Unit IV
• Single-Phase and Three-Phase Controlled Rectifiers:
• Introduction
• Phase Angle Control
• 1ϕ Full-Wave Half- and Full-Controlled Rectifiers
• 3ϕ Uncontrolled and Half-Controlled Bridge Rectifiers
• All Converters with R and RL Load
Phase Controlled Rectifiers
• Introduction: Rectification is a process of converting an alternating
current or voltage into a direct current or voltage.
• This conversion can be achieved by a variety of circuits based on and
using switching devices.
• The
widely
used
switching
devices
are
diodes,
thyristors,
power-transistors, power MOSFETs, etc.
• The rectifier circuits can be classified broadly into three classes:
uncontrolled, fully-controlled and half-controlled.
• An uncontrolled rectifier uses only diodes and the d.c. output voltage
is fixed in amplitude by the amplitude of the a.c. supply.
2
Rectifiers contd……
• The fully-controlled rectifier uses thyristors as the rectifying elements
and the d.c. output voltage is a function of the amplitude of the a.c.
supply voltage and the point-on-wave at which the thyristors are
triggered (called firing-angle α).
• The half-controlled rectifier contains a mixture of diodes and
thyristors, allowing a limited control over the d.c. output voltage-level
than the fully-controlled rectifier.
• The half-controlled rectifier is cheaper than a fully-controlled rectifier
of the same ratings but has operational limitations.
3
Rectifiers contd……
• Uncontrolled and half-controlled rectifiers will permit power to flow
only from the a.c. system to the d.c. load and are, therefore, referred to
as unidirectional converters.
• However, with a fully-controlled rectifier, it is possible, by control of
the point-on-wave at which switching takes place, to allow power to
be transferred from the d.c. side of the rectifier back into the a.c.
system.
• When this occurs, operation is said to be in the inverting mode.
• The fully controlled converter may, therefore, be referred to as a
bidirectional converter.
4
Phase Angle Control
• Figure shows the technique of controlled conversion from ac to dc for a
half-wave circuit which uses a unidirectional switch.
• When this switch S is turned-on (closed), it conducts current in the
direction of the arrow.
• The output voltage waveform depends on the switch control waveform
and the pulse-triggered switch, such as SCR and GTO or level-triggered
switch such as BJT, MOSFET and IGBTs.
5
Phase Angle Control contd……
• Current pulses are required for triggering SCRs and GTOs, whereas
voltage pulses are required for MOSFETs and IGBTs.
• Usually the source has an inductance, such as the ac line inductance
or leakage inductance of transformer.
• For analysis purpose, we neglect this inductance.
• In a.c. circuits, the SCR can be turned ‘on’ by the gate at any angle,
with respect to the applied voltage.
• This firing angle is measured with respect to a given reference, at
which the firing pulses are applied to the thyristor gates.
6
Phase Angle Control contd……
• The reference point is the point at which the application of the gate
pulses results in the maximum mean positive d.c.-terminal voltage of
which the converter is capable.
• In other words, a firing-angle of 0° corresponds to the conditions
when each thyristor in the circuit is fired at the instant its anode
voltage-first becomes positive in each cycle.
• Under this condition, therefore, the converter operates in exactly the
same manner as if it was an uncontrolled rectifier circuit.
• The ‘α’ is the symbol for the firing angle.
7
Phase Angle Control contd……
• Hence, the most efficient method to control the turning “on” of a
thyristor is achieved by varying the firing-angle of thyristor.
• Such a method of control is called as phase-angle control.
• This phase-angle control is a highly efficient means of controlling the
average-power to loads such as lamps, heaters, motors, d.c. supplies,
etc.
• Those converters, which work on the principle of natural
commutation, are called ‘line-commutated converters’.
8
Phase Angle Control contd……
• Figure shows various waveforms obtained using phase-angle control
for half-wave-controlled converter circuit.
