 myself, Sunil Kalshiti, Aisil Professor, Department of Electronics Engineering, Walchand Institute of Technology, Solapur. Today, I am going to explain the voltage commutated chopper, learning outcome. At the end of the session, students can analyze voltage commutated chopper. This is the basic circuit arrangement for the voltage commutated chopper. Here, the T1 acts as a main power device. TA acts as an auxiliary resistor. L, D and C acts as a commutating component. The voltage commutated chopper is also called as a impulse commutated chopper. This commutation circuit compromises an auxiliary SCR, TA, diode D, L and C. The complete chopper circuit has outlined with dotted box. The main power switch is the SCR T1, whereas TA is the auxiliary resistor. The auxiliary resistor is used to turn off the main power device. Mode zero operation. Now, for this mode, main thyristor T1 is a non-conducting set. Apply the gate pulse to the auxiliary thyristor. So, auxiliary thyristor TA conducts and assumes that the capacitor C is in a fully discharging state. Therefore, capacitor C starts charges and the charging path is vs, c, ta, load, back to vs. and capacitor C starts charges. As long as C charges, the current flows through the circuitry and auxiliary thyristor TA remains in a conducting state. According to the property of capacitor, capacitor C charges up to its peak input voltage. At the end of this mode, capacitor C gets fully charged, so the current becomes zero and auxiliary thyristor TA turns off. So, at the end of this mode, capacitor C gets fully charged up to vs. Mode on operation. For this mode, T is greater than zero, less than T1. The main SCR T1 is triggered at T is equal to zero. The source current flows in the two paths. Load current I0 consults one path and the commutation current Ic in the other path. With the triggering of SCR T1, the load gets connected to the supply and the load voltage vs is equal to EDC. The T carries the load current as well as the discharging current. And here in this duration, thyristor TA remains in non-conducting state and capacitor C starts discharges and the discharging path is the CTLD. The load current I0 flows through the path EDC T1 load EDC. Whereas, the commutating current Ic flows through the path CT1LDC. The capacitor current first rises from zero to maximum value. When the voltage across C is zero, at T is equal to T1 by 2. At T is equal to T by 2. As Ic decreases to zero, capacitor is charged to reverse voltage minus EDC at T is equal to T1 as shown in the figure. This reverses the voltage on capacitor is held constant by diode D. Mode 2 operation. In this mode, thyristor T remains in continuously conducting state and thyristor TA remains non-conducting state. Now in this mode, capacitor will not discharge because capacitor now before the end of this mode, capacitor C gets charged and the polarity of capacitor lower plate is positive and upper plate is negative. Now capacitor try to discharge in this direction, but because of the diode D. Because of diode D, because of this diode D, it will the capacitor will not discharge. It holds its charge, therefore capacitor will not discharge. During this mode, the condition existing at T is equal to T1 is continued. Only T1 is conducting and the load voltage is maintained equal to EDC. This mode ends at T is equal to T2 when auxiliary SCR TA is fired. So in this mode, the thyristor T carries the load current only. Mode 3. T is greater than T2 less than T3. Before end of this mode, capacitor C gets fully charged. Now this time to turn off the main power device, apply the gate pulse to the auxiliary thyristor TA. So the previously charged capacitor gets discharged and the discharging path is VSC TA load and capacitor starts discharges. And this reverse voltage is appears across the main power device. So because of this reverse voltage, the main power device T turns off and all the current flows in this direction. VSC TA load and capacitor draws the heavy current at the starting. Therefore the very less current flows through the T and this current is less than holding value. That's why thyristor T turns off and the sufficient reverse voltage is appears across the T. Therefore thyristor T turns off. Now capacitor C starts discharges and the discharging path is VSC TA load. Once the capacitor C gets fully discharged, then again start charges with upper plate is positive and lower plate is negative. As long as C charges, thyristor TA remains in conducting state. Once the capacitor C gets fully charged, then TA turns off. So at the end of this mode, thyristor TA turns off. And during this mode, the load voltage is the sum of supply voltage and the reverse capacitor voltage. More for T is greater than T3 and less than T4. After T is equal to T3, the freewheeling diode starts conducting and load current decays. During this energy transfer mode, capacitor C charges beyond the supply voltage. At the end of this mode, capacitor attains peak voltage. When IC falls to zero, TA is turned off. Why? Capacitor is overcharged in the mode 4. In the mode 4, because of this inductor, because of inductive load and because of inductive source, capacitor is slightly overcharged. Because the property of capacitor, capacitor is always charges up to its peak input value. Because of inductive source, inductive load and inductor real, the capacitor is overcharged. Mode 5. T is greater than T4, less than T5. During this mode, IL continuously decays through FD, freewheeling diode. The overcharged capacitor forces current through the VS, freewheeling diode LD. Due to this, capacitor C gets discharged to the voltage close to VS. So, this is the discharging path, freewheeling diode LDC and due to this, C gets discharged to the voltage close to VS. Mode 6. T is greater than T5, less than T6. During this mode, the load current continuously decays through the freewheeling diode. This mode ends when the main thyristor is referred at the beginning of next cycle at T is equal to T5. These are the waveforms of the voltage commutated chopper. Now, these are the waveforms for gate pulse, main T1, gate pulse for auxiliary thyristor. These are the waveforms for the load current. These are the waveforms for charging and discharging current. These are the waveforms for IT1. These are the waveforms for freewheeling diode. These are the waveforms for voltage across capacitor. These are the waveforms for voltage across auxiliary thyristor. These are the waveform across auxiliary thyristor and these are the waveform across the load voltage. Why this chopper is termed as a voltage commutated chopper? In this chopper, the main thyristor is turned off by applying the charged capacitor across the main conducting device. Here in this mode, the charged capacitor is placed across T means indirectly the reverse voltage appears across the T and the thyristor T turns off. That's why the name is the voltage commutated chopper. Why voltage commutated chopper operates only on load? If load is not connected, then capacitor will not charge, so capacitor will not get sufficient energy to turn off the main power device. That's why this chopper operates only on load. These are references. Thank you.