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MODULE -2
Single-Phase Controlled
Rectifiers
1
INTRODUCTION TO CONTROLLED
RECTIFIERS


Controlled rectifiers are line commutated ac to
dc power converters which are used to convert a
fixed voltage, fixed frequency ac power supply
into variable dc output voltage.
Type of input: Fixed voltage, fixed frequency ac
power supply. Type of output: Variable dc output
voltage
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APPLICATIONS OF
PHASE CONTROLLED RECTIFIERS
☞DC motor control in steel mills, paper and
textile mills employing dc motor drives.
 ☞AC fed traction system using dc traction motor.
 ☞Electro-chemical and electro-metallurgical
processes.
 ☞Magnet power supplies.
 ☞Reactor controls.
 ☞Portable hand tool drives.
 ☞Variable speed industrial drives.
 ☞Battery charges.
 ☞High voltage DC transmission.
 ☞Uninterruptible power supply systems (UPS).

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DIFFERENT TYPES OF SINGLE
PHASE CONTROLLED RECTIFIERS
Single Phase Controlled rectifiers are further subdivided
into different types
☞Half wave controlled rectifier which uses a single
thyristor device (which provides output control only in one
half cycle of input ac supply, and it provides low dc output).
☞Full wave Controlled rectifiers (which provide
higher dc output)
☞Full wave controlled rectifier using a center
tapped transformer (which requires two thyristors).
☞Full wave bridge Controlled rectifiers (which do
not require a center tapped transformer)
☞Single phase semi-converter (half controlled bridge
converter, using two SCR’s and two diodes, to provide single
quadrant operation).
☞ Single Phase Full Converter (fully controlled
bridge converter which requires four SCR’s, to provide two
quadrant operations).

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THREE PHASECONTROLLED RECTIFIERS ARE
OF DIFFERENT TYPES
☞Three phase half wave controlled
rectifiers
 ☞Three phase full wave controlled
rectifiers
 ☞Semi converter (half controlled bridge
converter).
 ☞Full converter (fully controlled bridge
converter).

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INTRODUCTION

Diodes of rectifier circuits are replaced by thyristors.

In thyristor based rectifiers, output voltage can be
controlled. So they are termed as controlled rectifiers.

Controlled rectifiers produce variable DC output, whose
magnitude is varied by Phase control.
Phase Control
DC output from rectifier is controlled by controlling
duration of the conduction period by varying the point at
which gate signal is applied to SCR.

Main drawback of phase control is Radio Frequency
Interference (RFI)
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Controlled rectifiers are of two types,
1- Fully Controlled rectifiers

DC current is unidirectional, but DC voltage has
either
polarity. With one polarity, flow of power is
from AC
source to DC load---Rectification.
With the reversal of DC voltage by the load, flow of
power is from DC load to AC source---Inversion.
2- Half controlled rectifiers
Half of SCRs are replaced by diodes.
DC output current and voltage are unidirectional. i.e.,
flow of power is from AC source to DC load.
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HALF-WAVE CONTROLLED
RECTIFIERS
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With Resistive Load
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CALCULATION OF VOUT(AVG)
VO  dc   Vdc
1

2
2
v
O
.d  t ;
0
vO  Vm sin  t for  t   to 

VO  dc   Vdc
VO  dc 
1

2
1

Vm sin  t .d  t 

2 

 V
m
sin  t .d  t 
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
VO dc
VO dc
VO dc
VO dc
Vm

sin t.d t 

2 
Vm

2

 cos t





Vm
   cos  cos  ; cos  1
2
Vm
 1 cos  ; Vm  2VS
2
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Power Electronics
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Power Electronics
CONTROL CHARACTERISTIC
VO(dc)
Vdm
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0.6Vdm
0.2 Vdm
0
60
120
180
Trigger angle in degrees
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voltage with respect to Vdm , the
Vdc
Vn 
Vdm
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Normalized output voltage
Power Electronics
Normalizing the dc output
Vm
1  cos  
2


Vm

Vdc 1
Vn 
 1  cos    Vdcn
Vdm 2
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With an Inductive (RL) Load
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With Inductive Load and
Freewheeling Diode
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FULL-WAVE
CONTROLLED CENTERTAP RECTIFIERS
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With Resistive Load
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With an Inductive (RL) Load
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Control Characteristics for center-tap rectifier
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With Freewheeling Diode
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EXAMPLE 6.4
Explain with the help of waveforms the operation
of a full-wave center-tap rectifier with RL load for
the following firing angles:
(a) 0°
(b) 45°
(c) 90°
(d) 135°
(e) 180°
Assume highly Inductive Load
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Voltage and current waveforms for α=0°

During positive-half cycle of
source voltage, SCR1 is forward
biased and SCR2 is reverse
biased. During negative halfcycle, SCR2 is forward biased
and SCR1 is reverse biased. In
either case voltage across the
load is Vs.

