Three-phase to Single-phase Cyclo-converters - Module 4 AC to AC Voltage Converters Lesson 30 Notes | EduRev

Created by: Sanya Gill

: Three-phase to Single-phase Cyclo-converters - Module 4 AC to AC Voltage Converters Lesson 30 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 
30 
 
Three-phase to Single-
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 
30 
 
Three-phase to Single-
phase Cyclo-converters 
Version 2 EE IIT, Kharagpur 2
Instructional Objectives 
 
Study of the following: 
• The three-phase to single-phase cyclo-converter circuit, using two three-phase full-wave 
thyristorised bridge converters   
• The operation of the above cyclo-converter circuit, along with the voltage waveforms 
 
Introduction 
 In the last lesson - first one in the second half of this module, firstly, the basic principle of 
operation of the cyclo-converter circuits has been presented. This followed by the discussion of 
the circuit, and the operation of the single-phase to single-phase cyclo-converter circuit with both 
resistive and inductive loads, in detail. Two full-wave bridge converters (rectifiers) connected 
back to back, with four thyristors as power switching device in each bridge, are used. Also 
described are the advantages and disadvantages of the cyclo-converter. The dc link converter is 
introduced briefly, along with its advantages and disadvantages.  
 In this lesson - the second one in the second half, firstly, the three-phase to single-phase 
cyclo-converter circuit, using two three-phase full-wave thyristorised bridge converters, is 
presented. Then, the operation of the above cyclo-converter circuit, with both resistive and 
inductive loads, is described in detail, along with voltage waveforms. The mode of operation 
used is the non-circulating current one. The following are discussed in brief -.the circulating 
current mode of operation for the above, and also the cyclo-converter circuit, using  two three-
phase half-wave converters.  
Keywords: Three-phase to single-phase cyclo-converter, Voltage waveforms, Non-circulating 
current, and Circulating current modes of operation, Three-phase full-wave bridge, and half-
wave converters. 
 
Three-phase to Single-phase Cyclo-converter 
 The circuit of a three-phase to single-phase cyclo-converter is shown in Fig. 30.1. Two three-
phase full-wave (six-pulse) bridge converters (rectifier) connected back to back, with six 
thyristors for each bridge, are used. The ripple frequency here is 300 Hz, six times the input 
frequency of 50 Hz. So, low value of load inductance is needed to make the current continuous, 
as compared to one using single-phase bridge converters described in the previous lesson (#4.4) 
with ripple frequency of 100 Hz. Also, the non-circulating current mode of operation is used, 
where only one converter - bridge 1 (positive) or bridge 2 (negative), conducts at a time, but 
both converters do not conduct at the same time. It may be noted that each thyristor conducts for 
about ) 3 / ( 120 p ° , i.e., one-third of one complete cycle, whereas a particular thyristor pair, say 
1& 2 conduct for about ) 6 / ( 60 p ° , i.e., one-sixth of a cycle. The thyristors conduct in pairs as 
stated, one (odd-numbered) thyristor in the top half and the other (even-numbered) one in the 
bottom half in two different legs. Two thyristors in one leg are not allowed to conduct at a time, 
which will result in short circuit at the output terminals. The sequence of conduction of the 
thyristors is 1 & 6, 1 & 2, 3 & 2, and so on. When thyristor 1 is triggered, the conducting 
thyristor (#5) in top half, being reverse biased at that time, turns off. Similarly, when thyristor 2 
is triggered, the conducting thyristor (#6) in bottom half, being reverse biased at that time turns 
Version 2 EE IIT, Kharagpur 3
Page 4


 
 
 
 
 
 
 
 
 
Module 
4 
 
AC to AC Voltage 
Converters 
Version 2 EE IIT, Kharagpur 1
 
 
 
 
 
 
 
 
 
 
Lesson 
30 
 
Three-phase to Single-
phase Cyclo-converters 
Version 2 EE IIT, Kharagpur 2
Instructional Objectives 
 
Study of the following: 
• The three-phase to single-phase cyclo-converter circuit, using two three-phase full-wave 
thyristorised bridge converters   
• The operation of the above cyclo-converter circuit, along with the voltage waveforms 
 