9
Phase Angle Control contd……
• In a pulse-triggered switch, the falling edge of the control pulse
(VC) lies at an angle α + δ, as shown in figure (c), where δ is a
short angle.
• Once the switch is turned-on, it remains ON even if the
triggering pulse has subsided.
• It can be turned-off only when the current through it is reduced
below the holding current (in case of SCR).
• In a level triggered switch, the falling edge lies at an angle π, as
shown in figure (d).
10
1Φ Half-Controlled FWR
• The two different versions of half-controlled bridge-circuit with
resistive load are shown in the figure.
• Half-controlled converters are also known as semi-converter.
11
1Φ Half-Controlled FWR contd……
• In a symmetrical configuration, the cathodes of two SCRs are at the same
potential so their gates can be connected and a single gate-pulse can be
used for triggering either SCR.
• The SCR, which is forward-biased at the instant of firing, will turn-on.
• In asymmetrical configuration, separate-triggering circuits are to be used.
• Now,
consider
the
symmetrical
configuration
of
half-controlled
bridge-circuit.
• During the positive half-cycle of the a.c. supply, thyristor T1 and diode D1
are forward-biased and are in the forward-blocking mode.
12
1Φ Half-Controlled FWR contd……
• The different-waveforms of symmetrical converter-circuit is shown in
the figure.
13
1Φ Half-Controlled FWR contd……
• When the SCR T1 is triggered, at a firing-angle α, the current flow
through the path L - T1 – R - D1 - N.
• As shown in the figure, the load-current will flow until it is
commutated by reversal of supply voltage at ωt = π.
• During the negative half-cycle of the a.c. supply, thyristor T2 and
diode D2 are forward-biased.
• When SCR T2 is triggered at an angle (π + α), the current would flow
through the path N - T2 – A – R – B - D2 - L.
• This current is continuous till angle 2π, when SCR T2 is turned-off.
14
1Φ Half-Controlled FWR contd……
• The voltage and current-relations are derived as follows:
• Average d.c. load-voltage:
• Average load-current:
15
1Φ Half-Controlled FWR contd……
• RMS load-voltage: The RMS load voltage for a given firing angle α
is given by
16
1Φ Half-Controlled FWR contd……
• Half Controlled Bridge Rectifier
with R-L Load: Figure shows
half-controlled
bridge
converter
(symmetrical configuration) with
inductive
load,
corresponding
waveforms.
17
1Φ Half-Controlled FWR contd……
• Consider the symmetrical circuit configuration.
• As shown in the figure, thyristor T1 is turned-on at a firing angle α in each
positive half-cycle.
• From this instant α, supply voltage appears across output terminals AB,
through thyristor T1 and diode D1.
• Current flows through the path L - T1 – A – L – R – B - D1 - N.
• Here, the filter inductance L is assumed to be sufficiently large as to
produce continuous load current.
• This current Id is taken to be constant.
• Hence, during positive half-cycle, thyristor T1 and diode D1 conduct.
18
1Φ Half-Controlled FWR contd……
• Now, when the supply voltage reverses at ωt = π, the diode D2 is
forward biased since diode D1 is already conducting.
• The diode D2 then turns ON, and the load current passes through D2
and T1.
• The supply voltage reverse-biases D1 and turns it off.
• Thus, the load-current freewheels through the path R – B - D2 - T1 – A
- L during the interval from π to (π + α) in each supply-cycle.
• During the negative half-cycle, at the instant (π + α), a
triggering-pulse is applied to the forward-biased thyristor T2.
19
1Φ Half-Controlled FWR contd……
• Thyristor T2 is turned ON.
• As thyristor T2 is turned ON, the supply voltage reverse-biases T1 and
then turns it OFF by the line commutation.
• Therefore, the load-current flows through T2 and D2, the above-cycle
repeats and the waveforms obtained are as shown in the figure.
• Here, we have seen that the conduction period of thyristors and diodes
are equal, therefore this circuit is called as the symmetrical
configuration.
