Transcript Chopper-Controlled DC Drives By Dr. Ungku Anisa Ungku Amirulddin Department of Electrical Power Engineering College of Engineering Dr.

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
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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
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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)
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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
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DC – DC Converter Fed Drives
- Step Down Class A Chopper

Motoring
 Provides positive output
voltage and current
 Average power flows from
source to load (motor)
 Switch (S) operated
periodically with period T
Q2
Q1
Q3
Q4
T
Ia
S
Ra
Va
V
D
La
Ea
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DC – DC Converter Fed Drives
- Step Down Class A Chopper
S
Ia
Motoring
S is ON (0  t  ton)
Ra
Ia
Va
V
Ra
La
D
Va
V
Ea
La
Ea
dia
Raia  La
 E V
dt
Dr. Ungku Anisa, July 2008
•Va = V
•Ia flows to motor
•|Ia| increases
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Duty
Interval
( ia  )
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DC – DC Converter Fed Drives
- Step Down Class A Chopper
S
Motoring
Ia
S if OFF (ton  t  T)
Ra
Ia
Va
V
Ra
Va
ID
Ea
La
Ea
dia
Raia  La
E 0
dt
Dr. Ungku Anisa, July 2008
D
La
•Va = 0
•Ia freewheels through
diode DF
•|Ia| decreases
EEEB443 - Control & Drives
Freewheeling
Interval
( ia  )
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DC – DC Converter Fed
- Step Down Class A Chopper
Motoring
ton
where T  chopper period
 Duty cycle  
T
 Under steady-state conditions:
Motor side: Va  Ra I a  E
Chopper side, average armature
voltage: Va  V
Therefore,
Duty
Interval
( ia  )
Freewheeling
Interval
( ia  )
V  Va  Ra I a  E
 Hence, average armature current:
Ia 
Dr. Ungku Anisa, July 2008
V  E
Ra
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T
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DC – DC Converter Fed Drives
- Step Up Class B Chopper 
Regenerative Braking
 Provides positive output voltage and
negative average output current
 Average power flows from load (motor) to
source
Ia
D
Ra
Va
V
Switch (S)
operated
periodically
with period T
Dr. Ungku Anisa, July 2008
S
La
Ea
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Q2
Q1
Q3
Q4
T
•Possible for speed
above rated speed
and down to nearly
zero speed
•Application:
• Battery operated
vehicles
• Regenerated
power stored in
battery
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DC – DC Converter Fed Drives
- Step Up Class B Chopper
Regenerative Braking
D
S is ON (0  t  ton)
Ia
Ra
S
Ra
Va
 Va = 0 (diode blocks V)
 ia increases due to E
Va
La
Ea
dia
Raia  La
E
dt
Dr. Ungku Anisa, July 2008
(since E > Va)
 Mechanical energy
converted to electrical
(i.e. generator)
 Energy stored in La
 Any remaining energy
dissipated in Ra and S
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Ia
V
La
S
Ea
Energy Storage
Interval
( ia  )
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DC – DC Converter Fed Drives
- Step Up Class B Chopper
Regenerative Braking
D
S if OFF (ton  t  T)
Ia
Ra  ia decreases
Va
V
in
negative direction
La  Energy stored in La &
energy supplied by
machine are fed to
Ea
the source
dia
Raia  La
V  E
dt
Dr. Ungku Anisa, July 2008
Ra
 ia flows through diode
D and source V
EEEB443 - Control & Drives
Ia
Va
V
La
S
Ea
Duty
Interval
( ia  )
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DC – DC Converter Fed Drives
- Step Up Class B Chopper
Regenerative Braking
 Duty cycle   ton where T  chopper period
T
Negative because
current flows from
motor to source
 Under steady-state conditions
Generator side: Va  E  Ra I a
Chopper side, average armature
voltage: Va  1   V
 Therefore,
Energy Storage
Interval
( ia  )
Duty
Interval
( ia  )
1   V  Va  E  Ra I a
 Hence, average armature current:
Ia 
Dr. Ungku Anisa, July 2008
E  1   V
Ra
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T
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DC – DC Converter Fed Drives
- Two-quadrant Control
No Speed
 Combination of Class A & B choppers
Reversal
 Forward motoring Q1 - T1 and D2 (Class A)

 Forward braking Q2 – T2 and D1 (Class B)
+
Q2
Q1
Q3
Q4
T
T1
D1
V
+
Va
T2
Dr. Ungku Anisa, July 2008
D2
-
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always +ve   always +ve
• Ia can be +ve or –ve
• Do not fire both switches
together  short circuit at
supply
• Va
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DC – DC Converter Fed Drives
- Two-quadrant Control
 Forward motoring Q1 - T1 and D2 (Class A)
 D2 conducting: Va = 0 (ia )
 T1 conducting: Va = V (ia )
+
+
T1
T1
D1
ia
ia
V
V
+
+
T2

D2
T2
Va

-
Average Va = 1V,
1 = (ton T1 / T ), 2 = 0
T1 chopping
ON & OFF
Dr. Ungku Anisa, July 2008
T2 always
OFF
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D1
Average
Va
D2
Va
-
Ea
•Average Va positive
•Average Va made larger
than back emf Ea
•Ia positive
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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 )
+
+
T1
T1
D1
ia
ia
V
V
+
+
T2

D2
T2
Va

-
Average Va =(1 - 2)V,
1 = 0, 2 = (ton T2 / T )
T1 always
OFF
Dr. Ungku Anisa, July 2008
T2 chopping
ON & OFF
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D1
D2
Va
-
Average
Va
Ea
•Average Va positive
•Average Va made smaller
than back emf Ea
•Ia negative (motor acts as
generator)
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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:   ton T 1 where T  chopper period
T
 Average armature voltage is:
Dr. Ungku Anisa, July 2008
EEEB443 - Control & Drives
Average Va =V
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DC – DC Converter Fed Drives
- Two-quadrant Control: Example
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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
Q2
Q3
T1
D1
D3
Q1
Q4
T
T3
+ Va ia
T4
D4
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EEEB443 - Control & Drives
D2
T2
Note:
Polarity of Va and
direction of Ia
indicated are
assumed
positive.
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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)
Leg B
 Both switches in each
leg, are alternately
+
D1
D3
T1
T3
switched
+ V  If T1 = ON, T4 = OFF
 If T4 = ON, T1 = OFF
a
ia
Vdc
T4
D4
D2
T2
Dr. Ungku Anisa, July 2008
EEEB443 - Control & Drives
Leg A
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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

T1

Current decreases
Energy returned to supply
Q4 operation
Dr. Ungku Anisa, July 2008
EEEB443 - Control & Drives
Q1
Q3
Q4
D1
D3
+ Va -
Current decreases
conducts current

Q2
T
+
T3
ia
 Va = -Vdc when D3 and D4 Vdc


T4
D4
T3 and
T4 off
D2
T2
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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
Q2
Q1
Q3
Q4
T
 Va = 0 when current
freewheels through
T4 and D2

Current decreases
 Va = Vdc when D1 and D2
conducts current



+
Current decreases
Energy returned to
supply
Q2 operation
Dr. Ungku Anisa, July 2008
EEEB443 - Control & Drives
T1
D3
+ Va -
T3
ia
Vdc
T4
-
D1
D2
D4
T1 and
T2 off
T2
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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
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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
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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.
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