Introduction 
 In the last lesson - first one in the second half of this module, firstly, the basic principle of 
operation of the cyclo-converter circuits has been presented. This followed by the discussion of 
the circuit, and the operation of the single-phase to single-phase cyclo-converter circuit with both 
resistive and inductive loads, in detail. Two full-wave bridge converters (rectifiers) connected 
back to back, with four thyristors as power switching device in each bridge, are used. Also 
described are the advantages and disadvantages of the cyclo-converter. The dc link converter is 
introduced briefly, along with its advantages and disadvantages.  
 In this lesson - the second one in the second half, firstly, the three-phase to single-phase 
cyclo-converter circuit, using two three-phase full-wave thyristorised bridge converters, is 
presented. Then, the operation of the above cyclo-converter circuit, with both resistive and 
inductive loads, is described in detail, along with voltage waveforms. The mode of operation 
used is the non-circulating current one. The following are discussed in brief -.the circulating 
current mode of operation for the above, and also the cyclo-converter circuit, using  two three-
phase half-wave converters.  
Keywords: Three-phase to single-phase cyclo-converter, Voltage waveforms, Non-circulating 
current, and Circulating current modes of operation, Three-phase full-wave bridge, and half-
wave converters. 
 
Three-phase to Single-phase Cyclo-converter 
 The circuit of a three-phase to single-phase cyclo-converter is shown in Fig. 30.1. Two three-
phase full-wave (six-pulse) bridge converters (rectifier) connected back to back, with six 
thyristors for each bridge, are used. The ripple frequency here is 300 Hz, six times the input 
frequency of 50 Hz. So, low value of load inductance is needed to make the current continuous, 
as compared to one using single-phase bridge converters described in the previous lesson (#4.4) 
with ripple frequency of 100 Hz. Also, the non-circulating current mode of operation is used, 
where only one converter - bridge 1 (positive) or bridge 2 (negative), conducts at a time, but 
both converters do not conduct at the same time. It may be noted that each thyristor conducts for 
about ) 3 / ( 120 p ° , i.e., one-third of one complete cycle, whereas a particular thyristor pair, say 
1& 2 conduct for about ) 6 / ( 60 p ° , i.e., one-sixth of a cycle. The thyristors conduct in pairs as 
stated, one (odd-numbered) thyristor in the top half and the other (even-numbered) one in the 
bottom half in two different legs. Two thyristors in one leg are not allowed to conduct at a time, 
which will result in short circuit at the output terminals. The sequence of conduction of the 
thyristors is 1 & 6, 1 & 2, 3 & 2, and so on. When thyristor 1 is triggered, the conducting 
thyristor (#5) in top half, being reverse biased at that time, turns off. Similarly, when thyristor 2 
is triggered, the conducting thyristor (#6) in bottom half, being reverse biased at that time turns 
Version 2 EE IIT, Kharagpur 3
off. This sequence is repeated in cyclic order. So, natural or line commutation takes place in this 
case. Otherwise, the procedure is similar to the one as discussed in the previous lesson.  
  
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
-
l
o
a
d
i
N i
P
+
P
1 P
3
P
4 P
6
N
1 N
3
N
4 N
6
3-phase 
ac 
supply  
Fig. 30.1: Three-phase to single-phase cycloconverter   
i
O
P
5
P
2
N
5
N
2
A 
B 
C 
3-phase 
ac 
supply  
A 
B 
C 
 The procedure to be followed in the triggering of the thyristors in sequence in the two bridge 
converters has been briefly given earlier. The readers are requested to go through two lessons 
(#2.5-2.6) in module 2 (AC-DC Converters), or any standard text book. As given in the earlier 
lesson (#4.4), the firing angle ( a) of two converters is first decreased starting from the initial 
value of  to the final value of , and then again increased to the final value of , as 
shown in Fig. 30.2.  Also, for positive half cycle of the output voltage waveform, bridge 1 is 
used, while bridge 2 is used for negative half cycle. The two half cycles are combined to form 
one complete cycle of the output voltage, the frequency being decided by the number of half 
cycles of input voltage waveform used for each half cycle of the output. As more no. of segments 
of near 
° 90 ° 0 ° 90
) 6 / ( 60 p ° is used, the output voltage waveform becomes near sinusoidal, with its 
frequency also being reduced.  
 The initial value of firing angle delay is kept at ° ˜ 90
1
a ,  such the average value (dc) of the 
output voltage in this interval of  near ) 6 / ( 60 p ° [ 0 . 0 90 cos cos
1
= ° = ? a
av
V ], is zero. It may 
be noted that the next thyristor in sequence is triggered at ° < 90
2
a , as the firing angle is 
decreased for each segment, to obtain higher voltage ve V
av
+ = ?
2
cos a , to form the sine wave 
at the output. This can be observed from the points, M, N, O, P, Q, R & S, shown in Fig. 30.2. 
From these segments, the first quarter cycle of the output voltage waveform from  to , is 
obtained. The second quarter cycle of the above waveform from  to , is obtained, using 
the segments starting from the points, T, U, V, W, X &Y (fig. 30.2). It may be noted that the 
firing angle delay at the point, Y is 
° 0 ° 90
° 90 ° 180
° = 90 a , and also the firing angle is increased from  (T) 
to (Y) in this interval. When the firing angle delay is , the average value of the segment is 
° 0
° 90 ° 0
0 . 1 0 cos cos = ° = ? a
av
V . The two quarter cycles form the positive half cycle of the output 
voltage waveform. In this region, the bridge 1 (positive) is used. To obtain the negative half 
cycle of the output voltage waveform ( - ), the other bridge converter (#2) termed 
negative (N) is used in the same manner as given earlier, i.e. its firing angle delay ( a) is first 
decreased starting from the initial value of  to the final value of , and then again increased 
° 180 ° 360
° 90 ° 0
Version 2 EE IIT, Kharagpur 4
Page 5


