Controlled Rectifier

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2. By:
Kashan Ahmed Memon 14EL-02
Faraz Ahmed Jat 14EL-42
Abdul Shakoor Khuhro 14EL-26
Sajjad Ali Khaskheli 14EL-04
M.Ammar Ali Memon 14EL-86
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3.  A controlled rectifier is a circuit that is used
for converting AC supply into unidirectional
DC supply & can control the power fed to
the load. This process of
converting alternating current (AC) to direct
current (DC) is also called as controlled
rectification.
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4.  A silicon controlled rectifier is a
semiconductor device that acts as a true
electronic switch
 It can change AC into DC and at the same
time can control the amount of power fed to
the load.
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5. Leakage Current
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6.  Because it is silicon device and is used as
rectifier and that rectification can be
controlled.
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7.  4 layers
 3 terminals
◦ Anode, Cathode, Gate
 Combination of rectifier (pn) and transistor
(npn)
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9.  SCR is solid state equivalent of Thyratron
◦ 3 terminals (Grid, plate & cathode)
 Gate equivalent to grid
 Anode equivalent to plate
 Cathode equivalent to cathode of thyratron
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11.  When gate is open
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12.  When gate is positive with respect to cathode
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13.  Forward Characterstics
 Reverse Characterstics
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14.  1. Single Phase Half Wave Controlled Rectifier
with R Load
 2. Single Phase Half Wave Controlled Rectifier
with RL Load
 3. Single Phase Full Wave Controlled Rectifier
with R Load
 4. Single Phase Full Wave Controlled Rectifier
with RL Load
 5. Three Phase Controlled Rectifier
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15.  The basic function of single phase controlled
rectifier is to convert a single phase AC input
voltage to controllable DC output voltage
 Types
◦ Single Phase Half Wave Controlled Rectifier
◦ Single Phase full wave/Bridge Controlled Rectifier
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16.  Resistive R Load
 Resistive+Inductive RL Load
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17.  Consists of:-
o AC Supply
o Thyristor
o Load
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18.  The circuit consist of a thyristor T, a voltage source Vs and a
resistive load R.
 During the positive half cycle of the input voltage, the thyristor T is
forward biased but it does not conduct until a gate signal is applied
to it.
 When a gate pulse is given to the thyristor T at ωt = α, it gets
turned ON and begins to conduct.
 When the thyristor is ON, the input voltage is applied to the load.
 During the negative half cycle, the thyristor T gets reverse biased
and gets tuned OFF.
 So the load receives voltage only during the positive half cycle only.
The average value of output voltage can be varied by varying the
firing angle α.
 The waveform shows the plot of input voltage, gate current, output
voltage, output current and voltage across thyristor.
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20.  The circuit consist of a thyristor T, a voltage source
Vs, an inductive load L and a resistive load R.
 During the positive half cycle of the input voltage, the
thyristor T is forward biased but it does not conduct
until a gate signal is applied to it.
 When a gate pulse is given to the thyristor T at ωt =
α, it gets turned ON and begins to conduct.
 When the thyristor is ON, the input voltage is applied
to the load but due to the inductor present in the
load, the current through the load builds up slowly.
 During the negative half cycle, the thyristor T gets
reverse biased but the current through the thyristors
is not zero due to the inductor.
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21.  The current through the inductor slowly decays to
zero and when the load current (i.e the current
through the thyristor) falls below holding current, it
gets turned off.
 So here the thyristor will conduct for a few duration
in the negative half cycle and turns off at ωt = β. The
angle β is called extinction angle.
 The duration from α to β is called conduction angle.
 So the load receives voltage only during the positive
half cycle and for a small
 duration in negative half cycle.
 The average value of output voltage can be varied by
varying the firing angle α.
 The waveform shows the plot of input voltage, gate
current, output voltage, output current and voltage
across thyristor.
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22.  Resistive R Load
 Resistive Inductive (R-L) Load
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24.  INTRODUCTION:-
 3-Phase converters are 3-Phase controlled rectifiers.
 Used to convert AC input power into dc output power
across the load.
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25.  Operate from 3-Phase AC supply voltage.
 Provide Higher DC output voltage and Power.
 High output voltage ripple frequency
APPLICATION:
 Used in High Power Variable Speed Industrial
DC Drives
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26.  3-Phase Full Wave Controlled Rectifier
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28.  The 3-phase half wave converter combines three single phase half
wave controlled rectifiers in one single circuit feeding a common
load.
 The thyristor S1 in series with one of the supply phase windings ‘a-
n’ acts as one half wave controlled rectifier. The second thyristor S2
in series with the supply phase winding ‘b-n’ acts as the second half
wave controlled rectifier. The third thyristor S3 in series with the
supply phase winding acts as the third half wave controlled rectifier.
Figure shows three phase fully controlled rectifier.
 When thyristor S2 is triggered at ωt=(5π/6α), S1 becomes reverse
biased and turns-off. The load current flows through the thyristor
and through the supply phase winding ‘b-n’. When S2 conducts the
phase voltage vbnappears across the load until the thyristor S3 is
triggered.
 The 3-phase input supply is applied through the star connected
supply transformer as shown in the figure. The common neutral
point of the supply is connected to one end of the load while the
other end of the load connected to the common cathode point.
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29.  When the thyristor S1 is triggered at ωt=(π/6 + α)=(30° + α) , the phase
voltage Van appears across the load when S1 conducts. The load current
flows through the supply phase winding ‘a-n’ and through thyristor S1 as
long as S1 conducts.
 When the thyristor S3 is triggered at ωt=(3π/2 + α)=(270°+α) , S2 is
reversed biased and hence S2 turns-off. The phase voltage Van appears
across the load when S3 conducts.
 When S1 is triggered again at the beginning of the next input cycle the
thyristor S3 turns off as it is reverse biased naturally as soon as S1 is
triggered. The figure shows the 3-phase input supply voltages, the output
voltage which appears across the load, and the load current assuming a
constant and ripple free load current for a highly inductive load and the
current through the thyristor T1.
 For a purely resistive load where the load inductance ‘L = 0’ and the trigger
angle α >(π/6), the load current appears as discontinuous load current and
each thyristor is naturally commutated when the polarity of the
corresponding phase supply voltage reverses. The frequency of output ripple
frequency for a 3-phase half wave converter is fs, where fs is the input
supply frequency.The 3-phase half wave converter is not normally used in
practical converter systems because of the disadvantage that the supply
current waveforms contain dc components.
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30.  Power Control
 Switching
 Zero Voltage Switching
 Over-Voltage Protection
 Pulse Circuits
 Battery Charging Regulator
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A commonly used circuit for
controlling power in load RL using
two SCRs is shown in figure.
Potentiometer R controls the angle
of conduction of the two SCRs. The
greater the resistance of the pot,
lesser will be the voltage across
capacitors C1 and C2 and hence
smaller will be the time duration of
conduction of SCR1 and SCR2 during
a cycle