20
1Φ Half-Controlled FWR contd……
• Figure shows half-controlled
bridge converter (asymmetric
configuration) with inductive
load, corresponding waveforms.
21
1Φ Half-Controlled FWR contd……
• Now consider the asymmetric configuration.
• During the positive half-cycle of the a.c.-supply, thyristor T1 and
diode D1 are forward-biased.
• As shown, thyristor T1 is turned ON at firing angle α.
• Current flows through the path L - T1 – A – L – R – B - D1 - N.
• Hence, T1 and D1 conduct from α to π.
• Similarly, T2 and D2 conduct from (π + α) to 2π in each negative
half-cycle of the a.c. supply.
• The freewheeling action is provided by diodes D1 and D2 from 0 to α
and from π to (π + α) in each supply cycle.
22
1Φ Half-Controlled FWR contd……
• In this converter configuration, the conduction periods of
thyristors and diodes are unequal.
• Hence, this circuit configuration is known as the asymmetrical
configuration.
• We have seen that, the thyristor conducts for a longer interval in
the symmetrical circuit configuration.
• Therefore, the thyristors used in this circuit must have a higher
average
current-rating
compared
to
those
in
the
asymmetrical-circuit configuration.
23
1Φ Half-Controlled FWR contd……
• The average d.c. output voltage, neglecting diode and SCR drop, is
given by
• In comparing the operation of the half-controlled 2-pulse circuit with
that of the fully-controlled circuit, the following points are evident:
• Since half the thyristors are replaced by diodes, a half-controlled
converter costs less than a fully-controlled converter.
24
1Φ Half-Controlled FWR contd……
• The periods of the negative-voltage “swing” at the d.c. terminals obtained
with fully-controlled bridge-circuit are replaced by “freewheeling”
periods of zero voltage in the half-controlled circuit.
• The elimination of the negative swings of voltage at the d.c. terminals is
advantageous because it results in a reduction of the ripple voltage, with
correspondingly reduced filtering requirements.
• In half-controlled bridge-circuit, the average d.c. terminal voltage can be
continuously controlled from maximum to virtually zero with an
increased control range of the firing angle α.
25
1Φ Half-Controlled FWR contd……
• Due
to
the
bridge-circuit,
freewheeling
power
action
factor
with
is
half-controlled
improved
in
half-controlled-converters.
• The a.c. supply-current is more distorted due to its zero periods
with half-controlled circuit, compared to fully-controlled
bridge-circuit.
• Half-controlled converters are widely used in main line a.c.
traction where large d.c. motors are supplied from a single-phase
a.c. supply.
26
1Φ Full-Wave Controlled Rectifier
• There are two basic configurations of full-wave controlled
rectifiers.
• They are:
– Mid-point converters (M-2 Connection)
– Bridge converters (B-2 Connection)
• Mid-point Converters (M-2 Connection):
• In a single phase full-wave controlled-rectifier circuit with
mid-point configuration, two SCRs and a single-phase-transformer
with centre-tapped secondary windings are employed.
27
1Φ Full-Wave Controlled Rectifier contd……
• These converters are also referred to as two pulse converters, as
two triggering pulses or two sets of triggering pulses are to be
generated during every cycle of the supply, to trigger the various
SCRs.
• Single-phase full-wave circuit with transformer mid-point
configuration are generally used for rectifiers of low ratings.
28
1Φ Full-Wave Controlled Rectifier contd……
• With Resistive Load: Figure illustrates a 2-pulse mid-point converter
circuit with resistive-load.
29
1Φ Full-Wave Controlled Rectifier contd……
• This type of full-wave rectifier circuit uses two SCRs connected to the
centre-tapped secondary of a transformer, as shown.
• The input signal is coupled through the transformer to the
centre-tapped secondary.
