Power Electronics 2011

7/29/2019 Power Electronics 2011

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Electric Power and

Power Electronics

Part II- Power Electronics

2010-2011


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ReferencePower Electronics : Circuits, Devices and

Applications, M. H. Rashid, Prentice Hall,

Third Edition, 2004.


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Course notes

3


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IntroductionPower electronics may be defined as the applications ofsolid-

state electronics forcontrol and conversion of electrical power.

Power electronics are based primarily on the switching of the

power semiconductor devices.

Power electronics combine power, electronics and control.

Power electronics have already found an important place in

modern technology and are now used in a great variety of

high- power products, including heat controls, light controls,

motor controls, power supplies, and high voltage direct current

systems.


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Course ContentsThe following topics will be covered in this course:

1. Power Electronic Devices

2. Power Electronic Circuits

DC-DC converters

AC-AC converters

AC-DC converters

DC-AC converters

3. Power Electronic Application Uninterruptible power supply (UPS).

Motor speed control (Electrical Drives)


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Chapter 1

Power Electronic Devices


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Power Electronic devices Switching devices are common to all

power electronic circuits

These devices control current: Ideal

switch turn ON allow current flow with

no resistance and OFF no current flow,

much like valves control the flow of fluids.


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Ideal Switch

Vsw

iR

vs

vsw

ivt

+

- vs

R

vs

Switch is opened

Switch is closed


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Power Electronic DevicesThese devices can be divided broadly into threemain types:

1. Power diodes2. Transistors

Bipolar Junction Transistors (BJT)

Power MOSFETs

Insulated Gate Bipolar Transistors (IGBTs)

3. Thyristors

SCR, GTO, Triac
http://localhost/var/www/apps/conversion/tmp/scratch_4/PE_CH3_s3.ppthttp://localhost/var/www/apps/conversion/tmp/scratch_4/PE_CH3_s3.ppthttp://localhost/var/www/apps/conversion/tmp/scratch_4/PE_CH3_s3.ppthttp://localhost/var/www/apps/conversion/tmp/scratch_4/PE_CH3_s3.ppt


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1- Power DiodesGeneral characteristics: A passive switch

Single-quadrant switch:

can conduct positive on-state current can block negative off-state voltage

Conducts when its anode voltage is higherthan that of the cathode (VA> VC)

Forward voltage drop (when on) is very low(typically 0.5 and 1.2V)

If VC > VA the diode is said to be in Blockingmode.

off

on

i

Instantaneous

i-vcharacteristic

v

i+

v

_

Symbol

Anode

Cathode


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Stud-mounted type

Disk type


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General purpose

Rating up to 6000V, 4500A

High speed (or fast recovery)

Rating up to 6000V, 1100A

Reverse recovery time 0.1 to 5ms

Essential for high-frequency switching

Types of Power Diodes


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R

)sin( tVv sms

Lv

Li

A diode as a half-wave uncontrolled rectifiersv

t

0

smV

Lv

t


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Transistors-2

Bipolar Junction Transistor (BJT)-i

N

N

P

(C)

(B)

(E)

Collector

Emitter

Base

(C)

(B)

(E)


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Main Features of BJT Current controlled device

Highbase current must be present during the closingperiod

Can operate at high frequencies

High base losses

Available at a relatively low power rating in the

range of 400V, 250A. The driving circuit must be capable of producing a

large base current for as long as the transistor isclosed. Such a circuit is large, of low efficiency, and

complex to build.


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POWER MOSFET-ii

Off (Vgs=0)

On (Vgs>0)i

Instantaneousi-v

characteristic

v

i+

V

Symbol

Gate

Drain

Source

_

MOSFET M etal-OxideSemiconductorFieldEffectTransistor


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Main Features of MOSFETAn active switch controlled by terminal Gate

Voltage controlled device

Low gate losses

Typical switching frequencies are tens andhundreds of kHz

Available at a relatively low power rating inthe range of 1000V, 100A.