 
 
 
 
 
 
 
 
 
Module 
4 
 
AC to AC Voltage 
Converters 
Version 2 EE IIT, Kharagpur 1
 
 
 
 
 
 
 
 
 
 
Lesson 
30 
 
Three-phase to Single-
phase Cyclo-converters 
Version 2 EE IIT, Kharagpur 2
Instructional Objectives 
 
Study of the following: 
• The three-phase to single-phase cyclo-converter circuit, using two three-phase full-wave 
thyristorised bridge converters   
• The operation of the above cyclo-converter circuit, along with the voltage waveforms 
 
Introduction 
 In the last lesson - first one in the second half of this module, firstly, the basic principle of 
operation of the cyclo-converter circuits has been presented. This followed by the discussion of 
the circuit, and the operation of the single-phase to single-phase cyclo-converter circuit with both 
resistive and inductive loads, in detail. Two full-wave bridge converters (rectifiers) connected 
back to back, with four thyristors as power switching device in each bridge, are used. Also 
described are the advantages and disadvantages of the cyclo-converter. The dc link converter is 
introduced briefly, along with its advantages and disadvantages.  
 In this lesson - the second one in the second half, firstly, the three-phase to single-phase 
cyclo-converter circuit, using two three-phase full-wave thyristorised bridge converters, is 
presented. Then, the operation of the above cyclo-converter circuit, with both resistive and 
inductive loads, is described in detail, along with voltage waveforms. The mode of operation 
used is the non-circulating current one. The following are discussed in brief -.the circulating 
current mode of operation for the above, and also the cyclo-converter circuit, using  two three-
phase half-wave converters.  
Keywords: Three-phase to single-phase cyclo-converter, Voltage waveforms, Non-circulating 
current, and Circulating current modes of operation, Three-phase full-wave bridge, and half-
wave converters. 
 
Three-phase to Single-phase Cyclo-converter 
 The circuit of a three-phase to single-phase cyclo-converter is shown in Fig. 30.1. Two three-
phase full-wave (six-pulse) bridge converters (rectifier) connected back to back, with six 
thyristors for each bridge, are used. The ripple frequency here is 300 Hz, six times the input 
frequency of 50 Hz. So, low value of load inductance is needed to make the current continuous, 
as compared to one using single-phase bridge converters described in the previous lesson (#4.4) 
with ripple frequency of 100 Hz. Also, the non-circulating current mode of operation is used, 
where only one converter - bridge 1 (positive) or bridge 2 (negative), conducts at a time, but 
both converters do not conduct at the same time. It may be noted that each thyristor conducts for 
about ) 3 / ( 120 p ° , i.e., one-third of one complete cycle, whereas a particular thyristor pair, say 
1& 2 conduct for about ) 6 / ( 60 p ° , i.e., one-sixth of a cycle. The thyristors conduct in pairs as 
stated, one (odd-numbered) thyristor in the top half and the other (even-numbered) one in the 
bottom half in two different legs. Two thyristors in one leg are not allowed to conduct at a time, 
which will result in short circuit at the output terminals. The sequence of conduction of the 
thyristors is 1 & 6, 1 & 2, 3 & 2, and so on. When thyristor 1 is triggered, the conducting 
thyristor (#5) in top half, being reverse biased at that time, turns off. Similarly, when thyristor 2 
is triggered, the conducting thyristor (#6) in bottom half, being reverse biased at that time turns 
Version 2 EE IIT, Kharagpur 3
off. This sequence is repeated in cyclic order. So, natural or line commutation takes place in this 
case. Otherwise, the procedure is similar to the one as discussed in the previous lesson.  
  
