1. EEEB283 Electrical Machines & Drives
Chopper-Controlled DC Drives By
Dr. Ungku Anisa Ungku Amirulddin
Department of Electrical Power Engineering
College of Engineering
Dr. Ungku Anisa, July 2008
EEEB443 - Control & Drives
Dr. Ungku Anisa, July 2008

2. Outline Introduction DC – DC Converter Fed Drives
Step Down Class A Chopper
Step Up Class B Chopper
Two-quadrant Control
Four-quadrant Control
References
Dr. Ungku Anisa, July 2008
EEEB443 - Control & Drives

3. Power Electronic Converters for DC Drives
Power electronics converters are used to obtain variable voltage
Highly efficient
Ideally lossless
Type of converter used is depending on voltage source :
AC voltage source  Controlled Rectifiers
Fixed DC voltage source
 DC-DC converters (switch mode converters)
Dr. Ungku Anisa, July 2008
EEEB443 - Control & Drives

4. DC – DC Converter Fed Drives
To obtain variable DC voltage from fixed DC source
Self-commutated devices preferred (MOSFETs, IGBTs, GTOs) over thyristors
Commutated by lower power control signal
Commutation circuit not needed
Can be switched at higher frequency for same rating
Improved motor performance (less ripple, no discontinuous currents, increased control bandwidth)
Suitable for high performance applications
Regenerative braking possible up to very low speeds even when fed from fixed DC voltage source
Dr. Ungku Anisa, July 2008
EEEB443 - Control & Drives

5. DC – DC Converter Fed Drives - Step Down Class A ChopperQ1 Q2 Q3 Q4 
Motoring
Provides positive output voltage and current
Average power flows from source to load (motor)
Switch (S) operated periodically with period T V S D Ra La Ea Va Ia
Dr. Ungku Anisa, July 2008
EEEB443 - Control & Drives

6. DC – DC Converter Fed Drives - Step Down Class A ChopperRa La Ea Va Ia
Motoring
S is ON (0  t  ton) Ra La Ea Va Ia V
Va = V
Ia flows to motor
|Ia| increases
Duty Interval
( ia  )
Dr. Ungku Anisa, July 2008
EEEB443 - Control & Drives

7. DC – DC Converter Fed Drives - Step Down Class A ChopperRa La Ea Va Ia
Motoring
S if OFF (ton  t  T) Ra La Ea Va Ia ID
Va = 0
Ia freewheels through diode DF
|Ia| decreases
Freewheeling Interval
( ia  )
Dr. Ungku Anisa, July 2008
EEEB443 - Control & Drives

8. DC – DC Converter Fed - Step Down Class A Chopper
Motoring
Duty cycle
Under steady-state conditions:
Motor side:
Chopper side, average armature voltage:
Therefore,
Hence, average armature current:
Duty Interval
( ia  )
Freewheeling Interval
( ia  )
 T
Dr. Ungku Anisa, July 2008
EEEB443 - Control & Drives

9. DC – DC Converter Fed Drives - Step Up Class B ChopperQ1 Q2 Q3 Q4 
Regenerative Braking
Provides positive output voltage and negative average output current
Average power flows from load (motor) to source V S D Ra La Ea Va Ia
Possible for speed above rated speed and down to nearly zero speed
Application:
Battery operated vehicles
Regenerated power stored in battery
Switch (S) operated periodically with period T
Dr. Ungku Anisa, July 2008
EEEB443 - Control & Drives

10. DC – DC Converter Fed Drives - Step Up Class B ChopperRa La Ea Va Ia
Regenerative Braking
S is ON (0  t  ton)
Va = 0 (diode blocks V)
ia increases due to E
(since E > Va)
Mechanical energy converted to electrical (i.e. generator)
Energy stored in La
Any remaining energy dissipated in Ra and S Ra La Ea Va Ia S
Energy Storage Interval
( ia  )
Dr. Ungku Anisa, July 2008
EEEB443 - Control & Drives

11. DC – DC Converter Fed Drives - Step Up Class B ChopperRa La Ea Va Ia
Regenerative Braking
S if OFF (ton  t  T)
ia flows through diode D and source V
ia decreases in negative direction
Energy stored in La & energy supplied by machine are fed to the source Ra La Ea Va Ia V
Duty
Interval
( ia  )
Dr. Ungku Anisa, July 2008
EEEB443 - Control & Drives

12. DC – DC Converter Fed Drives - Step Up Class B Chopper
Regenerative Braking
Duty cycle
Under steady-state conditions
Generator side:
Chopper side, average armature voltage:
Therefore,
Hence, average armature current:
Negative because current flows from motor to source
Energy Storage Interval
( ia  )
Duty
Interval
( ia  )
 T
Dr. Ungku Anisa, July 2008
EEEB443 - Control & Drives

13. DC – DC Converter Fed Drives - Two-quadrant Control
No Speed Reversal
Combination of Class A & B choppers
Forward motoring Q1 - T1 and D2 (Class A)
Forward braking Q2 – T2 and D1 (Class B) T Q1 Q2 Q3 Q4  D2 + Va - T1 D1 T2 V
Va always +ve   always +ve
Ia can be +ve or –ve
Do not fire both switches
together  short circuit at
supply
Dr. Ungku Anisa, July 2008
EEEB443 - Control & Drives

14. DC – DC Converter Fed Drives - Two-quadrant Control
Forward motoring Q1 - T1 and D2 (Class A)
T1 conducting: Va = V (ia )
D2 conducting: Va = 0 (ia ) + V  + V  T1 T1 D1 D1 ia ia + Va - + Va - D2 D2 T2 T2
Average Va = 1V, 1 = (ton T1 / T ), 2 = 0
Average Va Ea
T1 chopping
ON & OFF
T2 always
OFF
Average Va positive
Average Va made larger than back emf Ea
Ia positive
EEEB443 - Control & Drives
Dr. Ungku Anisa, July 2008

