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# Control Circuit for Three-phase to Three-phase Cyclo-converters - AC to AC Voltage Converters Electronics and Communication Engineering (ECE) Notes | EduRev

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## Electronics and Communication Engineering (ECE) : Control Circuit for Three-phase to Three-phase Cyclo-converters - AC to AC Voltage Converters Electronics and Communication Engineering (ECE) Notes | EduRev

``` Page 1

Module
4

AC to AC Voltage
Converters
Version 2 EE IIT, Kharagpur 1
Page 2

Module
4

AC to AC Voltage
Converters
Version 2 EE IIT, Kharagpur 1

Lesson
32

Control Circuit for Three-
phase to Three-phase
Cyclo-converters
Version 2 EE IIT, Kharagpur 2
Page 3

Module
4

AC to AC Voltage
Converters
Version 2 EE IIT, Kharagpur 1

Lesson
32

Control Circuit for Three-
phase to Three-phase
Cyclo-converters
Version 2 EE IIT, Kharagpur 2
Instructional Objectives

Study of the following:
• The control circuits used for the three-phase to three-phase cyclo-converters using two three-
phase converters, to generate the firing pulses for the thyristors
• The functional blocks, including the circuit and waveforms

Introduction
In the last lesson - third one in the second half of this module, firstly, the circuit along with
the operation of the three-phase to three-phase cyclo-converter, are described in brief. Two three-
phase half-wave converters, with three thyristors as power switching devices in each converter,
are needed, per phase, thus, using six such converters having a total of 18 thyristors. The mode
of operation is non-circulating current one, in which only one converter is conducting at a time.
Lastly, the analysis of the output waveform is presented.
In this lesson - the fourth and final one in the second half, the complete control circuit for the
three-phase to three-phase cyclo-converter, is presented in detail, showing how the firing pulses
are generated to trigger the thyristors. The function of the various blocks, with their respective
functions, and also circuit diagrams as needed, is described.
Keywords: The control circuit for the three-phase to three-phase cyclo-converter, functional
blocks.
Control Circuit for Cyclo-converters
The function of the control circuit used in this case is to deliver correctly timed, properly
shaped, firing pulses to the gates of the thyristors in the power converter (rectifier/inverter)
circuits, so as to generate a voltage of desired wave shape at the output terminals of a cyclo-
converter. The functional block diagram of the control circuit for the three-phase to three-phase
cyclo-converter, in the non-circulating current mode of operation, is shown in Fig. 32.1. The
same control circuit is applicable to the cyclo-converter operating in circulating current mode,
but the block designated as converter group selection will not be present in this case. There are
four functional blocks in the circuit as given here.
1. Synchronising circuit
2. Reference voltage sources
3. Logic and triggering circuit
4. Converter group selection circuit
Version 2 EE IIT, Kharagpur 3
Page 4

Module
4

AC to AC Voltage
Converters
Version 2 EE IIT, Kharagpur 1

Lesson
32

Control Circuit for Three-
phase to Three-phase
Cyclo-converters
Version 2 EE IIT, Kharagpur 2
Instructional Objectives

Study of the following:
• The control circuits used for the three-phase to three-phase cyclo-converters using two three-
phase converters, to generate the firing pulses for the thyristors
• The functional blocks, including the circuit and waveforms

Introduction
In the last lesson - third one in the second half of this module, firstly, the circuit along with
the operation of the three-phase to three-phase cyclo-converter, are described in brief. Two three-
phase half-wave converters, with three thyristors as power switching devices in each converter,
are needed, per phase, thus, using six such converters having a total of 18 thyristors. The mode
of operation is non-circulating current one, in which only one converter is conducting at a time.
Lastly, the analysis of the output waveform is presented.
In this lesson - the fourth and final one in the second half, the complete control circuit for the
three-phase to three-phase cyclo-converter, is presented in detail, showing how the firing pulses
are generated to trigger the thyristors. The function of the various blocks, with their respective
functions, and also circuit diagrams as needed, is described.
Keywords: The control circuit for the three-phase to three-phase cyclo-converter, functional
blocks.
