Detailed Notes: Cyclo Converters | Power Electronics - Electrical Engineering (EE) PDF Download

 Page 1


CYCLOCONVERTERS 
 
 
 
 
In industrial applications, two forms of electrical energy are used: direct current (dc) and 
altern ating current (ac). Usually constant voltage constant frequency single - phase or three - phase 
ac is readily available. However, for different applications, different forms, magnitudes and/or 
frequencies are required. There are four different conversions betw een dc and ac power sources. 
These conversions are done by circuits called power converters. The converters are classified as: 
1 - rectifiers: from single - phase or three - phase ac to variable voltage dc 
2 - choppers: from dc to variable voltage dc 
3 - inverters: from dc to variable magnitude and variable frequency, single - phase or three - phase ac 
4 - cycloconverters: from single - phase or three - phase ac to variable magnitude and 
variable frequency, single - phase or three - phase ac 
The first three classes are explained i n other articles. This article explains what cycloconverters 
are, their types, how they operate and their applications. 
 
Traditionally, ac - ac conversion using semiconductor switches is done in two different ways: 1 - in 
two stages (ac - dc and then dc - ac) as in dc link converters or 2 - in one stage (ac - ac) 
cycloconverters (Fig. 1). Cycloconverters are used in high power applications driving induction 
and synchronous motors. They are usually phase - controlled and they traditionally use thyristors 
due to their ea se of phase commutation.  
 
Fig.1 Block diagram of a cycloconverter 
Page 2


CYCLOCONVERTERS 
 
 
 
 
In industrial applications, two forms of electrical energy are used: direct current (dc) and 
altern ating current (ac). Usually constant voltage constant frequency single - phase or three - phase 
ac is readily available. However, for different applications, different forms, magnitudes and/or 
frequencies are required. There are four different conversions betw een dc and ac power sources. 
These conversions are done by circuits called power converters. The converters are classified as: 
1 - rectifiers: from single - phase or three - phase ac to variable voltage dc 
2 - choppers: from dc to variable voltage dc 
3 - inverters: from dc to variable magnitude and variable frequency, single - phase or three - phase ac 
4 - cycloconverters: from single - phase or three - phase ac to variable magnitude and 
variable frequency, single - phase or three - phase ac 
The first three classes are explained i n other articles. This article explains what cycloconverters 
are, their types, how they operate and their applications. 
 
Traditionally, ac - ac conversion using semiconductor switches is done in two different ways: 1 - in 
two stages (ac - dc and then dc - ac) as in dc link converters or 2 - in one stage (ac - ac) 
cycloconverters (Fig. 1). Cycloconverters are used in high power applications driving induction 
and synchronous motors. They are usually phase - controlled and they traditionally use thyristors 
due to their ea se of phase commutation.  
 
Fig.1 Block diagram of a cycloconverter 
There are other newer forms of cycloconversion such as ac - ac matrix converters and high 
frequency ac - ac (hfac - ac) converters and these use self - controlled switches. These converters, 
however, are not popular yet. 
 
Some applications of cycloconverters are: 
• Cement mill drives 
• Ship propulsion drives 
• Rolling mill drives 
• Scherbius drives 
• Ore grinding mills  
• Mi ne winders 
 
1.Operation Principles: 
The following sections will describe the operation principles of the cycloconverter starting from 
the simplest one, single - phase to single - phase (1 ?- 1 ?) cycloconverter. 
1.1. Single - phase to Single - phase (1 ? ? - 1 ? ? ) Cycloconv erter: 
To understand the operation principles of cycloconverters, the single - phase to single - phase 
cycloconverter (Fig. 2) should be studied first. This converter consists of back - to - back 
connection of two full - wave rectifier circuits. Fig 3 shows the oper ating waveforms for this 
converter with a resistive load.  
 
The input voltage, v
s is an ac voltage at a frequency, f
i as shown in Fig. 3a. For easy 
understanding assume that all the thyristors are fired at a =0 ° firing angle, i.e. thyristors act like 
diodes . Note that the firing angles are named as a
P for the positive converter and a
N for the 
negative converter.  
 
Consider the operation of the cycloconverter to get one - fourth of the input frequency at the 
output. For the first two cycles of v
s,
 the positive converter operates supplying current to the 
load. It rectifies the input voltage; therefore, the load sees 4 positive half cycles as seen in Fig. 
3b. In the next two cycles, the negative converter operates supplying current to the load in the 
reverse direc tion. The current waveforms are not shown in the figures because the resistive load 
Page 3


CYCLOCONVERTERS 
 
 
 
 
In industrial applications, two forms of electrical energy are used: direct current (dc) and 
altern ating current (ac). Usually constant voltage constant frequency single - phase or three - phase 
ac is readily available. However, for different applications, different forms, magnitudes and/or 
frequencies are required. There are four different conversions betw een dc and ac power sources. 
These conversions are done by circuits called power converters. The converters are classified as: 
1 - rectifiers: from single - phase or three - phase ac to variable voltage dc 
2 - choppers: from dc to variable voltage dc 
3 - inverters: from dc to variable magnitude and variable frequency, single - phase or three - phase ac 
4 - cycloconverters: from single - phase or three - phase ac to variable magnitude and 
variable frequency, single - phase or three - phase ac 
The first three classes are explained i n other articles. This article explains what cycloconverters 
are, their types, how they operate and their applications. 
 
