1. EEEB2833 Electrical Machines & Drives
DC Drives By
Dr. Ungku Anisa Ungku Amirulddin
Department of Electrical Power Engineering
College of Engineering
Dr. Ungku Anisa, July 2008
EEEB283 - Electrical Machines & Drives
Dr. Ungku Anisa, July 2008

2. Outline Power Electronics Converters for DC Drives
Controlled Rectifier Fed DC Drives
Single Phase
Three Phase
DC – DC Converter Fed Drives
Step Down Class A Chopper
Step Up Class B Chopper
Two-quadrant Control
Four-quadrant Control
Closed-loop Control (Brief overview)
References
Dr. Ungku Anisa, July 2008
EEEB283 - Electrical Machines & Drives

3. Power Electronic Converters for DC Drives
To obtain variable voltage
Efficient
Ideally lossless
Depending on voltage source:
AC voltage source  Controlled Rectifiers
Fixed DC voltage source
 DC-DC converters (switch mode converters)
Dr. Ungku Anisa, July 2008
EEEB283 - Electrical Machines & Drives

4. Controlled Rectifier Fed DC Drives
To obtain variable DC voltage from fixed AC source
DC current flows in only 1 direction
Example of a drive system
Dr. Ungku Anisa, July 2008
EEEB283 - Electrical Machines & Drives

5. Controlled Rectifier Fed DC Drives
Contains low frequency AC ripple
To reduce ripple: extra inductance added in series with La
Slow response
Discontinuous current may occur if
La not large enough
Motor is lightly loaded
Half-wave rectifier is used
Effect of discontinuous current
Rectifier output voltage increases  motor speed increases
(poor speed regulation under open-loop operation)
Dr. Ungku Anisa, July 2008
EEEB283 - Electrical Machines & Drives

6. Controlled Rectifier Fed – Single-phase DC DrivesQ1 Q2 Q3 Q4 
Two-quadrant drive
Limited to applications up to 15 kW
During regeneration, Ea can be reversed by reversing field excitation
Dr. Ungku Anisa, July 2008
EEEB283 - Electrical Machines & Drives

7. Controlled Rectifier Fed – Single-phase DC Drives
supply + Va  ia
For continuous current:
Armature voltage
where Vm = peak voltage
Armature current
Field voltage
90o
180o 
Dr. Ungku Anisa, July 2008
EEEB283 - Electrical Machines & Drives

8. Controlled Rectifier Fed – Single-phase DC DrivesQ1 Q2 Q3 Q4 
Four-quadrant drive
Converter 1 for operation in 1st and 4th quadrant
Converter 2 for operation in 2nd and 3rd quadrant
Limited to applications up to 15 kW
Single-phase
supply + Va  ia
Converter 1
Converter 2
Dr. Ungku Anisa, July 2008
EEEB283 - Electrical Machines & Drives

9. Controlled Rectifier Fed – Single-phase DC Drives
For continuous current:
Armature voltage:
If Converter 1 operates
If Converter 2 operates
where
Vm = peak voltage
Armature current
Field voltage + Va 
Converter 1
Converter 2 ia
Dr. Ungku Anisa, July 2008
EEEB283 - Electrical Machines & Drives

10. Controlled Rectifier Fed – Three-phase DC DrivesQ1 Q2 Q3 Q4 
Two-quadrant drive
Limited to applications up to 1500 kW
During regeneration, Ea can be reversed by reversing field excitation
Dr. Ungku Anisa, July 2008
EEEB283 - Electrical Machines & Drives

11. Controlled Rectifier Fed – Three-phase DC Drives
supply + Va  ia
For continuous current:
Armature voltage
where VL-L, m = peak line-to-line voltage
Armature current
Field voltage
(assuming a three-phase supply is used for field excitation)
90o
180o 
Dr. Ungku Anisa, July 2008
EEEB283 - Electrical Machines & Drives

12. Controlled Rectifier Fed – Three-phase DC DrivesQ1 Q2 Q3 Q4 
Four-quadrant drive
Converter 1 for operation in 1st and 4th quadrant
Converter 2 for operation in 2nd and 3rd quadrant
3-phase
supply + Va  ia
Converter 1
Converter 2
Dr. Ungku Anisa, July 2008
EEEB283 - Electrical Machines & Drives