Control Circuit for Cyclo-converters
The function of the control circuit used in this case is to deliver correctly timed, properly
shaped, firing pulses to the gates of the thyristors in the power converter (rectifier/inverter)
circuits, so as to generate a voltage of desired wave shape at the output terminals of a cyclo-
converter. The functional block diagram of the control circuit for the three-phase to three-phase
cyclo-converter, in the non-circulating current mode of operation, is shown in Fig. 32.1. The
same control circuit is applicable to the cyclo-converter operating in circulating current mode,
but the block designated as converter group selection will not be present in this case. There are
four functional blocks in the circuit as given here.
1. Synchronising circuit
2. Reference voltage sources
3. Logic and triggering circuit
4. Converter group selection circuit
Version 2 EE IIT, Kharagpur 3
Synchronizing
and
modulating
signals
Logic
and
trigger
circuit
Converter
group
selection
Reference
signals
P-converter
N-converter
Fig. 32.1  Control circuit block diagram for a cycloconverter with
non-circulating current mode

e

e

e
0
i
0
i
0
e
r
e
r

Synchronising Circuit
The main function of the synchronising circuit is to derive low voltage signals to the control
circuit, which operates at low voltages. These low voltage signals must be synchronised to the
voltages applied to the main power circuit. Step-down transformers may be used for this purpose
with the filter circuit to avoid waveform distortion, if any. While deriving the modulating
voltages at the supply frequency, the phase shifting network may also be required. To determine
the instants at which the firing signals are to be produced, to be fed to the gates of the thyristors
in the two converter groups, the modulating signals are compared with the reference voltages.
Reference Voltage Sources
The reference signal is designed to control the output voltage in the sense that the output
voltages tend to follow the reference signal. It means that, if the amplitude and frequency of the
reference signal is varied, then the amplitude and frequency of the output voltage varies
automatically. In the case of three-phase to three-phase cyclo-converter, the reference signal does
additional function of shifting ,  &  , by phase shift of . The three-phase variable
frequency, variable voltage sine wave reference voltage can be designed in various ways. As the
frequency of the reference voltage signal is low, normally limited to
OA
e
OB
e
OC
e ° 120
3
2
16 Hz, one-third of the
line frequency of 50 Hz (may be higher (25 Hz) in some case), one of the design approach as
given here, is to use a mixer, wherein two signals having frequencies,  &  are mixed to
c
f
d
f
Version 2 EE IIT, Kharagpur 4
Page 5

Module
4

AC to AC Voltage
Converters
Version 2 EE IIT, Kharagpur 1

Lesson
32

Control Circuit for Three-
phase to Three-phase
Cyclo-converters
Version 2 EE IIT, Kharagpur 2
Instructional Objectives

Study of the following:
• The control circuits used for the three-phase to three-phase cyclo-converters using two three-
phase converters, to generate the firing pulses for the thyristors
• The functional blocks, including the circuit and waveforms

Introduction
In the last lesson - third one in the second half of this module, firstly, the circuit along with
the operation of the three-phase to three-phase cyclo-converter, are described in brief. Two three-
phase half-wave converters, with three thyristors as power switching devices in each converter,
are needed, per phase, thus, using six such converters having a total of 18 thyristors. The mode
of operation is non-circulating current one, in which only one converter is conducting at a time.
Lastly, the analysis of the output waveform is presented.
In this lesson - the fourth and final one in the second half, the complete control circuit for the
three-phase to three-phase cyclo-converter, is presented in detail, showing how the firing pulses
are generated to trigger the thyristors. The function of the various blocks, with their respective
functions, and also circuit diagrams as needed, is described.
Keywords: The control circuit for the three-phase to three-phase cyclo-converter, functional
blocks.
Control Circuit for Cyclo-converters
The function of the control circuit used in this case is to deliver correctly timed, properly
shaped, firing pulses to the gates of the thyristors in the power converter (rectifier/inverter)
circuits, so as to generate a voltage of desired wave shape at the output terminals of a cyclo-
converter. The functional block diagram of the control circuit for the three-phase to three-phase
cyclo-converter, in the non-circulating current mode of operation, is shown in Fig. 32.1. The
same control circuit is applicable to the cyclo-converter operating in circulating current mode,
but the block designated as converter group selection will not be present in this case. There are
four functional blocks in the circuit as given here.