Traditionally, ac - ac conversion using semiconductor switches is done in two different ways: 1 - in 
two stages (ac - dc and then dc - ac) as in dc link converters or 2 - in one stage (ac - ac) 
cycloconverters (Fig. 1). Cycloconverters are used in high power applications driving induction 
and synchronous motors. They are usually phase - controlled and they traditionally use thyristors 
due to their ea se of phase commutation.  
 
Fig.1 Block diagram of a cycloconverter 
There are other newer forms of cycloconversion such as ac - ac matrix converters and high 
frequency ac - ac (hfac - ac) converters and these use self - controlled switches. These converters, 
however, are not popular yet. 
 
Some applications of cycloconverters are: 
• Cement mill drives 
• Ship propulsion drives 
• Rolling mill drives 
• Scherbius drives 
• Ore grinding mills  
• Mi ne winders 
 
1.Operation Principles: 
The following sections will describe the operation principles of the cycloconverter starting from 
the simplest one, single - phase to single - phase (1 ?- 1 ?) cycloconverter. 
1.1. Single - phase to Single - phase (1 ? ? - 1 ? ? ) Cycloconv erter: 
To understand the operation principles of cycloconverters, the single - phase to single - phase 
cycloconverter (Fig. 2) should be studied first. This converter consists of back - to - back 
connection of two full - wave rectifier circuits. Fig 3 shows the oper ating waveforms for this 
converter with a resistive load.  
 
The input voltage, v
s is an ac voltage at a frequency, f
i as shown in Fig. 3a. For easy 
understanding assume that all the thyristors are fired at a =0 ° firing angle, i.e. thyristors act like 
diodes . Note that the firing angles are named as a
P for the positive converter and a
N for the 
negative converter.  
 
Consider the operation of the cycloconverter to get one - fourth of the input frequency at the 
output. For the first two cycles of v
s,
 the positive converter operates supplying current to the 
load. It rectifies the input voltage; therefore, the load sees 4 positive half cycles as seen in Fig. 
3b. In the next two cycles, the negative converter operates supplying current to the load in the 
reverse direc tion. The current waveforms are not shown in the figures because the resistive load 
current will have the same waveform as the voltage but only scaled by the resistance. Note that 
when one of the converters operates the other one is disabled, so that there is no current 
circulating between the two rectifiers. 
 
a a b b 
Fig. 2 Single - phase to single - phase cycloconverter 
 
Page 4


CYCLOCONVERTERS 
 
 
 
 
In industrial applications, two forms of electrical energy are used: direct current (dc) and 
altern ating current (ac). Usually constant voltage constant frequency single - phase or three - phase 
ac is readily available. However, for different applications, different forms, magnitudes and/or 
frequencies are required. There are four different conversions betw een dc and ac power sources. 
These conversions are done by circuits called power converters. The converters are classified as: 
1 - rectifiers: from single - phase or three - phase ac to variable voltage dc 
2 - choppers: from dc to variable voltage dc 
3 - inverters: from dc to variable magnitude and variable frequency, single - phase or three - phase ac 
4 - cycloconverters: from single - phase or three - phase ac to variable magnitude and 
variable frequency, single - phase or three - phase ac 
The first three classes are explained i n other articles. This article explains what cycloconverters 
are, their types, how they operate and their applications. 
 
Traditionally, ac - ac conversion using semiconductor switches is done in two different ways: 1 - in 
two stages (ac - dc and then dc - ac) as in dc link converters or 2 - in one stage (ac - ac) 
cycloconverters (Fig. 1). Cycloconverters are used in high power applications driving induction 
and synchronous motors. They are usually phase - controlled and they traditionally use thyristors 
due to their ea se of phase commutation.  
 
Fig.1 Block diagram of a cycloconverter 
There are other newer forms of cycloconversion such as ac - ac matrix converters and high 
frequency ac - ac (hfac - ac) converters and these use self - controlled switches. These converters, 
however, are not popular yet. 
 