Output is similar to
uncontrolled rectifier.

Each SCR conducts for 180° and
supplies current to the load for
this period
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Voltage and current waveforms for α=45°

Average DC output voltage
decreases.

If SCR1 is triggered at 45°,
SCR2 will conduct upto that
point, even though the source
voltage is zero, due to highly
inductive nature of load.

When SCR1 is turned on,
SCR2 is turned off.

Current to the load is
supplied by SCR1 and SCR2,
each conducting for 180°
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Voltage and current waveforms for α=90°

Average DC voltage is
zero, so there is no
transfer of power from AC
source to DC load.

Each SCR remains in
conduction for 180°

As firing angle is
increased from 0 to 90°,
the power supplied to
the DC load decreases,
becoming zero at α=90°
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Voltage and current waveforms for α=135°

Average DC voltage is
negative.

Load current still flows in
each SCR for 180° in its
original direction.

Load voltage has changed
polarity.

Power now flows from DC
load to AC source .

Circuit acts like an inverter.
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Voltage and current waveforms for α=180°

Average output DC
voltage is at its maximum
negative value.

SCRs remain in
conduction for 180°
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EXAMPLE 6.5
Show direction of power flow and operating
mode (rectifying or inversion) of center-tap
rectifier circuit with following firing angles:
A) α > 0°
B) α < 90°
C) α > 90°
D) α < 180°
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SOLUTION
 For
firing angle in the range 0° < α <
90°
1.
Average output voltage is positive.
2.
Converter operates in the rectifying mode.
3.
Power to the load is positive
4.
Power flow is from AC source to the DC load.

For firing angle in the range 90° < α < 180°
1.
Average output voltage is negative
2.
Converter operates in inversion mode
3.
Power to the load is negative
4.
Power flow is from DC load to AC source
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ASSIGNMENT # 2

In example 6.4, draw waveforms for voltage
across thyristor 2. i.e., VSCR2
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FULL-WAVE CONTROLLED
BRIDGE RECTIFIER
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With Resistive Load
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With an Inductive (RL) Load
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For L >>> R
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
1 
VO dc   Vdc 
  vO .d  t   ;
2  0

The o/p voltage waveform consists of two o/p
pulses during the input supply time period of
0 to 2 radians. Hence the Average or dc
o/p voltage can be calculated as
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2
Power Electronics
The average dc output voltage
can be determined by using the expression
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VO dc 
VO dc 
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VO dc 

2 
 Vdc 
  Vm sin  t.d  t  
2  

2Vm
 
 Vdc 
  cos  t 
2
2Vm
 Vdc 
cos 
Power Electronics
 

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Power Electronics
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By plotting VO(dc) versus ,
we obtain the control characteristic
of a single phase full wave fully
controlled bridge converter
(single phase full converter)
for constant & continuous
load current operation.
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Power Electronics
To plot the control characteristic of a
Single Phase Full Converter for constant
& continuous load current operation.
We use the equation for the average/ dc
output voltage
VO dc   Vdc 
2Vm

cos 
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38
Power Electronics
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Vdm
Power Electronics
Vdc  Vdm  cos 
VO(dc)
0.6Vdm
0.2 Vdm
0
-0.2Vdm

30
60
90
120
150
180
-0.6 Vdm
-Vdm
Trigger angle in degrees
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40
Power Electronics
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During the period from t =  to  the input voltage vS
and the input current iS are both positive and the
power flows from the supply to the load.
 The converter is said to be operated in the rectification
mode
Controlled Rectifier Operation
for 0 <  < 900

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Power Electronics
42
During the period from t =  to (+),
the input voltage vS is negative and the
input current iS is positive and the output
power becomes negative and there will be
reverse power flow from the load circuit to
the supply.
The converter is said to be operated in the
inversion mode.
Line Commutated Inverter Operation
for 900 <  < 1800
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Controlled Rectifier
Operation
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0< < 900
Power Electronics
TWO QUADRANT OPERATION
OF A SINGLE PHASE FULL CONVERTER
900< <1800
Line Commutated
Inverter Operation
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Control characteristics for bridge rectifier
44
With RL load and freewheeling diode
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HALF-CONTROLLED
OR
SEMICONTROLLED BRIDGE
RECTIFIERS
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

In fully-controlled rectifier, only rectification can
be obtained by connecting a freewheeling diode
across the output terminals of the rectifier.
Another method of obtaining rectification in
bridge rectifiers is replacing half of the SCRs
with diodes. These circuits are called
semicontrolled bridge rectifiers.
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Full-wave semicontrolled bridge
rectifier circuit
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Semicontrolled bridge rectifier with FWD
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DUAL CONVERTER
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Exercise Problems
6.1, 6.5, 6.6, 6.9, 6.12, 6.21, 6.22, 6.24
Also give analysis of waveforms in each case of all
above exercise problems