1. Synchronising circuit
2. Reference voltage sources
3. Logic and triggering circuit
4. Converter group selection circuit
Version 2 EE IIT, Kharagpur 3
Synchronizing
and
modulating
signals
Logic
and
trigger
circuit
Converter
group
selection
Reference
signals
P-converter
N-converter
Fig. 32.1  Control circuit block diagram for a cycloconverter with
non-circulating current mode

e

e

e
0
i
0
i
0
e
r
e
r

Synchronising Circuit
The main function of the synchronising circuit is to derive low voltage signals to the control
circuit, which operates at low voltages. These low voltage signals must be synchronised to the
voltages applied to the main power circuit. Step-down transformers may be used for this purpose
with the filter circuit to avoid waveform distortion, if any. While deriving the modulating
voltages at the supply frequency, the phase shifting network may also be required. To determine
the instants at which the firing signals are to be produced, to be fed to the gates of the thyristors
in the two converter groups, the modulating signals are compared with the reference voltages.
Reference Voltage Sources
The reference signal is designed to control the output voltage in the sense that the output
voltages tend to follow the reference signal. It means that, if the amplitude and frequency of the
reference signal is varied, then the amplitude and frequency of the output voltage varies
automatically. In the case of three-phase to three-phase cyclo-converter, the reference signal does
additional function of shifting ,  &  , by phase shift of . The three-phase variable
frequency, variable voltage sine wave reference voltage can be designed in various ways. As the
frequency of the reference voltage signal is low, normally limited to
OA
e
OB
e
OC
e ° 120
3
2
16 Hz, one-third of the
line frequency of 50 Hz (may be higher (25 Hz) in some case), one of the design approach as
given here, is to use a mixer, wherein two signals having frequencies,  &  are mixed to
c
f
d
f
Version 2 EE IIT, Kharagpur 4
obtain frequencies ( ). Then, a low pass filter is used to obtain a signal of required
frequency, ( ). The details are as follows.
d c
f f ±
d c
f f -

Mixer 1

Mixer 2
Mixer 3
Low
pass
filter

f
c
± f
d
f
c
- f
d

e
ra
Low
pass
filter

f
c
± f
d
f
c
- f
d

e
rb
Low
pass
filter

f
c
± f
d
f
c
- f
d

e
rc
Ring
counter
Astable
multi-
vibrator
Variable frequency
f
A
f
C
f
B
f
3
f
2
f
1
f
d
f
c
f
d
f
d
f
c
f
c
Fig. 32.2  Reference voltage generator block diagram.
Fixed
frequency
oscillator (f
c
)

The reference voltage generator block diagram is shown in Fig. 32.2. An astable multi-
vibrator is used to generate a square wave with frequency, (
d
f · 3 ), which is then fed to a three-
stage ring counter, whose output is three numbers of three-phase, square wave (
A
f ,
B
f &
C
f ) of
frequency , at a phase shift of . The fixed frequency oscillator ( ) produces three
outputs (
d
f ° 120
c
f
1
f ,
2
f &
3
f ), which may be taken as three-phase. Three mixers - one for each phase, as
stated earlier, are used to combine the fixed and variable frequencies. The output of each mixer
stage is a square wave with half-wave symmetry, consisting of a fundamental and a series of odd
harmonics. If all higher order higher harmonics are neglected, the output signal has only two
frequencies, sum or difference of the fixed and variable frequencies, as given earlier. Then, a low
pass filter is used to select the low frequency signal (
d c
f f - ), and also eliminate the high
frequency one ( ). Finally, the three reference signals obtained are in the form,
d c
f f +
) ) ( 2 ( sin t f f E r e
d c m ra
- · = p ,
) 120 ) ( 2 ( sin ° - - · = t f f E r e
d c m rb
p ,
) 120 ) ( 2 ( sin ° + - · = t f f E r e
d c m rc
p ,
where,  is the peak of the cosine modulating wave.
m
E
From the above equations, it can be observed that the amplitude and frequency of the reference
waveforms are controlled by varying r and  respectively, while the phase sequence of the
three-phase outputs is controlled by setting , greater or lower than . The amplitude is varied
by changing
d
f
d
f
c
f
r from 0 to 1, i.e. 0 1 = =r .

Version 2 EE IIT, Kharagpur 5
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