Some applications of cycloconverters are: 
• Cement mill drives 
• Ship propulsion drives 
• Rolling mill drives 
• Scherbius drives 
• Ore grinding mills  
• Mi ne winders 
 
1.Operation Principles: 
The following sections will describe the operation principles of the cycloconverter starting from 
the simplest one, single - phase to single - phase (1 ?- 1 ?) cycloconverter. 
1.1. Single - phase to Single - phase (1 ? ? - 1 ? ? ) Cycloconv erter: 
To understand the operation principles of cycloconverters, the single - phase to single - phase 
cycloconverter (Fig. 2) should be studied first. This converter consists of back - to - back 
connection of two full - wave rectifier circuits. Fig 3 shows the oper ating waveforms for this 
converter with a resistive load.  
 
The input voltage, v
s is an ac voltage at a frequency, f
i as shown in Fig. 3a. For easy 
understanding assume that all the thyristors are fired at a =0 ° firing angle, i.e. thyristors act like 
diodes . Note that the firing angles are named as a
P for the positive converter and a
N for the 
negative converter.  
 
Consider the operation of the cycloconverter to get one - fourth of the input frequency at the 
output. For the first two cycles of v
s,
 the positive converter operates supplying current to the 
load. It rectifies the input voltage; therefore, the load sees 4 positive half cycles as seen in Fig. 
3b. In the next two cycles, the negative converter operates supplying current to the load in the 
reverse direc tion. The current waveforms are not shown in the figures because the resistive load 
current will have the same waveform as the voltage but only scaled by the resistance. Note that 
when one of the converters operates the other one is disabled, so that there is no current 
circulating between the two rectifiers. 
 
a a b b 
Fig. 2 Single - phase to single - phase cycloconverter 
 
Fig. 3 Single - phase to single - ph ase cycloconverter waveforms 
a) input voltage 
b) output voltage for zero firing angle 
c) output voltage with firing angle p /3 rad. 
d) output voltage with varying firing angle 
The frequency of the output voltage, v
o in Fig. 3b is 4 times less than that of v
s , the input voltage, 
i.e. f
o /f
i =1/4. Thus, this is a step - down cycloconverter. On the other hand, cycloconverters that 
ha ve f
o /f
i >1 frequency relation are called step - up cycloconverters. Note that step - down 
cycloconverters are more widely used than the step - up ones.  
 
The frequency of v
o can be changed by varying the number of cycles the positive and the 
negative converters work. It can only change as integer multiples of f
i 
in 1 ?- 1 ? cycloconverters. 
 
With the above operation, the 1 ?- 1 ? cycloconverter can only supply a certain voltage at a certain 
firing angle a . The dc output of each rectifier is: 
22 cos dV V a
p
=          (1) 
where V is the input rms voltage. 
The dc value per half cycle is shown as dotted in Fig. 3d. 
Then the peak of the fundamental output voltage is 
1 422 () cos o vt V a
p p
=         (2) 
Equation 2 implies that the fundamental output voltage dep ends on a . For a =0 °, 
10 1 do do V V V = ×= where 
422 doV V p p
= . If a is increased to p /3 as in Fig. 3d, then 
1 0.5 o do VV=× . 
Thus varying a , the fundamental output voltage can be controlled. 
 
Constant a operation gives a cr ude output waveform with rich harmonic content. The dotted 
lines in Fig. 3b and c show a square wave. If the square wave can be modified to look more like 
a sine wave, the harmonics would be reduced. For this reason a is modulated as shown in Fig. 
3d. Now, the six - stepped dotted line is more like a sinewave with fewer harmonics. The more 
pulses there are with different a 's, the less are the harmonics. 
Page 5


CYCLOCONVERTERS 
 
 
 
 
In industrial applications, two forms of electrical energy are used: direct current (dc) and 
altern ating current (ac). Usually constant voltage constant frequency single - phase or three - phase 
ac is readily available. However, for different applications, different forms, magnitudes and/or 
frequencies are required. There are four different conversions betw een dc and ac power sources. 
These conversions are done by circuits called power converters. The converters are classified as: 
1 - rectifiers: from single - phase or three - phase ac to variable voltage dc 
2 - choppers: from dc to variable voltage dc 
3 - inverters: from dc to variable magnitude and variable frequency, single - phase or three - phase ac 
4 - cycloconverters: from single - phase or three - phase ac to variable magnitude and 
variable frequency, single - phase or three - phase ac 
The first three classes are explained i n other articles. This article explains what cycloconverters 
are, their types, how they operate and their applications. 
 
Traditionally, ac - ac conversion using semiconductor switches is done in two different ways: 1 - in 
two stages (ac - dc and then dc - ac) as in dc link converters or 2 - in one stage (ac - ac) 
cycloconverters (Fig. 1). Cycloconverters are used in high power applications driving induction 
and synchronous motors. They are usually phase - controlled and they traditionally use thyristors 
due to their ea se of phase commutation.  
 
Fig.1 Block diagram of a cycloconverter 
There are other newer forms of cycloconversion such as ac - ac matrix converters and high 
frequency ac - ac (hfac - ac) converters and these use self - controlled switches. These converters, 
however, are not popular yet. 
 
