Module # 3

CONTROLLED RECTIFIERS
 CONTROLLED RECTIFIERS
• diode rectifiers provide a fixed output voltage only.
• To obtain controlled output voltage, thyristors
are used instead of diodes.
• The output voltage of thyristor rectifiers is varied by
controlling the firing angle of thyristors.
• A phase-control thyristor is turned on by
applying a pulse to its gate.
• in case of inductive load, it is turned off by firing
another thyristor of the rectifier during the
negative half-cycle of input voltage.
 CONTROLLED RECTIFIERS

• Phase-controlled rectifiers are simple

and less expensive.
• Its efficiency are, in general, above 95%.
• these controlled rectifiers are also
called ac-dc converters.
• It’s used extensively in industrial applications,
especially in variable-speed drives, ranging from
fractional horsepower to megawatt power level.
 Uncontrolled (Diode) Rectification

Half-wave rectifier.
• Only the positive half-cycles of the supply
voltage are passed to the load.
• The output dc voltage is fixed.
• Replacing the diode in the circuit by a thyristor,
we can adjust the output d.c. voltage.
 Phase Controlled Rectification
(Controlled Rectifiers)

• By varying the firing angle , the

d.c. power fed to the load will be Vm/

controlled.

• The relationship between the

output voltage and  is shown
 phase-control converters

single-phase three-phase
converters converters

Semi converter

Full Converter

Dual Converter
Semi converter, Full Converter, and Dual Converter

• semiconverter is a one-quadrant
converter, it has one polarity of output

Voltage V
voltage and current.
• A full converter is a two-quadrant
converter and the polarity of its output
voltage can be either positive or
negative. However, the output current
of full converter has one polarity only.
• A dual converter can operate in four quadrants; and both the
output voltage and current can be either positive or negative.
• If two of full converters are connected back to back, a dual
converter is formed.
Another way of classifying converters

according to the number of load current

pulses per cycle of source voltage

One pulse Two pulse

Single Phase, Half Wave, Semi converter, One pulse,
With Resistive Load
•Let us consider the circuit with a resistive load.

• During the positive half-cycle of input voltage, the thyristor anode is positive with respect to
its cathode and the thyristor is said to be forward biased.

•When thyristor T1 is fired at t = , thyristor T1 conducts and the input voltage appears
across the load.

• When the input voltage starts to be negative at t = , the thyristor anode is negative with
respect to its cathode and thyristor T1 is said to be reverse biased, and it is turned off.

•The time after the input voltage starts to go positive until the thyristor is fired at t =  is
called the delay or firing angle, .
 Single Phase, Half Wave, Semiconverter, One pulse,
With Resistive Load

If Vm is the peak input voltage, the average output

voltage is given by :-
Vm
Vdc = (1 + Cos  )
2

• The maximum value of Vdc = Vm/ when  = 0.

• The minimum value of Vdc = 0 when  = .
 Single Phase, Half Wave, Semiconverter, One pulse,
With Resistive Load

• Why this converter is not used in industrial applications?

because it has high ripple content and low ripple frequency.
• What’s the main disadvantage of this converter?
one half cycle of the input wave is not used as load power.
• How this disadvantage can be eliminated?
by the use of full-wave or single phase two pulse converters
(either using center-tapped transformer, or using a bridge of
four thyristors)
 center-tapped transformer and two thyristors

T1
IO
waveform across the load
Vo VO RL

Time, t
T2

• Both the load voltage and current are positive so the circuit is a semi
converter.
• Two load voltage pulses are produced in one cycle of the input wave.
• The average dc voltage is twice that of the one pulse converter and is
given by
Vm
Vdc = (1 + Cos  ) Vdc can be varied from 2 Vm/ to zero
by varying  from 0 to .

 - + - +
Single-Phase (One pulse) Semi converter with Inductive Load
+ R L
+ Th
• After zero-cross of Vs, the current still flow, because ofVS + - + -
the inductor, (current lags behind voltage in an
inductor). + i
R L -
di + Th
VS
• Applying KVL, V s = Ri + L + VTF
dt i -
• At voltage zero crossing we can write:
di
L = Ri + VTF
dt
• If Vs remains zero, the energy stored in the
inductor should dissipate and the current will
become zero.
• if Vs goes negative, the current decreases rapidly.
• When i is ≤ the holding current, the thyristor will
switch off and the current and load voltage will
remain zero until the thyristor is fired again.
 Dm
Adding a Freewheeling Diode
+ - + -
• a freewheeling diode is added Rto decrease
L the Dm

ripple current in the load. +

+ - + - + T
• After zero crossing of Vs,
VS when the current starts
h
decreasing, the polarity
+ ofRthe induced
L
i + -
voltage
of the inductor Vreverses.
S
Th
• When Vs becomes zero, the iinductor- voltage
forward biases Dm, placing a short across the
load.
• The current through R, L and Dm continue to
flow due to the energy stored in the inductor.
• the voltage across the load remains equal
to the forward voltage drop of the diode,
until the energy stored in the inductor is
dissipated
 Adding a Freewheeling Diode
• when Vs starts going negative, the thyristor, thus reverse
biased and switches off.
• The load voltage goes to zero, but the load current
continues in the same direction.
• The negative voltage does not affect the load current.
• In many cases the next turning-on occurs before the decay
of the current.
• As further cycles are repeated, the average current
continues to increase and eventually reaches a stationary
value.
• The average dc voltage will be
Vm
Vdc = (1 + Cos  )
2
Single Phase, Two pulse, Semi converter
with RL Load
 Single Phase (Two Pulse) Semi converter With RL Load

