At t = T, the main thyristor is again triggered and the cycle repeats.

This simple chopper was extensively used at one time. It suffers from the fact that the capacitor has to be initially charged to begin the operation. Further, the voltage jumps to twice the value as soon as commutation is initiated by triggering Th₂.