13. Controlled Rectifier Fed – Three-phase DC Drives
Disadvantage:
Circulating current
Slow response
For continuous current:
Armature voltage:
If Converter 1 operates
If Converter 2 operates
where
VL-L, m = peak line-to-line voltage
Armature current
Field voltage + Va 
Converter 1
Converter 2 ia
Dr. Ungku Anisa, July 2008
EEEB283 - Electrical Machines & Drives

14. Controlled Rectifier Fed – Three-phase DC DrivesQ1 Q2 Q3 Q4 
Four-quadrant drive
One controlled rectifier with 2 pairs of contactors
M1 and M2 closed for operation in 1st and 4th quadrant
R1 and R2 closed for operation in 2nd and 3rd quadrant M1 M2 R1 R2
+ Va -
3-phase
supply
Dr. Ungku Anisa, July 2008
EEEB283 - Electrical Machines & Drives

15. 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
EEEB283 - Electrical Machines & Drives

16. DC – DC Converter Fed Drives - Step Down Class A ChopperQ1 Q2 Q3 Q4 
Motoring
Dr. Ungku Anisa, July 2008
EEEB283 - Electrical Machines & Drives

17. DC – DC Converter Fed Drives - Step Down Class A Chopper
Motoring
S is ON (0  t  ton)
Duty Interval
- ia 
Dr. Ungku Anisa, July 2008
EEEB283 - Electrical Machines & Drives

18. DC – DC Converter Fed Drives - Step Down Class A Chopper
Motoring
S if OFF (ton  t  T)
Freewheeling Interval
- ia 
Dr. Ungku Anisa, July 2008
EEEB283 - Electrical Machines & Drives

19. DC – DC Converter Fed - Step Down Class A Chopper
Motoring
Duty cycle
Under steady-state conditions
Motor side:
Chopper side:
Hence,
Duty Interval
- ia 
Freewheeling Interval
- ia 
average Va
average Ia kT
Dr. Ungku Anisa, July 2008
EEEB283 - Electrical Machines & Drives

20. DC – DC Converter Fed Drives - Step Up Class B ChopperQ1 Q2 Q3 Q4 
Regenerative Braking
Possible for speed above rated speed and down to nearly zero speed
Application:
Battery operated vehicles
Regenerated power stored in battery
Dr. Ungku Anisa, July 2008
EEEB283 - Electrical Machines & Drives

21. DC – DC Converter Fed Drives - Step Up Class B Chopper
Regenerative Braking
S is ON (0  t  ton)
Va = 0
ia increases due to E
Mechanical energy converted to electrical (i.e. generator)
Energy stored in La
Energy Storage Interval
- ia 
Dr. Ungku Anisa, July 2008
EEEB283 - Electrical Machines & Drives

22. DC – DC Converter Fed Drives - Step Up Class B Chopper
Regenerative Braking
S if OFF (ton  t  T)
ia flows through diode D and source V
Energy stored in La & energy supplied by machine are fed to the source
Duty
Interval
- ia 
Dr. Ungku Anisa, July 2008
EEEB283 - Electrical Machines & Drives

23. DC – DC Converter Fed Drives - Step Up Class B Chopper
Regenerative Braking
Duty cycle
Under steady-state conditions
Generator side:
Chopper side:
Hence,
Energy Storage Interval
- ia 
Duty
Interval
- ia 
average Va
average Ia
 T
Dr. Ungku Anisa, July 2008
EEEB283 - Electrical Machines & Drives

24. DC – DC Converter Fed Drives - Two-quadrant Control
Forward motoring Q1 - T1 and D2
Forward braking Q2 – T2 and D1 D2 + Va - T1 D1 T2 V
No Speed Reversal
Dr. Ungku Anisa, July 2008
EEEB283 - Electrical Machines & Drives