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Figure shows a circuit in which
two SCRs are used for making
and breaking an ac circuit. The
input voltage is alternating and
the trigger pulses are applied to
the gates of SCRs through the
control switch S. Resistance R is
provided in the gate circuit to
limit the gate current while
resistors R1 and R2 are to protect
the diodes D1 and D2 respectively

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 In some ac circuits it is
necessary to apply the
voltage to the load when the
instantaneous value of this
voltage is going through the
zero value. This is to avoid a
high rate of increase of
current in case of purely
resistive loads such as
lighting and furnace loads,
and thereby reduce the
generation of radio noise and
hot-spot temperatures in the
device carrying the load
current.

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
SCRs can be employed for
protecting other
equipment from over-
voltages owing to their
fast switching action. The
SCR employed for
protection is connected in
parallel with the load.
Whenever the voltage
exceeds a specified limit,
the gate of the SCR will
get energized and trigger
the SCR.

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 SCRs are used for producing
high voltage/current pulses
of desired waveform and
duration. The capacitor C is
charged during the positive
half cycle of the input
supply and the SCR is
triggered during the nega-
tive half-cycle

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
Diodes D1 and D2 are to
establish a full-wave
rectified signal across SCR1
and the 12 V battery to be
charged. When the battery
is in discharged condition,
SCR2 is in the off-state.
When the full-wave rectified
input is large enough to
give the required turn-on
gate current (controlled by
resistor R1), SCR1 will turn
on and the charging of the
battery will commence

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