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
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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) Dr. Ungku Anisa, July 2008
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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 Dr. Ungku Anisa, July 2008
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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
EEEB443 - Control & Drives
Duty Interval ( ia ) 6
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 ) 7
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
EEEB443 - Control & Drives
T 8
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 EEEB443 - Control & Drives
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 9
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
EEEB443 - Control & Drives
Ia
V
La
S
Ea
Energy Storage Interval ( ia ) 10
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 ) 11
DC – DC Converter Fed Drives - Step Up Class B Chopper Regenerative Braking
Duty cycle ton whereT 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
EEEB443 - Control & Drives
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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
-
EEEB443 - Control & Drives
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 EEEB443 - Control & Drives
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 EEEB443 - Control & Drives
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 whereT chopper period
T
Average armature voltage is: Dr. Ungku Anisa, July 2008
EEEB443 - Control & Drives
Average Va =V 16
DC – DC Converter Fed Drives - Two-quadrant Control: Example
Dr. Ungku Anisa, July 2008
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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
Dr. Ungku Anisa, July 2008
EEEB443 - Control & Drives
D2
T2
Note: Polarity of Va and direction of Ia indicated are assumed positive. 18
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
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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.
Dr. Ungku Anisa, July 2008
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