• During the positive half-cycle of the a.c. supply, i.e., when terminal A
of the transformer is positive with respect to terminal B, or the
secondary-winding terminal A is positive with respect to N, SCR1
(T1) is forward-biased and SCR2 (T2) is reverse-biased.
30
1Φ Full-Wave Controlled Rectifier contd……
• Since no triggering pulses are given to the gates of the SCRs, initially
they are in off-state.
• When SCR1 is triggered at a firing-angle α, current would flow from
terminal A through SCR1, the resistive load R and back to the
centre-tap of the transformer (i.e., terminal N).
• This current continuous to flow up to angle π when the line voltage
reverses its polarity and SCR1 is turned-off.
• Depending upon the value of α and the load circuit parameters, the
conduction angle of SCR1 may be any value between 0 and π.
31
1Φ Full-Wave Controlled Rectifier contd……
• During the negative half-cycle of the a.c. supply, the terminal B of the
transformer is positive with respect to N.
• SCR2 is forward-biased.
• When SCR2 is triggered at an angle (π + α), current would flow from
terminal B, through SCR2, the resistive load and back to centre-tap of
the transformer.
• This current continues till angle 2 π, then SCR2 is turned off.
• Here, it is assumed that both thyristors are triggered with the same
firing angle, hence they share the load current equally.
32
1Φ Full-Wave Controlled Rectifier contd……
• Each half of the input-wave is applied across the load.
• Thus, across the load, there are two pulses of current in the same
direction.
• Hence the ripple frequency across the load is twice that of the input
supply frequency.
• The voltage and current waveforms of this configuration are shown in
the next figure.
• It is clear from the figure that with purely resistive load, the load
current is always discontinuous.
33
1Φ Full-Wave Controlled Rectifier contd……
34
1Φ Full-Wave Controlled Rectifier contd……
• Average d.c. Output Voltage: The output d.c. voltage, Edc, across
the resistive load is given by
• Average-load Current: The average-load current is given by
35
1Φ Full-Wave Controlled Rectifier contd……
• RMS Load-voltage: The RMS load-voltage for a given firing angle α
is given by
36
1Φ Full-Wave Controlled Rectifier contd……
• With inductive Load: The circuit diagram of the single-phase
full-wave, or bi-phase half-wave controlled rectifier with R-L load is
shown in the figure.
37
1Φ Full-Wave Controlled Rectifier contd……
• The various voltage and current waveforms are shown.
38
1Φ Full-Wave Controlled Rectifier contd……
• With reference to the circuit diagram, thyristor T1 can be fired into the
on-state at any time after e1 goes positive.
• Once thyristor T1 is turned-on, current builds up in the inductive load,
maintaining thyristor T1 in the on-state up to the period when e1 goes
negative.
• However, once e1 goes negative, e2 becomes positive, and the firing of
thyristor T2 immediately turns on thyristor T2 which takes up the load
current, placing a reverse voltage on thyristor T1, its current being
commutated (transferred) to thyristor T2.
• The thyristor voltage, VT , waveform shows that it can be fired into
conduction at anytime when VT is positive.
39
1Φ Full-Wave Controlled Rectifier contd……
• The peak reverse (and forward) voltage that appears across the thyristor is
2Em, that is, the maximum value of the complete transformer secondary
voltage.
• The load-current may be continuous or discontinuous, depending on the
inductance value.
• The load current is continuous if inductance value is greater than its critical
value.
• It is discontinuous if inductance value is less than its critical value.
• The analysis given here assumes that the inductance is sufficiently large, so
that each thyristor conducts for a period of 180° (conduction of current is
continuous).
40
1Φ Full-Wave Controlled Rectifier contd……
• Also, both thyristors are triggered with the same delay angle, hence
they share the load current equally.
• As shown in waveforms, due to large inductance in the circuit and
continuous current conduction, the thyristors continue to conduct even
when their anode voltages are negative with respect to the cathode.
• The load current is shown to be constant d.c.
