 myself Sunil Karshati, Asin Professor, Department of Electronics Engineering, Valchan is of Technology, Solapur. Today, I am going to discuss the classification of choppers. Learning outcome. At the end of this session, students can classify the different chopper, classification chopper. Choppers are classified into five classes as shown below. Type A chopper, type B chopper, type C chopper, type D chopper and type E chopper. Type A and type B are the single quadrant chopper. Type C, type D are two quadrant chopper and type E is the four quadrant chopper. This is the circuit diagram for the class A chopper. The dotted rectangle represents the chopper switch with internal firing circuit, commutation circuit plus protection circuit. And FD is the freewheeling diode. When chopper is on, supply voltage will connected across the load. During T1, when gate pulse is applied to the chopper switch, it acts as a short. So, input is appears across the output. So, for this duration, the current flows through the VS chopper switch load. Assume that this direction of voltage is positive and the direction of current is positive and the chopper operates in first quadrant. During T1, when chopper switch turns off, then freewheeling diode conducts, then the direction of current remains same and our load voltage is zero. Still, chopper operates in first quadrant. The average value of output voltage and current are always positive. Class A chopper is the step down chopper in which power always flows from source to load. It is used to control the speed of the DC motor. These are the wave forms for voltage and current wave forms for class A chopper. During T1, chopper switch is in conducting state. So, input is appears across the output and current rises linearly. And during T off, the freewheeling action takes place, current is reduces, the load voltage is equal to zero. Class B chopper, this is the circuit diagram for the class B chopper. When separately excited DC motor is operating from fixed supply, regenerative braking is only possible for speed above rated speed. However, with this chopper, it is possible to obtain regenerative braking down to zero speed. This helps to save large amount of energy, especially in underground traction drives and in battery operated vehicle, where saved energy will help to travel the vehicles for larger distance. In this circuit, the motor works as a generator. When the chopper is on, V0 is equal to zero, E drives the current through L and R in the direction opposite to that shown in the figure. When chopper is on, the current flows through the EL chopper switch and the current flows in the anticlockwise direction. Assume that this direction of current is negative and for this, for this duration, the load voltage is zero. During on time, the inductance L stores the energy. When the chopper is off, the diode conducts and the part of energy stored in the inductor L is returned to the supply. Average output voltage is positive and average output current is negative. In this chopper, power flows from load to source. Class B chopper is used for regenerative braking of DC motor and it is the step up chopper. These are the waveforms for the class B chopper. When during on, when chopper switch is in conducting set, the current increases from Imin to Imax and for this duration V0 is equal to zero. And during off, the current reduces from Imax to Imin and V0 is equal to Vs plus Ldi by dt. Why? Class A chopper is called as a step down chopper and a class B chopper called as a step up chopper. For the class A, output voltage is less than input. That's why class A is called as a step down chopper and in class B, output is greater than input. That's why it is called as a step up chopper. Class C chopper. This is the circuit diagram of class C chopper. It is the combination of class A and class B chopper. Chopper switch 1, D2 acts as a class A chopper and chopper switch 2, D1 acts as a class B chopper. For first quadrant operation, chopper switch 1 is on or the freewheeling diode D2 conducts. For second quadrant operation, chopper switch 2 is on or D1 conducts. When chopper switch 1 is on, the load current is positive and output voltage is equal to V and the load receives the power from source. When chopper switch 1 is in conducting state, the current flows from source Vs, chopper switch 1, L, E back to Vs. Assume that this direction of voltage is positive and the direction of current is positive and chopper operates in first quadrant. And when chopper switch turns off, the freewheeling action takes place. The current flows through the D and load and here the load voltage is zero and the load current is still positive. When chopper switch 2 is triggered, the voltage E forces current to flow in opposite direction through L and chopper switch 2 and the output voltage is zero. Here, for this duration, when chopper switch 2 conducts, then current flows through the EL chopper switch 2 and assume that this direction of current is negative. On turning of chopper switch 2, the energy stored in the inductance drives the current through the diode D1 and the supply output voltage is V. The input current becomes negative and power flows from load to source. Average output voltage is positive and average output current can take both positive and negative values. Chopper switch CH1 and CH2 should not be turned on simultaneously as it would result in short circuiting the supply. Class C chopper can be used both for DC motor control and regenerative braking of DC motor. Class C chopper can be used as a step up or step down chopper. These are the wave form, voltage and current wave form for class C chopper, class D chopper. When both chopper switch 1 and chopper switch 2 are triggered simultaneously, the output voltage V0 is equal to V and output current I0 flows through the load in the direction shown in the figure. When chopper switch 1, chopper switch 2 both are in conducting set, the current flows through the VS CH1 load CH2. Assume that this direction of voltage is positive and the direction of current is positive, the chopper operates in first quadrant. When both CH1 and CH2 are turned off, the load current I0 continues to flow in the same direction through the load D1 and D2. Due to the energy stored in the inductorial, but output voltage V0 is negative, V0 is minus V. Now when CH1 and CH2 both are turned off, the current flows through the load D2 VS D1. Assume that the direction of current is same, but the load voltage gets reversed. The average load voltage is positive if chopper on time is more than off time and average output voltage is negative if on time is less than off time. Hence the direction of load current is always positive, but the load voltage can be either positive or negative. Class T is the two quadrant chopper. These are the waveforms for class T when on time is greater than off time. When on time is greater than off time, average load voltage is positive. These are the waveforms when off time is greater than on time and average voltage is negative. Class C chopper. Class C is a four quadrant chopper. When CH1 and CH4 are triggered, when CH1 and CH4 are triggered, the direction of current is VS CH1 load CH4. Assume that this direction of voltage is positive and the direction of current is positive. This gives the first quadrant operation. When both CH1 and CH4 are off, the energy stored in the inductor L drives I0 through the D2 and D3 in the same direction, but output voltage V0 is minus V. Therefore, chopper operates in fourth quadrant. When CH2 and CH3 are triggered, when CH2 and CH3 are triggered, the current flows through the VS CH3 load CH2. Assume that this direction of current is exactly opposite. Assume that this direction of current is negative and the load voltage is negative. Since both I0 and V0 are negative, the chopper operates in third quadrant. When both CH2 and CH3 are off, the load current I0 continues to flow in the same direction D1 and D4 and the output voltage V0 is equal to V. Therefore, the chopper operates in second quadrant as V0 is positive, but I0 is negative. These are references. Thank you.