chopper-controlled dc drives by dr. ungku anisa ungku amirulddin department of electrical power...
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Chopper-Controlled DC DrivesByDr. Ungku Anisa Ungku AmirulddinDepartment of Electrical Power EngineeringCollege of Engineering
Dr. Ungku Anisa, July 2008 1EEEB443 - Control & Drives
OutlineIntroduction DC – DC Converter Fed Drives
Step Down Class A ChopperStep Up Class B ChopperTwo-quadrant ControlFour-quadrant Control
References
Dr. Ungku Anisa, July 2008 EEEB443 - Control & Drives 2
Power Electronic Converters for DC DrivesPower electronics converters are used to obtain
variable voltageHighly efficientIdeally lossless
Type of converter used is depending on voltage source :AC voltage source Controlled RectifiersFixed DC voltage source
DC-DC converters (switch mode converters)
Dr. Ungku Anisa, July 2008 EEEB443 - Control & Drives 3
DC – DC Converter Fed Drives To obtain variable DC voltage from fixed DC sourceSelf-commutated devices preferred (MOSFETs, IGBTs,
GTOs) over thyristorsCommutated by lower power control signalCommutation circuit not neededCan be switched at higher frequency for same rating
Improved motor performance (less ripple, no discontinuous currents, increased control bandwidth)
Suitable for high performance applicationsRegenerative braking possible up to very low speeds
even when fed from fixed DC voltage source
Dr. Ungku Anisa, July 2008 EEEB443 - Control & Drives 4
DC – DC Converter Fed Drives- Step Down Class A ChopperMotoringProvides positive output
voltage and currentAverage power flows from
source to load (motor)Switch (S) operated
periodically with period T
Dr. Ungku Anisa, July 2008 EEEB443 - Control & Drives 5
T
Q1Q2
Q3 Q4
V
S
D
Ra
La
Ea
Va
Ia
DC – DC Converter Fed Drives- Step Down Class A Chopper
S is ON (0 t ton)
Dr. Ungku Anisa, July 2008 EEEB443 - Control & Drives 6
Motoring
VEdt
diLiR aaaa
Duty Interval( ia )
•Va = V•Ia flows to motor•|Ia| increases
V
S
D
Ra
La
Ea
Va
Ia
Ra
La
Ea
Va
Ia
V
DC – DC Converter Fed Drives- Step Down Class A Chopper
S if OFF (ton t T)
Dr. Ungku Anisa, July 2008 EEEB443 - Control & Drives 7
Motoring
0 Edt
diLiR aaaa
Freewheeling Interval( ia )
•Va = 0•Ia freewheels through diode DF
•|Ia| decreases
Ra
La
Ea
Va
Ia
ID
V
S
D
Ra
La
Ea
Va
Ia
DC – DC Converter Fed- Step Down Class A ChopperMotoringDuty cycleUnder steady-state conditions:
Motor side: Chopper side, average armature voltage:
Therefore,
Hence, average armature current:
Dr. Ungku Anisa, July 2008 EEEB443 - Control & Drives 8
period chopper where TT
ton
EIRVV aaa
aa R
EVI
T
Freewheeling Interval
( ia )
Duty Interval
( ia )
EIRV aaa
VVa
DC – DC Converter Fed Drives- Step Up Class B Chopper
Dr. Ungku Anisa, July 2008 EEEB443 - Control & Drives 9
T
Q1Q2
Q3 Q4
•Possible for speed above rated speed and down to nearly zero speed
•Application:• Battery operated
vehicles• Regenerated
power stored in battery
V S
D Ra
La
Ea
Va
Ia
Regenerative BrakingProvides positive output voltage and
negative average output currentAverage power flows from load (motor) to
source
Switch (S) operated periodically with period T
DC – DC Converter Fed Drives- Step Up Class B Chopper
S is ON (0 t ton)
Dr. Ungku Anisa, July 2008 EEEB443 - Control & Drives 10
Regenerative Braking
Energy Storage Interval( ia )
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
Edt
diLiR aaaa
V S
D Ra
La
Ea
Va
Ia
Ra
La
Ea
Va
Ia
S
DC – DC Converter Fed Drives- Step Up Class B Chopper
S if OFF (ton t T)
Dr. Ungku Anisa, July 2008 EEEB443 - Control & Drives 11
Regenerative Braking
Duty Interval
( ia )
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
EVdt
diLiR aaaa
V S
D Ra
La
Ea
Va
Ia
Ra
La
Ea
Va
Ia
V
