1. Two-quadrant converterVa T1 +
Vdc  D1 ia Q2 Q1 Ia + Va - D2 T2
T1 conducts  va = Vdc

2. Two-quadrant converterVa T1 +
Vdc  D1 ia Q2 Q1 Ia + Va - D2 T2
D2 conducts  va = 0
T1 conducts  va = Vdc
Quadrant 1 The average voltage is made larger than the back emf

3. Two-quadrant converterVa T1 +
Vdc  D1 ia Q2 Q1 Ia + Va - D2 T2
T2 conducts  va = 0
Quadrant 2 The average voltage is made smaller than the back emf, thus forcing the current to flow in the reverse direction

4. Two-quadrant converterVa T1 +
Vdc  D1 ia Q2 Q1 Ia + Va - D2 T2
D1 conducts  va = Vdc

5. Four-quadrant Chopper
Chopper fed DC drive
Four-quadrant Chopper Va +
Vdc  D1 D3 Q1 Q3
+ Va  Ia D2 Q2 D4 Q4
Forward Motoring
Q1 & Q4 is ON
Current Flow : Vdc + _ Q1 _Motor_Q4_Vdc-
Current Ia & Va are positive
Operates in First Quadrant

6. Four-quadrant Chopper
Chopper fed DC drive
Four-quadrant Chopper +
Vdc  D1 D3 Q1 Q3
+ Va  D2 Q2 D4 Q4
Q1 is OFF & Q4 is ON. Inductor current has to flow in the same Direction.
Diode D2 is FB
Inductor Current freewheels through D2 & Q4
Output Voltage is Zero

7. Four-quadrant Chopper
Chopper fed DC drive
Four-quadrant Chopper +
Vdc  D1 D3 Q1 Q3
+ Va  D2 Q2 D4 Q4
Q4 is OFF. Q2 is ON. Load is not connected with the source.
Back Emf drives the current through Q2 & D4

8. Four-quadrant Chopper
Chopper fed DC drive
Four-quadrant Chopper Va +
Vdc  D1 D3 Q1 Q3
+ Va  Ia D2 Q2 D4 Q4
Forward Braking
Q2 is OFF. Diode D1 is FB
Current flows through D4 & D1
Current Ia is negative & Va is positive.
Operates in second quadrant.

9. Four-quadrant Chopper
Chopper fed DC drive
Four-quadrant Chopper +
Vdc  D1 D3 Q1 Q3
- Va + D2 Q2 D4 Q4
Q3 & Q2 is ON
Current Flow : Vdc + _ Q3 _Motor_Q2_Vdc-
Current Ia & Va are negative
Operates in third Quadrant

10. Four-quadrant Chopper
Chopper fed DC drive
Four-quadrant Chopper +
Vdc  D1 D3 Q1 Q3
- Va + D2 Q2 D4 Q4
Q3 is OFF. Q2 is ON. Current has to be continuous.
Diode D4 is FB
Current flows through Q2,D4 & (Eb,La,Ra)

11. Four-quadrant Chopper
Chopper fed DC drive
Four-quadrant Chopper +
Vdc  D1 D3 Q1 Q3
- Va + D2 Q2 D4 Q4
Q4 Is ON & Q2 is OFF.
Back emf Drives the current through Q4 – D2 - MOTOR

12. Four-quadrant Chopper
Chopper fed DC drive
Four-quadrant Chopper +
Vdc  D1 D3 Q1 Q3
- Va + D2 Q2 D4 Q4
Q4 IS Turned Off,
D3 is FB
Current Flows through Va+ - D3 – D2 – Va-
Va is negative. But current Ia is positive
Operates in fourth quadrant