• During the positive half cycle of the input voltage thyristor T1 is forward
biased and fired at ωt = , the load is connected to input supply through T1
and D2 from  to .
• During . To .+  the input voltage is negative and free wheeling diode Dm
is forward biased.
• Dm conducts to provide the continuity of current in the inductive load.
• The load current is now transferred from T1 and D2 to Dm.
• Thyristor T1 and Diode D2 are turned off.
• During the negative half cycle of the input voltage , thyristor T2 is forward
biased and firing at ωt = .+  .

• The free wheeling diode Dm will reverse bias and its turned off.

• The Load is connected to the supply through T2 and D1

Single Phase
Single Phase (Two
(Two Pulse)
Pulse) Semiconverter
Semiconverter With
With RL
RL Load
Load

• The converter operates in the first quadrant where the

output voltage and current have positive polarity.
• This converter is used in applications up to 15 kW,
where one-quadrant operation is acceptable.
• The average output voltage can be found from

Vdc = (1+ Cos  )

Vm

• Maximum value of Vdc equals (2Vm /) at  = 0.
• Minimum value of Vdc equals 0 at  = .
Single-phase Full Converter With Inductive Load
 Single Phase Full Converters
• During the positive half cycle of the input voltage thyristors T1
and T2 forward biased and fired at ωt =.
• The load is connected to the input supply through T1 and T2.
• Due to the inductive load, thyristors T1 and T2 will continue to
conduct beyond ωt =  (ie. ωt = .+ ) even the input voltage is
already negative.
• During the negative half cycle of the input voltage thyristors T3
and T4 are forward biased .
• Firing of thyristors T3 and T4 will apply the input voltage across
T1 and T2 as reverse blocking voltage and T1 and T2 will be
turned off.
• The load current will be transferred from T1 and T2 to T3 and T4.
 Single Phase Full Converters
• The output voltage polarity can reverse (+ ive & - ive), but
the current direction cannot reverse (only + ive).
• It operates in first and fourth quadrant.
• In the first quadrant the electrical power flows from the
supply to the load, while in the fourth quadrant the power
is fed back to the supply from the load.
• if the load is a rotating d.c. machine (motor) in the 1st
quadrant , it must work as a d.c. generator in the 4th
quadrant.
• As the load during the fourth quadrant operates as a
generator delivers power back to the source, it must have
storage elements in it.
 Single-phase Full Converter With Inductive Load

• from  to , the output voltage and output current are

positive; the power flows from the supply to the load. the
converter is said to be operated in rectification mode.
• from  to  + , the output voltage is negative and the
output current is positive; and there will be reverse power
flow from the load to the supply, the converter is operated in
inversion mode.
• This converter is extensively used in industrial applications
up to 15 kW.
• Depending on the value of , the average output voltage
could be either positive or negative and it provides two-
quadrant operation.
 Single-phase Full Converter With Inductive Load

• The average output voltage can be found from :-

2Vm
Vdc = cos 


• The maximum value of Vdc is (2Vm /) at  = 0.

• The minimum value of Vdc is (–2 Vm / ) at  = .
 Variable-Speed Drive for DC Motor

Field coil and armature are energized separately

 Variable-Speed Drive for DC Motor

UJT Oscillator

• The armature voltage is supplied through a controlled rectifier,

hence the motor speed is dependent on the firing angle .
• Field is usually supplied at its rated voltage via a diode rectifier.
 Variable-Speed Drive for DC Motor

• The field coil is supplied a fixed full-wave rectifier

voltage for its excitation.
• The same full-wave rectified voltage is also
supplied to the motor but with firing delay  in
every cycle
• In this way the average voltage supply to the
armature will be Vm
Va = (1 + Cos  )

• which can be varied between 2Vm and 0 by
varying  from 0 to  
 Variable-Speed Drive for DC Motor
• Variation of the average voltage will vary the speed of
the motor.
• The firing angle can be adjusted through a UJT
oscillator circuit shown as dotted-line box .
• The oscillator circuit produces pulses which are used to
trigger the thyristor
• Large inductance of the motor and freewheeling diode
will make the motor current continuous
• The magnitude of both the average motor armature
voltage and current is dependent on the firing angle .
• The function of the Snubber circuit is the same as
explained in module 2
Waveforms across the field coil and motor

Voltage applied to the field coil Voltage applied to the motor

Motor current

 Time, t
Firing angle