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iii- Insulated Gate Bipolar Transistor(IGBT)

Equivalent circuit

Symbol

(C)

(G)

(E)


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Easy to drive similar to MOSFET

Typical switching frequencies:

3 -30kHzcompared with MOSFET:

slower switching times,

lower on-resistance,

useful at higher power rating

(up to 1700V, 2400 A)

IC

VCE

VG2

VG3

VG1>VG2>VG3

GV = 0

Main Features of IGBT


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3-Thyristorsi- Silicon Controlled Rectifiers (SCR)

Equivalent circuit

Symbol

Cathode (K)

Gate (G)

Anode (A)

N

N N

N

P

P

P

P P

N

Anode (A)

Cathode (K)

Anode (A) Anode (A)

Cathode (K)Cathode (K)

IA IA

IA

Ic1

Ic2

Q1

Q1Q2

Q2

GateGate

IG


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The SCR: high voltage and current ratings (6500V,4200A)

low cost, passive turn-off transition.


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Closing Conditions of SCR

1. Positive anode to

cathode voltage

(VAK)

2. Gate pulse is applied

(Ig)

Anode (A)

Cathode (K)

Gate (G)

Closing angle isa


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Thyristor commutation techniques

Commutation is the process of turning off a thyristor. There are

many techniques to commutate a thyristor. However, these can

be broadly classified into two types:

1- Natural or line commutation:If the voltage source is ac, the thyristor current goes through a

natural zero, and a reverse voltage appears across the thyristor.

The device is then automatically turned off.

2- Forced commutation:If the voltage source is dc, the forward current of the thyristor is

forced to be zero by an additional circuitry called commutation

circuit to turn off the thyristor. The commutation circuit

normally consists of a capacitor, an inductor and one or more

thyristor(s) and/or diode(s).


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R

)sin( tVv sms

Lv

Li

A thyristor as a half-wave controlled rectifiersv

t

0

smV

Lv

ta2a

a t

ig

a2

=


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ii- Gate Turn-Off Thyristor (GTO) Thyristors are suitable for ac line operation

systems.

Thyristors are NOT suitable for dc line operationsystems because of the turn-off problems.

GTO is the solution, a GTO is an SCR fabricatedusing modern techniques.

Negative gate current is able to completelyreverse-bias the gate-cathode junction.

GTO requires positive current impulse at the gatefor turn-on and negative impulse for turn-off.


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GTO: General Characteristics Maximum controllable current (MCC) is

highest anode current that can be turned

off under gate control. GTO is designed for an allowable peak

current that is less than the MCC by a

safety factor.Symbol

Gate (G)

Anode (A)

Cathode (K)

Turn-on positive gate current pulse is higherthan that of a normal SCR.


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RLv

Li

Vs

Vs

Vs

ig

GTO

Lv

GTOturn-onandturn-off.


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The GTO: intermediate

ratings (less than SCR,

somewhat more than

IGBT).

Slower than IGBT.

Difficult to drive.A (200 V, 160 A) GTO


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iii-TRIACS

i

BVR

Thirdquadrant

First

quadrant

v

BVf

i-v characteristics

Gate

G

Equivalent circuit

Symbol

MT1

MT1

MT2

MT2


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sv

t0

smV

a

a

Lv

ta2

a t

ig

a

2

a

A triac as an ac voltage controller

R

)sin( tVv sms

Lv

Li

)sin( tVv sms

Lv

L

i

1T

2T

R


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Classification1. Uncontrolled turn on and turn off (e.g. diode)

2. Controlled turn on and uncontrolled turn off

(e.g. SCR)

3. Controlled turn on and off (e.g. BJT, MOSFET,IGBT, GTO)

4. Continuous gate signal requirement (e.g. BJT,MOSFET, IGBT)

5. Pulse gate requirement (e.g. SCR, GTO)


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Power Electronic

Circuits

Chapter 2


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The main taskof power electronics is to control and convert

electrical power from one form to another.

The four main forms of conversion are:

- DC-to DC conversion,

- AC-to-AC conversion,

- AC-to-DC conversion, and

- DC-to-AC conversion.

Power electronic circuits


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"Electronic Power Converter" is the term that is used to

refer to a power electronic circuit that converts voltage and

current from one form to another. These converters can be

classified as:1- Chopper that converts a dc voltage to another dc

voltage,

2- AC voltage controller converting an ac voltage to

another ac voltage,3- Rectifier converting an ac voltage to a dc voltage, and

4- Inverter converting a dc voltage to an ac voltage.


