power electronics 2011
TRANSCRIPT
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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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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
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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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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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)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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For (/3)
)]3
cos(1[33
6/sin33/
1
max
6/5
6/max
a
a
V
dVVav
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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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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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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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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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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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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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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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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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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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THREE-PHASE INVERTER
Phase Bridge Inverter-Three
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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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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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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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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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Separately ExcitedMotors
Basic Characteristics of DC Motors
IfR
f
Vf
I
Ra
a
Vt
Ea
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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
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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
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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
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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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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
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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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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
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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
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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
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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
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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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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
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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
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122
Closed-loop control of dc drives
DC Motor
Speed sensing
ConverterSpeed
controller
+
-
Vr Ve Vc Va
TLPower supply
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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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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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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
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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
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Circuit arrangements
PWM
Inverter
6 step
Inverter
Controlled
rectifier
Diode
rectifier