DC – DC Converter Fed Drives- Step Up Class B ChopperRegenerative BrakingDuty cycle
Under steady-state conditionsGenerator side: Chopper side, average armature voltage:
Therefore,
Hence, average armature current:
Dr. Ungku Anisa, July 2008 EEEB443 - Control & Drives 12
periodchopper where TT
ton
aaa IREVV 1
a
a R
VEI
1
T
Duty Interval
( ia )
Energy Storage Interval
( ia )
aaa IREV
VVa 1
Negative because current flows from
motor to source
DC – DC Converter Fed Drives- Two-quadrant ControlCombination of Class A & B choppersForward motoring Q1 - T1 and D2 (Class A)Forward braking Q2 – T2 and D1 (Class B)
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D2
+Va
-
T1D1
T2
D2
+
V
-
T
Q1Q2
Q3 Q4
No Speed Reversal
• Va always +ve always +ve• Ia can be +ve or –ve • Do not fire both switches together short circuit at supply
DC – DC Converter Fed Drives- Two-quadrant ControlForward motoring Q1 - T1 and D2 (Class A)
T1 conducting: Va = V (ia ) D2 conducting: Va = 0 (ia )
Dr. Ungku Anisa, July 200814
T1
T2
D1
+
Va
-
D2
ia
+
V
T1
T2
D1
+
Va
-
D2
ia
Average Va
Ea
+
V
Average Va = 1V, 1 = (ton T1 / T ), 2 = 0
EEEB443 - Control & Drives
T2 alwaysOFF
T1 choppingON & OFF
•Average Va positive•Average Va made larger
than back emf Ea •Ia positive
DC – DC Converter Fed Drives- Two-quadrant ControlForward braking Q2 – T2 and D1 (Class B)
D1 conducting: Va = V (ia ) T2 conducting: Va = 0 (ia )
T1
T2
D1
+
Va
-
D2
ia
T1
T2
D1
+
Va
-
D2
ia
Average Va
Ea
•Average Va positive•Average Va made smaller
than back emf Ea
•Ia negative (motor acts as generator)
+
V
+
V
Average Va =(1 - 2)V, 1 = 0, 2 = (ton T2 / T )
15Dr. Ungku Anisa, July 2008 EEEB443 - Control & Drives
T1 alwaysOFF
T2 choppingON & OFF
DC – DC Converter Fed Drives- Two-quadrant ControlFor 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:Dr. Ungku Anisa, July 2008 EEEB443 - Control & Drives 16
periodchopper where1 TT
t Ton
Average Va =V
DC – DC Converter Fed Drives- Four-quadrant ControlOperation in all four quadrants
Va and Ia can be controlled in magnitude and polarityPower flow can be in either direction
Speed and torque can be reversed
+ Va -
T1D1
T2D2
D3
D4
T3
T4
T
Q1Q2
Q3 Q4
ia
Note:Polarity of Va and direction of Ia indicated are assumed positive.
Dr. Ungku Anisa, July 2008 18EEEB443 - Control & Drives
DC – DC Converter Fed Drives- Four-quadrant ControlWhen 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 stateConverter has two legs (Leg A & Leg B)Both switches in each
leg, are alternately switchedIf T1 = ON, T4 = OFFIf T4 = ON, T1 = OFF
+ Va -T1
D1
T2D2
D3
D4
T3
T4
+
Vdc
-
Dr. Ungku Anisa, July 2008 19EEEB443 - Control & Drives Leg A
Leg B
ia
DC – DC Converter Fed Drives- Four-quadrant ControlPositive 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 D4conducts current Current decreases Energy returned to supply Q4 operation
+ Va -T1
D1
T2D2
D3
D4
T3
T4
T
Q1Q2
Q3 Q4
+
Vdc
-
Dr. Ungku Anisa, July 2008 20EEEB443 - Control & Drives
ia
T3 and T4 off
DC – DC Converter Fed Drives- Four-quadrant ControlNegative 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 D2conducts current Current decreases Energy returned to
supply Q2 operation
T
Q1Q2
Q3 Q4
+
Vdc
-
Dr. Ungku Anisa, July 2008 21EEEB443 - Control & Drives
ia
+ Va -T1
D1
T2D2
D3
D4
T3
T4
T1 and T2 off
DC – DC Converter Fed Drives- Four-quadrant ControlFor 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 switchingUnipolar switching
Switching scheme determines output voltage swing betweenVdc and -Vdc or Vdc and 0.
Dr. Ungku Anisa, July 2008 EEEB443 - Control & Drives 22
DC – DC Converter Fed DrivesOperation 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 23
ReferencesRashid, 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 24EEEB443 - Control & Drives