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Choppers-1

DC Conversion)-to-(DC

Choppers are widely used for traction motor control, marine

hoists, forklift trucks and mine haulers. They provide smooth

acceleration control, high efficiency and fast dynamic response.

They are used also as dc voltage regulators.The choppers can be step-down or step-up chopper.

1. Step-down (Buck) chopper:

where the output voltage of the chopper is lower than the

input voltage.2. Step-up (Boost) chopper:

where the output voltage is higher than the input voltage.


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down Chopper-Step

Timet

ton

VS

Vl

Timet

ton

I

VCE

VS

I

+

-

Vl

s

t

son

sav VKVt

dtVVon

0

1

tt

K is Chopper

duty cycle


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The average load voltage is:

(Where Kis the chopper duty cycle)

The power supplied to the load is:

Where Iav is the average load current.

The equivalent input resistance

sav VKV

avsavavload IVKIVP

K

R

R

VK

V

I

VR

s

s

av

seq

Chopper Performance Parameters

R

VK

R

VI savav


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The duty cycle k can be varied from 0 to 1 as follows:

1- Constant frequency operation:

The chopping frequency (or chopping periodt

) is keptconstant and the on-time ton is varied. The width of the

pulse is varied and this type of control is known as pulse

width modulation (PWM)

2- Variable frequency operation:

The chopping frequency is varied. Either on-time ton or

off-time toff is kept constant. This is called frequency

modulation.


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Example

msKt

oK

VKV

on

sav

0834.02.0417.0

417.12

5

t

?;5;12

)(5

onavs tVVVV

frequencyswitchingkHzf

msf

2.05000

11t

Solution

Step-down (Buck) chopper


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Step-Up

Chopper

TIL

dtdiLvL

When the chopper is turned on, the

voltage across the inductor is:

This gives the peak-to-peak

ripple current in the inductor

12 ss

s

iiIwhere

TkLVI

ia

iC

D

va

+

_VsC

L

a

b

ON OFF

is1

is2

is kT T

ic

is


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k

Vv

k

kV

kTT

ILVv

dtdiLVv

so

sso

so

1

)1

1()(

When the chopper is turned off

The average output voltage is:

1k

Vo

Vs

2Vs

0.5


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Example

mst

k

ok

k

VV

on

so

12.02.06.06.0

6.0

4.30

121

1

t

?;30;12

)(5

onostVVVV

frequencyswitchingkHzf

msf

2.05000

11t

Solution

Step-up (Boost) chopper


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2- AC to AC ConvertersTypes of Ac to Ac converters :

AC Voltage Controllers control the output

rms voltage using SCR-type switches.They are two types:

On-Off Control

Phase-angle Control

AC V lt C t ll


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AC Voltage Controllers

(AC Voltage Regulator)

LZ

)sin( tVv sms

Lv

Li

1T

2T

LZ)sin( tVv sms Lv

Li

Triac


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Off Control-On-i

periodoneduringcyclehalfofnumber:N

onswitchduringcycleshalfofnumber:n

R

)sin( tVv sms

Lv

Li

sv

t0

smV

n

Lv

t

N

N

nVV

rmsLrms

2

smV

N

n

2

smV

KK: is called the duty cycle


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Example

A single-phase ac voltage controller uses on-off control for heating a

resistive load of R = 4 and the rms input voltage is 240 V. If the

desired output power is 3.6 kW, determine the duty cycle K.

Solution:

25.0

5.0240

120

1204*36002

k

V

V

kVkV

RPVR

VP

s

L

sL

LLL

L

LL


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angle control-Phase-ii

2sin-2

2

)sin(12

aa

a

smLrms

smLrms

VV

dVV

1. Resistive Load

R

)sin( tVv sms

Lv

Li

t

sv

t0

smV

a

a

Lv

a2


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Example

A single-phase ac voltage controller uses phase control has a resistive

load of R = 5 and the input voltage vs = 170 sin 314t . For delay angle

= 90

a- Sketch the waveforms for the output voltage and output current.

b- Calculate the values of the rms output voltage, rms output current

and output power.

