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

 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  
Load   
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  
Load   
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