• Now, the output d.c. voltage Edc can be obtained as
41
1Φ Full-Wave Controlled Rectifier contd……
• Some conclusions have been made from this equation– The highest value of this voltage will be when the firing angle is
zero, i.e., α = 0°.
– This voltage is zero when α = 90°.
• Meaning that, the load voltage will contain equal positive and
negative areas, giving zero output voltage.
– This voltage is negative maximum when α = 180°.
42
1Φ Full-Wave Controlled Rectifier contd……
• The circuit waveforms for α = 30° are shown.
43
1Φ Full-Wave Controlled Rectifier contd……
• The circuit waveforms for α = 90° are shown.
44
1Φ Full-Wave Controlled Rectifier contd……
• The circuit waveforms for α = 150° are shown.
45
1Φ Full-Wave Controlled Rectifier contd……
• The following points can be observed from these curves:
– For α = 30°, more power is fed into the load than is received
back into the supply.
– As firing-angle α increases, the average d.c. output voltage
decreases.
– For α = 90°, the load voltage contains equal positive and
negative areas, giving zero output-voltage.
– For firing-angle α > 90°, the d.c. voltage goes negative.
46
1Φ Full-Wave Controlled Rectifier contd……
– At α = 180°, the negative d.c. voltage is maximum.
– The period for which a thyristor is reverse-biased reduces
as the firing angle increases to 180°.
– To turn OFF a thyristor, it must be reverse biased for
greater than its turn-off time.
– Therefore, the maximum firing-angle must always be less
than 180°.
47
1Φ Full-Wave Controlled Rectifier contd……
• Effect of Freewheeling Diode: Figure shows the full-wave centre tap
phase-controlled
thyristor-circuit
with
inductive
load
and
freewheeling diode.
48
1Φ Full-Wave Controlled Rectifier contd……
• The load voltage and current-waveforms are shown.
49
1Φ Full-Wave Controlled Rectifier contd……
• As shown in the figure, the thyristors are triggered at angle α.
• The variable d.c. voltage at the load is obtained by varying this firing
angle α.
• From the same figure, it is also clear that as the supply voltage goes
through zero at 180°, the load voltage cannot be negative since the
freewheeling diode, Df, starts conducting and clamps the load voltage
to zero volts.
• A constant load current is maintained by freewheeling current through
the diode.
50
1Φ Full-Wave Controlled Rectifier contd……
• The conduction period of thyristors and diode is also shown in the
figure.
• The stored-energy in the inductive load circulates current through the
feedback-diode in the direction shown in the figure.
• The rate of decay of this current depends upon the time-constant of
the load.
• The average d.c. output voltage can be calculated as
51
1Φ Full-Wave Controlled Rectifier contd……
• The d.c. load-current is given by
• It is also observed from the figure, that the freewheeling diode, D F,
carries the load-current during the firing-angle α when the thyristors
are not conducting.
• Hence, the current through the diode DF is given by
52
1Φ Full-Wave Controlled Rectifier contd……
• Bridge-Configurations (B-2 Connection): An alternative-circuit
arrangement of a two-quadrant converter, operating from a
single-phase supply, is a fully controlled bridge-circuit as shown in
the figure.
53
1Φ Full-Wave Controlled Rectifier contd……
• In the bridge circuit, diagonally opposite pair of thyristors are made to
conduct, and are commutated, simultaneously.
• The related voltage and current waveforms for this circuit are shown
in the figure.
54
1Φ Full-Wave Controlled Rectifier contd……
• During the first positive half-cycle, SCRs T1 and T2 are forward
biased and if they are triggered simultaneously, then current flows
through the path L - T1 – R - T2 - N.
• Hence, in the positive half cycle, thyristors T1 and T2 are conducting.
• During the negative half-cycle of the a.c. input, SCRs T3 and T4 are
forward biased and if they are triggered simultaneously, current flows
through the path N - T3 – R - T4 - L.