905170 aRVsm

WRIP

AR

VI

VVVb

Lrms

LrmsLrms

smLrms

14455.)17()(

175

85

85sin

2-2

21702sin-2

2

22

aa


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LZ

AvAi0

Bv

Cv

LZLZ

Bi0

Ci0

1

2

3

N

1T

2T

3T

LZ

AvAi0

Bv

Cv

LZLZ

Bi0

Ci0

1

2

3

N

1T

2T

3TN

AvAi0

Bv

Cv

Bi0

Ci0

1

2

3

1T

2T

3T

LZ

LZ

LZ

Av

Bv

Cv2T

1

32

1T

3T

LZ

LZ

LZ

3-PHASE AC REGULATORS


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3- Rectifiers

Rectifiers can be classified as controlledand uncontrolled rectifiers.

Uncontrolled rectifier circuits are built withdiodes only.

Controlled rectifiers can be further divided intosemi-controlled and fully-controlled rectifiers.Fully-controlled rectifier circuits are built withSCRs and semi-controlled rectifier circuits arebuilt with both diodes and SCRs .


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There are several rectifier circuit configurations. The popular

rectifier configurations are listed below:

- Single-phase half-wave uncontrolled rectifier,

- Single-phase full-wave uncontrolled rectifier,

- Three-phase half-wave uncontrolled rectifier,

- Three-phase full-wave uncontrolled rectifier,

- Single-phase half-wave controlled rectifier,- Single-phase full-wave controlled rectifier,

- Three-phase half-wave controlled rectifier,

- Three-phase full-wave controlled rectifier,


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wave uncontrolled-phase half-Single-1rectifier

R

vii

v

vvv

Vv

d

s

ss

sms

00

00if0

0if

)sin(

Resistive Load:

R

D

vs

id

+ vd -

Vo

+

_

0 0.005 0.01 0.015 0.02-400

-300

-200

-100

0

100

200

300

400

io

vo

vs

sm

sm

VdVV sin

2

1

0

0


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R-L Load:

L

D

vs

id

+ vd -

vo

+

_

R

0 0.005 0.01 0.015 0.02-400

-300

-200

-100

0

100

200

300

400

vo

io

vd

R L L d ith f h li di d


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R-L Load with freewheeling diode:

L

D1

vs

id

+ vd -

vo

+

_

R

D2

L

D1

vs

id

+ vd -

vo

+

_

R

D2

Mode 2

L

D1

vs

id

+ vd -

vo

+

_

R

D2

Mode 1

0 0.005 0.01 0.015 0.02-400

-300

-200

-100

0

100

200

300

400

vo

io

vd


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wave uncontrolled-phase full-Single-2rectifier

Resistive load: Mode 1: 0


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wave uncontrolled-phase half-Three-3

rectifier

D2va

n

id

R Vo

+

_

D1

D3vbn

vcn

Single-phase:

High output voltage ripple

Low ripple frequency (2fs)

Limitations

Limitations can be overcome or minimized using multiphase

(3f) input sources.


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0 30 270

-1.5

-1

-0.5

0

0.5

1

1.5

Angle o

Voltages

vo

vbn vcnvan

150

390D1 D2 D3

30 150 270 390 Angleo

2

33sin

2

3 6/5

6/0

smsm

VdVV


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wave uncontrolled-phase full-Three-4rectifier

opCN

o

pBN

pAN

Vv

Vv

Vv

240sin2

120sin2

sin2

v0

io

+

_

D1

D4

D3

D6

D5

D2

A

B

C

iAiB

iC

o

LLCA

o

LLBC

o

LLAB

Vv

Vv

Vv

210sin2

90sin2

30sin2

pLLVV 3

i


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io

v0+

_

D1

D4

D3

D6

D5

D2

A

BC

iA

iBiC

Mode 1:

CABCAB vvv &

D1 & D6 conduct

0ABv

)2/6/(,6/sin20 LLAB Vvv

-30 30 90 210 270 330-1.5

-1

-0.5

0

0.5

1

1.5

Angleo

Voltag

es

vo

vBC vCAvAB

15

0

390

D1D6


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io

v0+

_

D1

D4

D3

D6

D5

D2

A

B

C

iA

iBiC

Mode 2:

BCABCA vvv &

D1 & D2 conduct

0ACv

-30 30 90 210 270 330-1.5

-1

-0.5

0

0.5

1

1.5

Angle o

Voltages

vo

vBC vCAvAB

15

0

390

D1

D6

D2

D1


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-30 30 90 210 270 330-1.5

-1

-0.5

0

0.5

1

1.5

Angleo

Voltage

s

vo

vBC vCAvAB

150 390

D1D6

D3D2

D2D1

D4D3

D5D4

D6D5

- Therefore the output voltage v0 is periodical with a period of

60o as shown. (six-pulse)


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mspLL

LL

VVV

dVV

33

3

2323

6/sin23/

1 2/6/

0

-The average output voltage can be calculated over one

period from /6 to /2 (mode 1).


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wave controlled rectifier-phase half-Single-5

i vtvs

+

-

ta

vs

vti

)tsin(Vv maxs

a

a

2tfor0v

tfor)sin(Vv

t0for0v

t

maxt

t

t

2


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i

a

vs

a

tdtVVav )(sin2

1max

a

tdvtdvV stav2

1

2

12

0

)cos1(2max a

VVav

R

VI avav

t

vt


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)cos1(2max a

V

VavVav

a

max

V

2

Vmax

2


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1Example

A single phase, half wave SCR circuit is used to reduce the dc voltage

across a nonlinear resistance. The elements of the resistance changethe resistive value according to the following equation:

The voltage of the a c side is 110 V(rms). Calculate the dc current and

dc power of the resistance when the triggering angle is adjusted to 90o.

52.0 2dcVR


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VV

Vdc 75.24)]90cos(1[2

1102)cos1(

2

m

a

6.1275)75.24(2.052.0

22

dcVR

AR

VI dcdc 2.0

6.127

75.24

Solution:

WIVP dcdcdc 95.42.0*75.24*


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wave controlled rectifier-phase full-Single-6

S1 S3

i2

S4

vs

Ci1

vt

S2

C


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71

S1 S3

i2

S4

vs

D

Ci1

vt

S2

vti2

t

vs

a

vti1

+a


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)cos1()sin(11 max

max a

a

a

V

tdtVtdvV sav

vti2

t

vs

a

vti1

)cos1(max a

V

Vav

+a


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wave controlled-phase half-Three-7

rectifier

S2

van

id

Vo

+

_

S1

S3vbn

vcn

No delay a = 0


74/128

7430 150 270 390 Angle o

0 30 270-1.5

-1

-0.5

0

0.5

1

1.5

Angle o

Voltag

es

vo

vbn vcnvan

15

0

390

S1 S2 S3

Triggering

Delayed Triggering (/6)

1 5


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0 30+a 270 +a-1.5

-1

-0.5

0

0.5

1

1.5

Angle o

Volta

ges

vo

vbn vcnvan

150 +a 390 +a

S1 S2 S3a

30

Triggering

)cos(2

33sin

2

3 6/5

6/

a

a

a

sm

smav

VdVV


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For (/6)

)]

6

cos(1[

2

3sin

2

3

6/a

a

smsmav

VdVV

0 30+a 270 +a-1.5

-1

-0.5

0

0.5

1

1.5

Angle o

Voltages

vbn vcnvan

150 +a 390 +a

S1 S2 S3a

30

30+a 150 +a


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wave controlled-phase full-Three-8

rectifier

van

vbn

vcn

S1 S3 S5

S4 S6 S2

ZL

c

b

a

vL

+

-

3-phase AC/DC

1.5

v


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78

3-phase, AC/DC

Conversion

van

vbn

vcn

S1 S3 S5

S4 S6 S2

ZLvL

+

-

No delay a = 0

-30 30 90 210 270 330-1.5

-1

-0.5

0

0.5

1

Angleo

Volta

ges

vo

vBC vCAvAB

150 390

S1S6

S3S2

S2S1

S4S3

S5S4

S6S5

Triggering

S6S5

Delayed Triggering (/3)