Some applications of cycloconverters are: 
• Cement mill drives 
• Ship propulsion drives 
• Rolling mill drives 
• Scherbius drives 
• Ore grinding mills  
• Mi ne winders 
 
1.Operation Principles: 
The following sections will describe the operation principles of the cycloconverter starting from 
the simplest one, single - phase to single - phase (1 ?- 1 ?) cycloconverter. 
1.1. Single - phase to Single - phase (1 ? ? - 1 ? ? ) Cycloconv erter: 
To understand the operation principles of cycloconverters, the single - phase to single - phase 
cycloconverter (Fig. 2) should be studied first. This converter consists of back - to - back 
connection of two full - wave rectifier circuits. Fig 3 shows the oper ating waveforms for this 
converter with a resistive load.  
 
The input voltage, v
s is an ac voltage at a frequency, f
i as shown in Fig. 3a. For easy 
understanding assume that all the thyristors are fired at a =0 ° firing angle, i.e. thyristors act like 
diodes . Note that the firing angles are named as a
P for the positive converter and a
N for the 
negative converter.  
 
Consider the operation of the cycloconverter to get one - fourth of the input frequency at the 
output. For the first two cycles of v
s,
 the positive converter operates supplying current to the 
load. It rectifies the input voltage; therefore, the load sees 4 positive half cycles as seen in Fig. 
3b. In the next two cycles, the negative converter operates supplying current to the load in the 
reverse direc tion. The current waveforms are not shown in the figures because the resistive load 
current will have the same waveform as the voltage but only scaled by the resistance. Note that 
when one of the converters operates the other one is disabled, so that there is no current 
circulating between the two rectifiers. 
 
a a b b 
Fig. 2 Single - phase to single - phase cycloconverter 
 
Fig. 3 Single - phase to single - ph ase cycloconverter waveforms 
a) input voltage 
b) output voltage for zero firing angle 
c) output voltage with firing angle p /3 rad. 
d) output voltage with varying firing angle 
The frequency of the output voltage, v
o in Fig. 3b is 4 times less than that of v
s , the input voltage, 
i.e. f
o /f
i =1/4. Thus, this is a step - down cycloconverter. On the other hand, cycloconverters that 
ha ve f
o /f
i >1 frequency relation are called step - up cycloconverters. Note that step - down 
cycloconverters are more widely used than the step - up ones.  
 
The frequency of v
o can be changed by varying the number of cycles the positive and the 
negative converters work. It can only change as integer multiples of f
i 
in 1 ?- 1 ? cycloconverters. 
 
With the above operation, the 1 ?- 1 ? cycloconverter can only supply a certain voltage at a certain 
firing angle a . The dc output of each rectifier is: 
22 cos dV V a
p
=          (1) 
where V is the input rms voltage. 
The dc value per half cycle is shown as dotted in Fig. 3d. 
Then the peak of the fundamental output voltage is 
1 422 () cos o vt V a
p p
=         (2) 
Equation 2 implies that the fundamental output voltage dep ends on a . For a =0 °, 
10 1 do do V V V = ×= where 
422 doV V p p
= . If a is increased to p /3 as in Fig. 3d, then 
1 0.5 o do VV=× . 
Thus varying a , the fundamental output voltage can be controlled. 
 
Constant a operation gives a cr ude output waveform with rich harmonic content. The dotted 
lines in Fig. 3b and c show a square wave. If the square wave can be modified to look more like 
a sine wave, the harmonics would be reduced. For this reason a is modulated as shown in Fig. 
3d. Now, the six - stepped dotted line is more like a sinewave with fewer harmonics. The more 
pulses there are with different a 's, the less are the harmonics. 
1.2. Three - Phase to Single - Phase (3 ? ? - 1 ? ? ) Cycloconverter: 
There are two kinds of three - phase to single - phase (3 ?- 1 ?) cycloconverters: 3 ?- 1 ? half - wave 
cycloconverter (Fig. 4) and 3 ?- 1 ? bridge cycloconverter (Fig. 5). Like the 1 ?- 1 ? case, the 3 ?- 1 ? 
cycloconverter applies rectified voltage to the load. Both positive and negative converters can 
generate voltages at either polarity, but the positive converter can only supply positive current 
and the negative converter can only supply negative current. Thus, the cycloconverter can 
operate in four quadrants: (+v, +i) and ( - v, - i) rectification modes and (+v, - i) and ( - v , +i) 
inversion modes. The modulation of the output voltage and the fundamental output voltage are 
shown in Fig. 6. Note that a is sinusoidally modulated over the cycle to generate a harmonically 
optimum output voltage. 
 
Fig. 4 3 ?- 1 ? half - wave cycloconverter 
 
Fig. 5 3 ?- 1 ? bridge cycloconverter