 Hello, myself, Sunil Kalshati, Aseem Professor, Department of Electronics Engineering, Walchand Institute of Technology, Swalapur. Today, I am going to discuss the load commutated and single SCR chopper, Learning.com. At the end of this session, students can analyze load commutated chopper and single SCR chopper. This is the circuit diagram of load commutated chopper. The 4 thyristor T1 to T4 are operated in two pairs, T1, T2 and T3, T4. When one pair is on, other is off. The load current is alternately carried by these two pairs. Only one capacitor is used as a commutating component. The load is involved in the process of commutation. Prior to firing of thyristor T1 and T2, at T is equal to 0, other pair T3, T4 was in conduction. Hence, C has been charged to the supply voltage Vs. Now, before mode 0, assume that the gate pulses are applied to the T3 and T4. Therefore, the current flows through the Vs, T4, C, T3 load and capacitor C starts charges. As long as C charges, T3, T4 remains in the conducting set and capacitor C gets fully charged up to Vs. So, at the end of this mode, the load plate becomes positive and upper plate becomes negative and capacitor C gets fully charged up to Vs. Now, mode 1, T is greater than 0, less than T1. With charged two Vs with the lower plate positive at T is equal to 0, T1, T2 are fired. Once T1, T2 turned on, the load current flows through the Vs, T1, C, T2 load. So, first, capacitor C starts discharges. So, at T is equal to 0, the Vl suddenly charges to Vs plus Vc is equal to 2Vs from 0. So, at T is equal to 0, so load voltage charges from 0 to 2Vs. After T is equal to 0, C discharges linearly to 0 and charges linearly with opposite polarity. So, at T is equal to T1, Vc becomes Vs. During this change in capacitor voltage, Vl falls from 2Vs to 0, at T is equal to T1, and Vl is equal to 0. At T is equal to T1, T3, T4 becomes forward biased by the capacitor voltage Vc is equal to Vs. That is, Vt3 is equal to Vt4 is equal to minus Vs, mode 2. T is greater than T1, less than T2. In this duration, all the 4 thyristor remains in non-conducting state. So, at the beginning of this mode, C is slightly overcharged, the load voltage becomes slightly negative, and the load current shifted from T1, T2 to freewheeling diode. During this mode, Vc is equal to Vs, so voltage across T3 is equal to voltage across T4 is equal to Vs, mode 3. T is greater than T2, less than T3. In this mode, T3, T4 are forward biased by capacitor voltage, Vc is equal to Vs. So, T3, T4 are triggered at T is equal to T2. Once T3, T4 turns on, the load voltage jumps from 0 to 2 Vs, T1, T2 becomes reverse biased by capacitor voltage, and the load current flows through Vs, T3, C, T4, load Vs. In this duration, the direction of load current is from Vs, T4, C, T3, load. So, capacitor discharges, then again charges with opposite polarity, lower plate is positive and upper plate is negative. So, C discharges linearly to 0 and charges linearly with opposite polarity. During this change in capacitor voltage, Vl falls from 2 Vs to 0, so at T is equal to T3, Vl is equal to 0, and the reverse bias is maintained across T1 and T2. At T is equal to T3, capacitor is slightly overcharged and load voltage becomes slightly negative. So, freewheeling diode gets forward biased and then conducts, and the load current shifted from T3, T4 to freewheeling diode. The next cycle begins at T is equal to T4, when T1, T2 are fired and a mode 1 repeats. These are the waveforms for gate pulse T1, T2, for gate pulses T3, T4, waveforms for load current, these are the waveforms for output voltage, waveforms for voltage across C. Now, these are the waveforms for freewheeling diode, these are the waveforms for charging and discharging current, and these are the waveforms for voltage across T1 and T2, why the chopper is termed as a load commutated chopper. In this chopper, the load is involved in the process of commutation, that is why the name is the load commutated chopper, single STR chopper, this is the basic arrangement for the single STR chopper, mode 0. When supply is given to circuit, since T is off, the load current flows through the L, C load and capacitor is charged with supply voltage Vs with polarity as shown, here the polarity of capacitor, I write plate is negative and left plate is positive. So, in this mode, capacitor C gets fully charged up to Vs, mode 1, T is greater than T1, less than T2. At T is equal to T1, the main power device T is fired, it carries the 2 current, load current as well as discharging current. The path for discharging current Ic is C, L and T, during this mode, the capacitor current rises, attains negative peak value and finally falls to 0 at T is equal to T2. During this mode, capacitor charge is reversed from plus Vs to minus Vs and during this mode T is on and Vl is equal to Vs, mode 2, T is greater than T2, less than T3. Since Vc is equal to minus Vs, C forces the current through the T in the opposite direction of load current. So, during this mode, the charging current increases in positive direction from 0 and when it becomes equal to Il, at T is equal to T3, thyristor current falls to 0, therefore the main power device T is turned off, mode 3. T is greater than T3, less than T4. At T is equal to T3, T is turned off, Vl shoots up due to L and C and discharging is continued to load. The Vc approaches towards 0 and Ic approaches towards the positive peak. At T is equal to T4, Ic attains positive peak and Vc becomes 0. Mode 4, T is greater than T4, less than T5. After T4, capacitor current decreases and capacitor charging with polarity as shown in the figure. During this mode, charging is continued till Vc becomes equal to Vs at T is equal to T5. Mode 5, T is greater than T5, less than T6. After T5, slight overcharging of capacitor results in forward biasing of freewheeling diode, therefore the load current maintained through the freewheeling diode continue till firing of T. These are the waveforms for single SCR chopper circuit. These are the waveforms for gate current, the waveforms for voltage across thyristor. These are the waveforms for charging and discharging current. These are the waveforms for voltage across capacitor. These are the waveforms for thyristor current and this is the waveform for load voltage. Which type of commutation takes place in the single SCR chopper and how? Here the current commutation takes place. Now in the mode 0, the capacitor C starts charges in this direction. In the mode 1, when thyristor T conducts, first capacitor C discharges in the clockwise direction, then charges with right plate positive with left plate is negative. Again capacitor discharges. Now this time the discharging current flows in the opposite direction of main thyristor. So when this current crosses the load current, the main thyristor T turns off. Here the current commutation takes place. These are references. Thank you.