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-30 30 90 210 270 330-1.5

-1

-0.5

0

0.5

1

1.5

Angleo

Voltages

vo

vBC vCAvAB

150 390

Triggering

a a a


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80

)6/36/(,6/sin3 max0 aa Vvv AB

The output voltage v0 is periodical with a period of 60o

The average output voltage can be calculated over one

period from /3+a to 2/3+a .

a

a

a

cos33

6/sin33/

1

max

6/3

6/max

V

dVVav


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81

For (/3)

)]3

cos(1[33

6/sin33/

1

max

6/5

6/max

a

a

V

dVVav


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82

For a three-phase full-wave controlled rectifier if the sourcevoltage is 208 V, calculate:

a- Maximum dc voltage across the load

b- The delay angle at which the dc voltage of the load

equals the peak phase voltage of the source

c- the dc load voltage for delay angle of 60

:2Example


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83

a

cos33

maxVVdc

Solution

VV

V Lph 1203

208

3

a- For maximum average voltage across the load a = 0

VVdc 69.280)0)(cos1202(33 b- The delay angle at which the average voltage of the load equals the

peak phase voltage of the source


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84

8.52

6046.033cos

cos33

maxmax

a

a

a

VVVdc

c- the average load voltage for delay angle of 60

V

VVdc

35.140

)60cos()1202(33

cos33

max

a

I t4


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85

Inverters-4)DC to AC Converters(

1. SINGLE-PHASE INVERTER

Converting a dc voltage to a single-phaseac voltage

2. THREE-PHASE INVERTER

Converting a dc voltage to a three-phaseac voltage

SINGLE PHASE INVERTER


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86

SINGLE-PHASE INVERTER

Half-Bridge

Full-Bridge

Vs/2

Vs/2

T1

T2

D1

D2

Load

Vs

T3

T2

D3

D2

Load

T1

T4

D1

D4

Bridge-Half


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87

Bridge-HalfResistive Load

Vs/2

-Vs/2

Vs/(2R)

v0

i0

T/2 T

s

s

sss

srms

VV

tnn

Vv

t

V

t

V

t

V

v

VV

45.0

)sin(2

)5sin(5

2

)3sin(3

2

)sin(

2

2

1

5,3,1

0

0

0

No even harmonics

Vs/2

Vs/2

T1

T2

D1

D2

Load

v0

i0


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88

lfundamentatheofrms

harmonicntheofrmsth

1 V

VHF nn

1

2

1

2

1

7,5,3

2

V

VV

V

V

THDo

n

Definitions:

Measure of closeness in shapebetween a waveform and its

fundamental.

(Harmonic Factor of nth harmonic)

(Total Harmonic Distortion)


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89

For half bridge

sss

sss

VVVVVV

tV

tV

tV

v

09.0,15.0,45.0

)5sin(5

2)3sin(

3

2)sin(

2

531

0

2.045.0

09.0

333.045.0

15.0

lfundamentatheofrmsharmonicntheofrms

1

55

1

33

th

1

s

s

s

s

nn

V

V

V

V

HF

V

V

V

VHF

VVHF

484.045.0

)45.05.0()(22

1

2

1

2

s

sso

V

VV

V

VVTHD

l ( hi hl ) d i d


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90

Purely (or highly) Inductive Load

Vs/2

-Vs/2

v0

i0

T/4 TT/23T/4

Imax

Imin

T1 OFF

T2 ONT1 ON

T2 OFF

D2 ON D1 ON

i0

v0

Quadrant 1

[0, T/4]

Quadrant 2

[T/4, T/2]

Quadrant 3

[T/2, 3T/4]

Quadrant 4

[3T/4, T]

Vs/2

Vs/2

T1

T2

D1

D2

Load

v0

i0


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91

Bridge-Full

Inductive Load

T1T2 D3D4 T3T4 D1D2T/4 T/2 3T/4 T

Same as half-bridge (Vs instead ofVs/2)

Vs

-Vs

v0i0

T/4 TT/23T/4

Imax

Imin T3T4T1T2 D3D4 D1D2

srms VV 0

Vs/2

Vs/2

T3

T2

D3

D2

Load

T1

T4

D1

D4

v0

i0


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92

THREE-PHASE INVERTER

Phase Bridge Inverter-Three


93/128

93

Power Electronic

Applications

Chapter 3


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94

1- Uninterruptible Power Supply (UPS).