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
-
l
o
a
d
i
N i
P
+
P
1 P
3
P
4 P
6
N
1 N
3
N
4 N
6
3-phase 
ac 
supply  
Fig. 30.1: Three-phase to single-phase cycloconverter   
i
O
P
5
P
2
N
5
N
2
A 
B 
C 
3-phase 
ac 
supply  
A 
B 
C 
 The procedure to be followed in the triggering of the thyristors in sequence in the two bridge 
converters has been briefly given earlier. The readers are requested to go through two lessons 
(#2.5-2.6) in module 2 (AC-DC Converters), or any standard text book. As given in the earlier 
lesson (#4.4), the firing angle ( a) of two converters is first decreased starting from the initial 
value of  to the final value of , and then again increased to the final value of , as 
shown in Fig. 30.2.  Also, for positive half cycle of the output voltage waveform, bridge 1 is 
used, while bridge 2 is used for negative half cycle. The two half cycles are combined to form 
one complete cycle of the output voltage, the frequency being decided by the number of half 
cycles of input voltage waveform used for each half cycle of the output. As more no. of segments 
of near 
° 90 ° 0 ° 90
) 6 / ( 60 p ° is used, the output voltage waveform becomes near sinusoidal, with its 
frequency also being reduced.  
 The initial value of firing angle delay is kept at ° ˜ 90
1
a ,  such the average value (dc) of the 
output voltage in this interval of  near ) 6 / ( 60 p ° [ 0 . 0 90 cos cos
1
= ° = ? a
av
V ], is zero. It may 
be noted that the next thyristor in sequence is triggered at ° < 90
2
a , as the firing angle is 
decreased for each segment, to obtain higher voltage ve V
av
+ = ?
2
cos a , to form the sine wave 
at the output. This can be observed from the points, M, N, O, P, Q, R & S, shown in Fig. 30.2. 
From these segments, the first quarter cycle of the output voltage waveform from  to , is 
obtained. The second quarter cycle of the above waveform from  to , is obtained, using 
the segments starting from the points, T, U, V, W, X &Y (fig. 30.2). It may be noted that the 
firing angle delay at the point, Y is 
° 0 ° 90
° 90 ° 180
° = 90 a , and also the firing angle is increased from  (T) 
to (Y) in this interval. When the firing angle delay is , the average value of the segment is 
° 0
° 90 ° 0
0 . 1 0 cos cos = ° = ? a
av
V . The two quarter cycles form the positive half cycle of the output 
voltage waveform. In this region, the bridge 1 (positive) is used. To obtain the negative half 
cycle of the output voltage waveform ( - ), the other bridge converter (#2) termed 
negative (N) is used in the same manner as given earlier, i.e. its firing angle delay ( a) is first 
decreased starting from the initial value of  to the final value of , and then again increased 
° 180 ° 360
° 90 ° 0
Version 2 EE IIT, Kharagpur 4
to the final value of , as given earlier. The two half cycles (positive and negative) together 
give one complete cycle ( - ) of the output voltage waveform.  
° 90
° 0 ° 360
 
 
Fig.  30.2  Output voltage waveforms for a three-phase to single phase 
cyclo-converter. 
Fabricated output voltage
 
Mean output voltage
 
e
0 
M
 
N
 
0
 
P
 
Q
 
R
 
S
 
T
 
U
 
V
 
W
 
X
 
Y
 
a = 90° a = 0° 
a = 90° 
? = ?t 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
  
 The load on the output of the cyclo-converter is assumed to be inductive (R-L). The load can 
also be capacitive. For inductive load, the output current (Fig. 30.3) lags behind the voltage by its 
phase angle, f (assumed to be positive). The load power factor is also +ve ( f cos ).  It may be 
noted that the current is unidirectional in a thyristor converter. As the current, being alternating 
in nature, flows in both directions in a complete cycle, two converters are connected in anti-
parallel. The positive (P) converter carries current during positive half cycle of output current, 
while the other, i.e. negative (N) one carries current in the negative half cycle. As discussed in 
the previous lesson (#29), P-converter acts as a rectifier, when the output voltage is positive, and 
as an inverter, when the output voltage is negative (Fig. 30.3). Similarly, N-converter acts as a 
rectifier, when the output voltage is negative, and as an inverter, when the output voltage is 
positive. It can thus be inferred, in general, that one of two converters would operate as rectifier, 
if its output voltage and current have the same polarity, and as an inverter, if these are of 
opposite polarity.       
Version 2 EE IIT, Kharagpur 5
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