55
1Φ Full-Wave Controlled Rectifier contd……
• Thyristors T1, T2 and T3, T4 are triggered at the same firing angle α in
each positive and negative half-cycles of the supply voltage,
respectively.
• When the supply voltage falls to zero, the current also goes to zero.
• Hence thyristors T1, T2 in positive half-cycle and T3, T4 in negative
half-cycle turn-off by natural commutation.
• The relations for Edc, Idc and Erms for this bridge configuration is
similar to the Mid-point Converter equations.
56
1Φ Full-Wave Controlled Rectifier contd……
• Fully Controlled Bridge Circuit with Inductive Load [R–L Load]:
The single phase fully controlled bridge circuit with R-L load is
shown in the figure.
57
1Φ Full-Wave Controlled Rectifier contd……
• Conduction does not takes place until the thyristors are fired and, in
order for current to flow, thyristors T1 and T2 must be fired together,
as must thyristor T3 and T4 in the next half-cycle.
• To ensuring simultaneous firing, both thyristors T1 and T2 are fired
from the same firing circuit.
• Inductance L is used in the circuit to reduce the ripple.
• A large value of L will result in a continuous steady current in the
load.
• A small value of L will produce a discontinuous load current for
large-firing angles.
58
1Φ Full-Wave Controlled Rectifier contd……
• The waveforms with two different firing-angles (α = 60º & 135º
respectively) are shown.
59
1Φ Full-Wave Controlled Rectifier contd……
• The voltage waveform at the d.c. terminals comprises a steady d.c.
component on to which is superimposed an a.c. ripple component,
having a fundamental frequency equal to twice that of a.c. supply.
• The input line-current has a square waveform of amplitude Id, and the
fundamental component of this waveform is in phase with the
input-voltage.
• As shown in figure, at firing angle α = 60°, thyristors T1 and T2 are
triggered.
• Current flows through the path L - T1 – A – L – R – B - T2 - N.
60
1Φ Full-Wave Controlled Rectifier contd……
• Supply voltage from this instant appears across output terminal and forces
the current through load.
• This load-current, Id , is assumed to be constant.
• This current also flows through the supply and the direction is from line to
neutral, which is taken positive, along with the applied voltage.
• Now, at instant π, voltage reverses.
• However, because of very large inductance L, the current is maintained in
the same direction at constant magnitude Id which keeps the thyristors T1
and T2 in conducting state and hence, the negative supply voltage appears
across output terminals.
61
1Φ Full-Wave Controlled Rectifier contd……
• At an angle π + α, thyristors T3 and T4 are fired.
• With this, the negative line voltage reverse biases thyristors T1
through T3, and T2 through T4 of commutating thyristors T1 and T2.
• The current flows through the path N - T3 – A – L – R – B - T4 - L.
• This continues in every half cycle and we get the output voltage as
shown in the figure.
• As shown, the line current is positive when T1, T2 are conducting and
negative when T3, T4 are conducting.
62
1Φ Full-Wave Controlled Rectifier contd……
• The average output d.c. voltage can be obtained as
• By controlling the phase-angle of firing pulses, applied to the gates of
the thyristors in the range 0°-180°, the average-value of the d.c.
voltage can be varied continuously from positive maximum to
negative maximum, assuming continuous current flow at the d.c.
terminals.
63
1Φ Full-Wave Controlled Rectifier contd……
• Because the average d.c. voltage is reversible even though the current
flow in the d.c. terminals is unidirectional, the power-flow in the
converter can be in either direction.
• Hence two modes of operation are possible with fully controlled
single-phase bridge circuit.
• Mode 1, Rectifying Mode: During the interval α to π, both supply
voltage Es and supply-current Is are positive;
– power, therefore, flows from a.c. source to load.
64
1Φ Full-Wave Controlled Rectifier contd……
• During the interval (π to π + α), Es is negative but Is is positive, the
load therefore returns some of its energy to the supply system.