It is used as standby ac source for critical loads. The UPS

configuration is as shown. The load is normally supplied from

the ac main supply and the rectifier maintains the full charge

of the battery. If the supply fails, the load is switched to theoutput of the inverter, which then takes over the main supply.

Rectifier

AC/DC

Inverter

DC/AC

Batteries

Critical

Load

Normally on

Normally off

Normally off

AC main

supply


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95

2- Electrical Drives

Electric drive systems involves controlling electric motors

using power electronic converters.

Motor Load

Command Signal

Controlunit

PowerSemiconductorconverter

Source

Sensingunit

M S d T


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96

Motors Speed-Torque

Characteristicsi) AC-Motors ii) DC-Motors

Separate

or shunt

Series

Compound

m

Tm

Induction Motorm

Tm

Tmax

Ts

m0 ms


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97

Speed Control of DC Motors

DC motors playa significant role in modern industrial drives.

DC motors have variable characteristics and are used extensivelyin variable-speed drives.

DC motors can provide a high starting torque and it is alsopossible to obtain speed control over a wide range.

The methods of speed control are normally simpler and lessexpensive than those of ac drives.

Both series and separately excited dc motors are normally used

in variable-speed drives, but series motors are employed fortraction applications.

Due to commutators, dc motors are not suitable for very highspeed applications and require more maintenance than do ac

motors.


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98

Separately ExcitedMotors

Basic Characteristics of DC Motors

IfR

f

Vf

I

Ra

a

Vt

Ea


99/128

99

aaat RIEV

f

f

fR

V=I

afd

fa

IIKT

IKE

daad TIEP

At steady-state & neglect saturation.

Ra, La

Ia

If

Rf

Vt

Vf

+

+

- -

Ea

Td,

Ia

+


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100

o

faat

fa

IKIRV

IKE

f

aa

f

t

IK

IR

IK

V

RaIf

Rf

Vt

Vf

+

+

--

Ea

,dT

afd IIKT df

a

f

t TIKR

IKV

2)(


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101

Speed-Torque CC of Separately Excited Motors

Torque

Speed

o

Tm

Load

Motor

Operating

Point

}

d

f

a

f

t TIK

R

IK

V2)(

o


102/128

102

SPEED CONTROL

aaatrIEV

mfa IKE

Since

So the speed of thed.c .motor can be controlled by controlling orVt

1- Armature Voltage Control

In this method If (i.e.f) is kept constant, and Vtis varied to change the speed.

Armature voltage control can control the speed

of the motor for speeds below rated speed but

not for speed above rated speed.

fa

aatm

IK

rIV


103/128

103

2- Field current control

In this method Vt remains fixed and

the speed is controlled by varying If .

This is normally achieved by using a

field rheostat as shown in the

following Figure for separately

excited d.c. motor. Field control can

control the speed of the motor for

speeds above base speed but not forspeeds below base speed.


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104

Operating modes


105/128

105

DC Drives

In Recent years, solid-state control have been used for

armature and field voltage control. Both can be achieved

using controlled rectifier or choppers.

DC drives can be classified in general into three types:1- Single phase drives

2- Three phase drives

3- DC-DC converter (chopper) drives

AC/DC

1f

3f

AC/DC

1f

3f

DC/DC

Arm.


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106

1- Single phase Drive

Single phase Drive may be subdivided into:

a) Single-phase half-wave converter drive

b) Single-phase full-wave converter drive

c) Single-phase duall converter drive


107/128


108/128

108

b- Single-phase full-wave converter drive:

aa

VV a cos

2max

ff

VV a

cos

2 max

Si l h d l t d i


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109

c- Single-phase dual converter drive:

1max

cos2

aa

VV a

12

2max cos

2

aa

aa

where

VV

aa

a

ff

VV a

cos

2 max

Quadrant

Va

Ia


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110

Example

A dc separately excited motor drives a constant

torque load of 18 NM. The motor is driven by a

full-wave converter through a 120 V ac supply.

