EEEB2833 Electrical Machines & Drives

DC DrivesByDr. Ungku Anisa Ungku AmirulddinDepartment of Electrical Power EngineeringCollege of Engineering

Dr. Ungku Anisa, July 2008 1EEEB283 - Electrical Machines & Drives

OutlinePower Electronics Converters for DC DrivesControlled Rectifier Fed DC Drives

Single Phase Three Phase

DC – DC Converter Fed DrivesStep Down Class A ChopperStep Up Class B ChopperTwo-quadrant ControlFour-quadrant Control

Closed-loop Control (Brief overview)References

Dr. Ungku Anisa, July 2008 EEEB283 - Electrical Machines & Drives 2

Power Electronic Converters for DC DrivesTo obtain variable voltageEfficientIdeally losslessDepending on voltage source:

AC voltage source Controlled RectifiersFixed DC voltage source

DC-DC converters (switch mode converters)


Controlled Rectifier Fed DC DrivesTo obtain variable DC voltage from fixed AC sourceDC current flows in only 1 directionExample of a drive system


Controlled Rectifier Fed DC DrivesContains low frequency AC ripple

To reduce ripple: extra inductance added in series with La

Slow responseDiscontinuous current may occur if

La not large enoughMotor is lightly loadedHalf-wave rectifier is used

Effect of discontinuous currentRectifier output voltage increases motor speed increases

(poor speed regulation under open-loop operation)Dr. Ungku Anisa, July 2008 EEEB283 - Electrical Machines & Drives 5

Controlled Rectifier Fed – Single-phase DC Drives

Two-quadrant driveLimited to applications up to 15 kWDuring regeneration, Ea can be reversed by reversing field

excitation


T

Q1Q2

Q3 Q4


For continuous current:Armature voltage

where Vm = peak voltage

Armature current

Field voltage


am

a

VV

cos

2

fm

f

VV

cos

2

a

aaa R

EVI

90o 180o

mV2

mV2

Single-phasesupply

+

Va

ia


Four-quadrant driveConverter 1 for operation in 1st and 4th quadrantConverter 2 for operation in 2nd and 3rd quadrantLimited to applications up to 15 kW


T

Q1Q2

Q3 Q4

Converter 1 Converter 2

Single-phasesupply

Single-phasesupply

+

Va

ia


For continuous current:Armature voltage:

If Converter 1 operates

If Converter 2 operates

where Vm = peak voltage

Armature current

Field voltage Dr. Ungku Anisa, July 2008 9EEEB283 - Electrical Machines & Drives

1cos2

am

a

VV

fm

f

VV

cos

2

a

aaa R

EVI

2cos2

am

a

VV

12 aa +

Va


ia

Controlled Rectifier Fed – Three-phase DC Drives

Two-quadrant driveLimited to applications up to 1500 kWDuring regeneration, Ea can be reversed by reversing field

excitation


T

Q1Q2

Q3 Q4


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)Dr. Ungku Anisa, July 2008 EEEB283 - Electrical Machines & Drives 11

am

a

VV

cos

3 L,-L

fm

f

VV

cos

3 L,-L

a

aaa R

EVI

90o

mV L,-L3

mV L,-L3

180o

3-phasesupply

+

Va

ia


Four-quadrant driveConverter 1 for operation in 1st and 4th quadrantConverter 2 for operation in 2nd and 3rd quadrant


T

Q1Q2

Q3 Q4


3-phasesupply

3-phasesupply

+

Va

ia



1, cos

3a

ma

VV

LL

fm

f

VV

cos

3 L,-L

a

aaa R

EVI

2cos3

am

a

VV

L,-L

12 aa

Disadvantage:• Circulating

current• Slow

response

+

Va


ia


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


T

Q1Q2

Q3 Q4

M1

M2

R1

R2+ Va -

3-phasesupply

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 ChopperMotoring


T

Q1Q2

Q3 Q4

DC – DC Converter Fed Drives- Step Down Class A Chopper

S is ON (0 t ton)


Motoring

VEdt

diLiR aaaa

Duty Interval

- ia

DC – DC Converter Fed Drives- Step Down Class A Chopper

S if OFF (ton t T)


Motoring

0 Edt

diLiR aaaa

Freewheeling Interval

- ia

DC – DC Converter Fed- Step Down Class A ChopperMotoring

Duty cycle

Under steady-state conditionsMotor side: Chopper side:

Hence,


period chopper where TT

tk on

EIRVkV aaa

aa R

EkVI

kT

Freewheeling Interval

- ia

Duty Interval

- ia

EIRV aaa kVVa

average Va

average Ia

DC – DC Converter Fed Drives- Step Up Class B ChopperRegenerative Braking


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

DC – DC Converter Fed Drives- Step Up Class B Chopper

S is ON (0 t ton)


Regenerative Braking

Energy Storage Interval

- ia

Va = 0ia increases due to EMechanical energy

converted to electrical (i.e. generator)

Energy stored in La

Edt

diLiR aaaa

DC – DC Converter Fed Drives- Step Up Class B Chopper

S if OFF (ton t T)


Regenerative Braking

Duty Interval

- ia

ia flows through diode D and source V

Energy stored in La & energy supplied by machine are fed to the source EV

dt

diLiR aaaa

DC – DC Converter Fed Drives- Step Up Class B ChopperRegenerative Braking

Duty cycle

Under steady-state conditionsGenerator side: Chopper side:

Hence,


period chopper where TT

tk on

aaa IREVVk 1

a

a R

VkEI

1

T

Duty Interval

- ia

Energy Storage Interval

- ia aaa IREV VkVa 1

average Va

average Ia

DC – DC Converter Fed Drives- Two-quadrant ControlForward motoring Q1 - T1 and D2Forward braking Q2 – T2 and D1


D2

+Va

-

T1D1

T2

D2

+

V

-

T

Q1Q2

Q3 Q4

No Speed Reversal

DC – DC Converter Fed Drives- Two-quadrant ControlForward motoring Q1

T1 conducting: Va = V

D2 conducting: Va = 0


T

Q1Q2

Q3 Q4

T1

T2

D1

+

Va

-

D2

ia

+

V

T1

T2

D1

+

Va

-

D2

ia

+

V

•Average Va positive•Average Va made larger

than back emf Eb •Ia positive

Va Eb

DC – DC Converter Fed Drives- Two-quadrant ControlForward braking Q2

D1 conducting: Va = V

T2 conducting: Va = 0

Dr. Ungku Anisa, July 2008 EEEB283 - Electrical Machines & Drives

T

Q1Q2

Q3 Q4

T1

T2

D1

+

Va

-

D2

ia

+

V

T1

T2

D1

+

Va

-

D2

ia

+

Vdc

VaEb

•Average Va positive•Average Va made

smaller than back emf Eb

•Ia negative

26

DC – DC Converter Fed Drives- Four-quadrant ControlOperation in all quadrantsSpeed can be reversed

+ Va -

T1D1

T2D2

D3

D4

T3

T4

T

Q1Q2

Q3 Q4


DC – DC Converter Fed Drives- Four-quadrant ControlForward 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

T2D2

D3

D4

T3

T4

T

Q1Q2

Q3 Q4

+

V

-

T3 and T4 off


DC – DC Converter Fed Drives- Four-quadrant ControlReverse 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

T2D2

D3

D4

T3

T4

T

Q1Q2

Q3 Q4

+

V

-

T1 and T2 off


Closed-loop ControlFeedback loops may be provided to satisfy one or more of the

following:ProtectionEnhancement of speed responseImprove steady-state accuracy

Variables to be controlled in drives:Torque – achieved by controlling currentSpeedPosition

Controllers are designed based on a linear averaged model


Closed-loop ControlVariables to be controlled in drives:

Torque – achieved by controlling currentCommonly employed current sensor:

Current shunt – no electrical isolation, cheap Hall effect sensor – provides electrical isolation

Speed is governed by torque:


dt

dJTT Le

firingcircuit

currentcontroller

controlled rectifier

+

Va

–

vciref+

-

e.g. With phase-controlled rectifier


Speed – with or without current loopCommonly 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



Speed control without current loop: Simple implementation Current can be too high may damage converter


Speedcontroller

Power Electronic Converters

* +

-

+va

vc

Tacho


Speed control with current loop: Two controllers required: speed and current Current limited by limiting ia*


Speedcontroller

Power Electronic Converters

* +

-

+va

vc

Tacho

Currentcontroller

ia*

ia

+

-

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

EEEB2833 Electrical Machines & Drives