15. DC – DC Converter Fed Drives - Two-quadrant Control
Forward braking Q2 – T2 and D1 (Class B)
D1 conducting: Va = V (ia )
T2 conducting: Va = 0 (ia ) + V  + V  T1 T1 D1 D1 ia ia + Va - + Va - D2 D2 T2 T2
Average Va =(1 - 2)V, 1 = 0, 2 = (ton T2 / T ) Ea
Average Va
T2 chopping
ON & OFF
Average Va positive
Average Va made smaller than back emf Ea
Ia negative (motor acts as generator)
T1 always
OFF
Dr. Ungku Anisa, July 2008
EEEB443 - Control & Drives

16. DC – DC Converter Fed Drives - Two-quadrant Control
For fast transition from motoring (Q1) to braking (Q2) and vice versa, both T1 and T2 are controlled simultaneously, i.e. within a period T:
T1 in ON and T2 is OFF between time 0 < t ≤ ton
If Ia is positive (Va > E), current flows from supply to motor via T1
If Ia is negative (E > Va), current flows from motor to supply via D1
T1 is OFF and T2 is ON between ton < t ≤ T
If Ia is positive, current circulates via D2
If Ia is negative, current circulates via T2
Duty ratio is given by:
Average armature voltage is:
Average Va =V
Dr. Ungku Anisa, July 2008
EEEB443 - Control & Drives

17. DC – DC Converter Fed Drives - Two-quadrant Control: Example
Dr. Ungku Anisa, July 2008
EEEB443 - Control & Drives

18. DC – DC Converter Fed Drives - Four-quadrant Control
Operation in all four quadrants
Va and Ia can be controlled in magnitude and polarity
Power flow can be in either direction
Speed and torque can be reversed T Q1 Q2 Q3 Q4 
+ Va - T1 D1 T2 D2 D3 D4 T3 T4 ia
Note:
Polarity of Va and direction of Ia indicated are assumed positive.
Dr. Ungku Anisa, July 2008
EEEB443 - Control & Drives

19. DC – DC Converter Fed Drives - Four-quadrant Control
When a switch is on (i.e. ‘ON state’) it may or may not conduct current depending on the direction of ia
If a switch conducts current, it is in a conducting state
Converter has two legs (Leg A & Leg B)
Both switches in each leg, are alternately switched
If T1 = ON, T4 = OFF
If T4 = ON, T1 = OFF
Leg B
+ Va - T1 D1 T2 D2 D3 D4 T3 T4 +
Vdc - ia
Dr. Ungku Anisa, July 2008
EEEB443 - Control & Drives
Leg A

20. DC – DC Converter Fed Drives - Four-quadrant Control
Positive Current (Ia > 0)
Va = Vdc when T1 and T2 are ON
Current increases
Q1 operation
Va = 0 when current freewheels through T2 and D4
Current decreases
Va = -Vdc when D3 and D4
conducts current
Energy returned to supply
Q4 operation
+ Va - T1 D1 T2 D2 D3 D4 T3 T4 +
Vdc - ia
T3 and T4 off
Dr. Ungku Anisa, July 2008
EEEB443 - Control & Drives

21. DC – DC Converter Fed Drives - Four-quadrant Control
Negative Current (Ia > 0)
Va = -Vdc when T3 and T4 are ON
Current increases in negative direction
Q3 operation
Va = 0 when current freewheels through T4 and D2
Current decreases
Va = Vdc when D1 and D2
conducts current
Energy returned to supply
Q2 operation
+ Va - T1 D1 T2 D2 D3 D4 T3 T4 +
Vdc - ia
T1 and T2 off
Dr. Ungku Anisa, July 2008
EEEB443 - Control & Drives

22. DC – DC Converter Fed Drives - Four-quadrant Control
For both positive and negative current, output voltage can swing between:
Vdc and -Vdc
Vdc and 0
Four quadrant chopper has two legs, so it requires two switching signals (one for each leg)
Depending on relationship between the two switching signals, 4-quadrant chopper has two switching schemes:
Bipolar switching
Unipolar switching
Switching scheme determines output voltage swing between
Vdc and -Vdc or Vdc and 0.
Dr. Ungku Anisa, July 2008
EEEB443 - Control & Drives

23. DC – DC Converter Fed Drives
Operation of DC motor drive depends on:
Direction of Ia (determined by torque, i.e. motoring or braking)
Polarity of Va and Ea (determined by speed, i.e. forward or reverse)
the duty cycle of the DC-DC Converter (either two-quadrant or four-quadrant)
Open loop control is achieved by changing the duty cycle manually as and when required
Dr. Ungku Anisa, July 2008
EEEB443 - Control & Drives

24. References
Rashid, M.H, Power Electronics: Circuit, Devices and Applictions, 3rd ed., Pearson, New-Jersey, 2004.
Dubey, G.K., Fundamentals of Electric Drives, 2nd ed., Alpha Science Int. Ltd., UK, 2001.
Krishnan, R., Electric Motor Drives: Modeling, Analysis and Control, Prentice-Hall, New Jersey, 2001.
Nik Idris, N. R., Short Course Notes on Electrical Drives, UNITEN/UTM, 2008.
Ahmad Azli, N., Short Course Notes on Electrical Drives, UNITEN/UTM, 2008.
Dr. Ungku Anisa, July 2008
EEEB443 - Control & Drives