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
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