• But the net power flow is from a.c. source to d.c. load because (π - α)
> α.
• Also above equation shows that if α < 90°, the voltage at the d.c.
terminals is positive, therefore, the power flows from a.c. side to d.c.
side and the converter operates as a rectifier.
• Mode 2, Inverting Mode: In the figure, the firing pulses are retarded
by an angle of 135°.
65
1Φ Full-Wave Controlled Rectifier contd……
• The d.c. terminal voltage waveforms now contains a mean negative
component, and the fundamental component of the a.c. line-current
waveforms lags the voltage by an angle of 135°.
• Since the mean d.c. terminal voltage is negative [α > 90°], the d.c. power,
and hence also the mean a.c. power, must also be negative.
• In other words, power is now being delivered from the d.c. side of the
converter to the a.c. side, and the converter is operating as a
“line-commutated inverter”.
• This is confirmed by the fact that the a.c. line current is displaced from the
a.c. voltage by an angle greater than 90°, which indicates that a mean
component of power is being delivered to the a.c. side of the circuit.
66
1Φ Full-Wave Controlled Rectifier contd……
• In order to achieve this situation in practice, it is necessary for a
source of d.c. voltage E whose voltage equals to the average d.c.
voltage (negative) of the converter must be connected in the output, as
shown in the figure.
67
1Φ Full-Wave Controlled Rectifier contd……
• It is this source of external voltage which drives the direct-current into
the converter against the “counter” voltage produced at its d.c.
terminals.
• If this d.c. voltage-source is not connected, the conduction will cease
somewhere before the angle (180° + α).
• Such an operation is used in the regenerative breaking mode of d.c.
drives and in high-voltage direct-current (HVdc) transmission.
• As both the types of phase-controlled converters have been studied,
the advantages of single-phase bridge converter over single-phase
mid-point converter can now be stated:
68
1Φ Full-Wave Controlled Rectifier contd……
• SCRs are subjected to a peak-inverse voltage of 2 Em in mid-point
converter and Em in fully controlled-bridge converter.
– Hence, for the same voltage and current ratings of SCRs, power
handled by mid-point configuration is about half of that handled by
bridge configuration.
• In mid-point configuration, each secondary should be able to supply
the load-power.
– As such, the transformer rating in mid-point converter is double
the load-rating.
– This, however, is not the case in the single-phase bridge converter.
69
1Φ Full-Wave Controlled Rectifier contd……
• Hence, bridge configuration is preferred over mid-point configuration.
• However, the choice between these two types depends primarily on
cost of the various components, available source voltage and the
load-voltage requirements.
• Midpoint converter is used in case the terminals on d.c. side have to
be grounded.
70
3Φ Uncontrolled Bridge Rectifier
• A three-phase bridge rectifier is commonly used in high-power
applications.
71
3Φ Uncontrolled Bridge Rectifier contd……
• This is a full-wave rectifier.
• It can operate with or without a transformer and gives six-pulse ripples on
the output voltage.
• The diodes are numbered in order of conduction sequences and each one
conducts for 120°.
• The conduction sequence for diodes is D1 - D2, D3 - D2, D3 - D4, D5 - D4,
D5 - D6, and D1 - D6.
• The pair of diodes which are connected between that pair of supply lines
having the highest amount of instantaneous line-to-line voltage will
conduct.
• The line-to-line voltage is √3 times the phase voltage of a three-phase
Y-connected source.
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3Φ Uncontrolled Bridge Rectifier contd……
• The waveforms and conduction times of diodes are shown in the
figure.
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3Φ Uncontrolled Bridge Rectifier contd……
• If Vm is the peak value of the phase voltage, then the instantaneous
phase voltages are given by
• Because the line–line voltage leads the phase voltage by 30°, the
instantaneous line–line voltages are given by
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3Φ Uncontrolled Bridge Rectifier contd……
• The average output voltage is found from
where Vm is the peak phase voltage.