25. DC – DC Converter Fed Drives - Two-quadrant Control
Average Va positive
Average Va made larger than back emf Eb
Ia positive
Forward motoring Q1
T1 conducting: Va = V T1 + V  D1
D2 conducting: Va = 0 ia + Va - D2 T2 T1 + V  D1 ia + Va - D2 T2 Va Eb
Dr. Ungku Anisa, July 2008
EEEB283 - Electrical Machines & Drives

26. DC – DC Converter Fed Drives - Two-quadrant Control
Average Va positive
Average Va made smaller than back emf Eb
Ia negative
Forward braking Q2
D1 conducting: Va = V T1 + V  D1
T2 conducting: Va = 0 ia + Va - D2 T2 T1 +
Vdc  D1 ia + Va - D2 T2 Eb Va
Dr. Ungku Anisa, July 2008
EEEB283 - Electrical Machines & Drives

27. DC – DC Converter Fed Drives - Four-quadrant Control
Operation in all quadrants
Speed can be reversed
+ Va - T1 D1 T2 D2 D3 D4 T3 T4
Dr. Ungku Anisa, July 2008
EEEB283 - Electrical Machines & Drives

28. DC – DC Converter Fed Drives - Four-quadrant Control
T3 and T4 off
Forward Motoring Q1
T1 and T2 on
Va = V
Ia increases
Reverse Braking Q4
(Regeneration)
T1 off but T2 still on
Va = 0
Ia decays thru T2 and D4
T1 and T2 off
Va = -V
Ia decays thru D3 and D4
Energy returned to supply
+ Va - T1 D1 T2 D2 D3 D4 T3 T4 + V -
Dr. Ungku Anisa, July 2008
EEEB283 - Electrical Machines & Drives

29. DC – DC Converter Fed Drives - Four-quadrant Control
T1 and T2 off
Reverse Motoring Q3
T3 and T4 on
Va = -V
Ia increases in reverse direction
Forward Braking Q2
(Regeneration)
T3 off but T4 still on
Va = 0
Ia decays thru T4 and D2
T3 and T4 off
Va = V
Ia decays thru D1 and D2
Energy returned to supply
+ Va - T1 D1 T2 D2 D3 D4 T3 T4 + V -
Dr. Ungku Anisa, July 2008
EEEB283 - Electrical Machines & Drives

30. Closed-loop Control
Feedback loops may be provided to satisfy one or more of the following:
Protection
Enhancement of speed response
Improve steady-state accuracy
Variables to be controlled in drives:
Torque – achieved by controlling current
Speed
Position
Controllers are designed based on a linear averaged model
Dr. Ungku Anisa, July 2008
EEEB283 - Electrical Machines & Drives

31. Closed-loop Control Variables to be controlled in drives:
Torque – achieved by controlling current
Commonly employed current sensor:
Current shunt – no electrical isolation, cheap
Hall effect sensor – provides electrical isolation
Speed is governed by torque:
e.g. With phase-controlled rectifier
firing
circuit
current
controller
controlled rectifier  + Va – vc
iref -
Dr. Ungku Anisa, July 2008
EEEB283 - Electrical Machines & Drives

32. Closed-loop Control Variables to be controlled in drives:
Speed – with or without current loop
Commonly employed speed/position sensor:
Tachogenerator – analog based
Digital encoder – digital based, converts speed to pulses
Torque is governed by speed demand:
Without current loop: no limit on current – can be too high
With current loop: current can be limited
Dr. Ungku Anisa, July 2008
EEEB283 - Electrical Machines & Drives

33. Power Electronic Converters
Closed-loop Control
Variables to be controlled in drives:
Speed control without current loop:
Simple implementation
Current can be too high  may damage converter
Speed
controller
Power Electronic Converters * + - va   vc
Tacho
Dr. Ungku Anisa, July 2008
EEEB283 - Electrical Machines & Drives

34. Power Electronic Converters
Closed-loop Control
Variables to be controlled in drives:
Speed control with current loop:
Two controllers required: speed and current
Current limited by limiting ia*
Speed
controller
Power Electronic Converters * + - va   vc
Tacho
Current
ia* ia
Dr. Ungku Anisa, July 2008
EEEB283 - Electrical Machines & Drives

35. 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
EEEB283 - Electrical Machines & Drives