6 Chopper Controlled DC Drives

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)


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


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


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)


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)


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:


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


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


S is ON (0 t ton)


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


S if OFF (ton t T)


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:


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)


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.


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

-


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

-


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.


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


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.


Documents

6 Chopper Controlled DC Drives