• The rms output voltage is
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3Φ Uncontrolled Bridge Rectifier contd……
• If the load is purely resistive, the peak current through a diode is Im = √3
Vm/R and the rms value of the diode current is
and the rms value of the transformer secondary current,
where Im is the peak secondary line current.
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3Φ Uncontrolled Bridge Rectifier contd……
• For a three-phase rectifier q = 6, the instantaneous output voltage is
given as
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3Φ Half Controlled Bridge Rectifier
• Circuit of three-phase half-controlled bridge converter contains three
thyristors in three arms and diodes in the other three arms.
• The circuit diagram of the three-phase symmetrical-half-controlled
bridge is shown in the figure.
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3Φ Half Controlled Bridge Rectifier contd……
• Here the asymmetrical configuration is not used, because it introduces
imbalance in line-currents on the a.c. side.
• The circuit can be looked at as a three-phase, half-wave diode circuit
in series with a three-phase, half-wave, phase-controlled thyristor
circuit.
• Three-phase semi converters are used in industrial applications upto
120 kW level, where one quadrant operation is required.
• The power-factor of this converter decreases as the delay angle
increases, but is better than that of three-phase half-wave converters.
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3Φ Half Controlled Bridge Rectifier contd……
• This converter has inherent free-wheeling action which improves its
power-factor.
• The free-wheeling action takes place between the two devices (SCR
and diode) in the same arm, i.e., between T1 D4, T3 D6 and T5 D2.
• With this free-wheeling action, the voltage drop across the load (V TH
+ VD) becomes approximately 2 V.
• Hence, this leads to following drawbacks:
– This voltage drop reduces the average output voltage.
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3Φ Half Controlled Bridge Rectifier contd……
– During the free-wheeling period, the conduction losses increase
which decreases the efficiency of the converter.
• This also makes the load current less continuous for the same
operating conditions.
– SCRs continue to conduct in the negative cycle of the line voltage
and hence the conduction period of each SCR is 180° (or π).
• This increases the average and rms current ratings of the SCRs.
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3Φ Half Controlled Bridge Rectifier contd……
• All these drawbacks can be reduced by placing an external
freewheeling diode across the load, as shown in the figure.
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3Φ Half Controlled Bridge Rectifier contd……
• This reduces the on-state voltage drop across the load and conduction
losses, and makes the load current more continuous.
• The free-wheeling diode also guarantees the commutation of each SCR at
the end of the corresponding half-cycle and hence the average and rms
current rating of the devices (SCRs and diodes) decreases.
• This helps to decrease the cost and cooling requirements of the converter.
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3Φ Half Controlled Bridge Rectifier contd……
• Operation with Resistive (R) Load: The related voltage and current
waveforms for the 3-Φ semi converter are shown.
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3Φ Half Controlled Bridge Rectifier contd……
• In the figure, the output voltage waveforms (for α = 0°, 30°, 60°),
supply current waveforms, (for α = 0° and 90°), and devices current
waveforms (for α = 90°) are shown.
• The following points can be observed from the figure:
• For α = 0, the output voltage waveform is a six pulse output.
• For α ≥ 30°, the output is only a 3-pulse and hence this converter is
known as 3-pulse converter.
• The output voltage waveform goes to zero after every pulse for α =
60° and for α > 60°, it remains zero for a finite time and is thus
discontinuous.
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3Φ Half Controlled Bridge Rectifier contd……
• The pulse width for α ≤ 60° is 120° and for α ≥ 60° is (180° α).
• The three-phase current waveforms are of 120° width in each
half-cycle for any α ≤ 60° and for any α ≥ 60°, the pulse-width
in each half-cycle starts decreasing and is given by (180° - α).
• The SCR and diode current waveforms are also 120° wide in
each cycle for α ≤ 60° and of duration (180° - α) for α > 60°.
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