Assume that K If= 2.5 and the armature resistance

is 2 . Calculate the triggering angle for the

motor to operate at 200 rev/min. The motor

current is continuous.


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111

Solution

83.51)cos(2120*2

76.66

)cos(2

76.662.7*236.52

2.75.2

18

36.52)60

2002(*5.2)

602(*

max

aa

a

VV

VIREV

AKI

TIIKIT

Vn

KIKIE

IREV

a

aaaa

f

aaf

ffa

aaaa


112/128


113/128

113

2- Three phase Drive

Three phase Drive may be subdivided into:

a) Three-phase half-wave converter drive

b) Three-phase full-wave converter drive

c) Three-phase duall converter drive


114/128

114

wave converter drive-phase half-Three-a

)cos(2

33 maxaa

VV a

S1

S2van

id

S3vbn

vcn

vaL

+

_

R

E

Rf

Vf

3-phase full-wave

converter

ff

VV a

cos33 max

3-phase ac supply


115/128

115

wave converter drive-phase full-Three-c

van

vbn

vcn

S1 S3 S5

S4 S6 S2

c

b

a

vaL

+

_

R

ERf

Vf

3-phase full-waveconverter

a b c

3-phase ac supply

ff

aa

VV

VV

a

a

cos33

cos33

max

max


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116

phase dual converter drive-Three-c

van

vbn

vcn

S1 S3 S5

S4 S6 S2

c

b

a

van

vbn

vcn

S4 S6 S2

S1 S3 S5

c

b

a

vaLa

+

_

Ra

Ea

RfVf

3-phase full-wave converter

a b c

3-phase ac supply2

max

1

max

cos33

cos

33

aa

aa

VV

V

V

a

a

ffV

V a cos33

max


117/128

117

DC converter (chopper) drives-DC-3

For 0 < t < kT Q1 is on

For kT < t < T Q1

is off, Ia

flows through Dm


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118

The average armature voltage is:

Where Kis the chopper duty cycle

The power supplied to the motor is:

Where Ia is the average armature current.

Assuming lossless chopper,

i.e. the average value of supply current is:

The equivalent input resistance

sa VKV

asaa IVKIVP 0

ssasi IVIVKPP 0

as IKI

a

s

s

seq

Ik

V

I

VR


119/128


120/128

120

Solution :

Ra = 1 , T (1/N)

Vs = 250 V

Chopper duty cycle K = 0.5

1

2

2

1

2

1

2

1

2

1

222

2

11

1

)(

200)(

2002508.0

80)0.1(45125

1252505.0

N

N

T

Tbut

I

I

Ik

Ik

T

T

IRIVE

VKVV

VRIVE

VKVV

a

a

a

a

aat

s

aat

s

f

f


121/128


122/128

122

Closed-loop control of dc drives

DC Motor

Speed sensing

ConverterSpeed

controller

+

-

Vr Ve Vc Va

TLPower supply


123/128

123

Speed Control of AC induction motors

Ac motors are lightweight, inexpensive, have low maintenancecompared with dc motors.

They required control of frequency, voltage, and current forvariable speed applications.

The power rectifiers, inverters, and ac voltage controllers can beused to meet the drive requirements. These power controllers

are complex, more expensive and require advanced feed-backcontrol techniques.

The advantages of ac drives outweigh the disadvantages.Therefore Ac drives are replacing dc drives and are used inmany industrial and domestic applications.

S d t l f i d ti


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124

The speed and the torque of induction

motors can be varied by one of the

following means:

1- Stator voltage control

2- Frequency control

Speed control of induction

motors


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125

iai

L

vA

N

T

1

T

4

vBNvC

N

T

3

T

6T

5

T

2

i

b

ic

NB

C

A

Inductionmotor

ac voltage controllerStator voltage control using-1

alpha = 100

TL

Speed control range


126/128


127/128

127

2- Stator Frequency Control

The induction motor speed is given by:

nm= (1-s) nswhere ns= 120 fs/P

fs= supply frequency

P= total no of poles


128/128

Circuit arrangements

PWM

Inverter

6 step

Inverter

Controlled

rectifier

Diode

rectifier

Power Electronics 2011

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Transcript of Power Electronics 2011