Download Introduction of Single Phase to Single Phase Cycloconverter

HOW-TO GUIDE
Introduction of Single Phase to Single
Phase Cycloconverter
Contents at a Glance
Introduction: ...................................................................3
THEORY: ..........................................................................4
TYPES OF CYCLOCONVERTER ............................................5
Operation Principles ........................................................5
 Single-phase to Single-phase (1-1) Cycloconverter..5
 Three-Phase to Single-Phase (3-1) Cycloconverter:10
 Three-Phase to Three-Phase (3-3) Cycloconverter:12
Applications of Cyclocontrollers ..................................... 14
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Introduction:
The Cycloconverter has been traditionally used only in very high power
drives, usually above one megawatt, where no other type of drive can be used.
Examples are cement tube mill drives above 5 MW, the 13 MW German-Dutch
wind tunnel fan drive, reversible rolling mill drives and ship propulsion drives. The
reasons for this are that the traditional cycloconverter requires a large number of
thyristors, at least 36 and usually more for good motor performance, together
with a very complex control circuit, and it has some performance limitations, the
worst of which is an output frequency limited to about one third the input
frequency .
Single Phase to
Single Phase
Cycloconverter
F I/P
F O/P
Block Diagram of Cycloconverter
The Cycloconverter has four thyristors divided into a positive and
negative bank of two thyristors each. When positive current flows in the load, the
output voltage is controlled by phase control of the two positive bank thyristors
whilst the negative bank thyristors are kept off and vice versa when negative
current flows in the load.
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An idealized output waveform for a sinusoidal load current and a 45
degrees load phase angle is shown in Figure 2. It is important to keep the non
conducting thyristor bank off at all times, otherwise the mains could be shorted
via the two thyristor banks, resulting in waveform distortion and possible device
failure from the shorting current. A major control problem of the Cycloconverter
is how to swap between banks in the shortest possible time to avoid distortion
whilst ensuring the two banks do not conduct at the same time. A common
addition to the power circuit that removes the requirement to keep one bank off
is to place a centre tapped inductor called a circulating current inductor between
the outputs of the two banks. Both banks can now conduct together without
shorting the mains. Also, the circulating current in the inductor keeps both banks
operating all the time, resulting in improved output waveforms. This technique is
not often used, though, because the circulating current inductor tends to be
expensive and bulky and the circulating current reduces the power factor on the
input.
THEORY:
In a
1
Cycloconverter, the output frequency is less than the supply
frequency. These converters require natural commutation which is provided by
AC supply. During positive half cycle of supply, thyristors
biased. First triggering pulse is applied to
P1
P1
and
N2
are forward
and hence it starts conducting.
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As the supply goes negative,
and
N1
are forward biased.
of supply,
N2
P2
P1
gets off and in negative half cycle of supply,
P2
is triggered and hence it conducts. In the next cycle
in positive half cycle and
N1
in negative half cycle are triggered.
Thus, we can observe that here the output frequency is 1/2 times the supply
frequency.
TYPES OF CYCLOCONVERTER
The Cycloconverter are classified into three types based on the type of input
ac supply applied to the circuit.
 Single Phase to Single phase Cycloconverter.
 Three Phase to Single Phase Cycloconverter.
 Three Phase to Three Phase Cycloconverter
Operation Principles
The following sections will describe the operation principles of the
cycloconverter starting from the simplest one, single-phase to single-phase (1f1f) cycloconverter.
 Single-phase to Single-phase (1-1) Cycloconverter
To understand the operation principles of cycloconverters, the singlephase 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 operating waveforms for this converter with a resistive load.
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The input voltage, vs is an ac voltage at a frequency, fi as shown in Fig. 3a.
For easy understanding assume that all the thyristors are fired at =0firing
angle, i.e. thyristors act like diodes. Note that the firing angles are named as P
for the positive converter and 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 vs, 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 direction. 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.
Fig:2 Single Phase to Single Phase Cyclo Converter
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Fig:3 Single Phase to Single Phase Cyclo Converter
a) Input Voltage
b) Output voltage for Zero Firing angle
c) Output voltage with firing angle π/3rad
d) Output voltage with varying firing angle
Thus by varying , the fundamental output voltage can be controlled.
Constant operation gives a crude 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 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 's, the less are the harmonics.
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a) Frequency Select =F
b) Frequency Select =F/2
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c) Frequency Select =F/3
d) Frequency Select =F/4
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 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 is sinusoidally modulated over the cycle to
generate a harmonically optimum output voltage.
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Fig. 6 3-1half-wave cycloconverter
waveforms
a) + converter output voltage
b) Cosine timing waves
c) – converter output voltage
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The polarity of the current determines if the positive or negative converter
should be supplying power to the load. Conventionally, the firing angle for the
positive converter is named P, and that of the negative converter is named N.
When the polarity of the current changes, the converter previously supplying the
current is disabled and the other one is enabled. The load always requires the
fundamental voltage to be continuous. Therefore, during the current polarity
reversal, the average voltage supplied by both of the converters should be equal.
Otherwise, switching from one converter to the other one would cause an
undesirable voltage jump. To prevent this problem, the converters are forced to
produce the same average voltage at all times.
 Three-Phase to Three-Phase (3-3) Cycloconverter:
If the outputs of 3 3-1converters of the same kind are connected in wye or
delta and if the output voltages are 2/3 radians phase shifted from each other, the
resulting converter is a threephase to three-phase (3-3) cycloconverter. The
resulting cycloconverters are shown in Figs. 7 and 8 with wye connections. If the three
converters connected are half-wave converters, then the new converter is called a 33half-wave cycloconverter. If instead, bridge converters are used, then the result is a
3-3bridge cycloconverter. 3-3half-wave cycloconverter is also called a 3-pulse
cycloconverter or an 18-thyristor cycloconverter. On the other hand, the 3-3bridge
cycloconverter is also called a 6-pulse cycloconverter or a 36-thyristor cycloconverter.
The operation of each phase is explained in the previous section.
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Fig. 7 3-3half-wave cycloconverter
Fig. 8 3-3bridge cycloconverter
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The three-phase cycloconverters are mainly used in ac machine drive
systems running threephase synchronous and induction machines. They are more
advantageous when used with a synchronous machine due to their output power
factor characteristics. A cycloconverter can supply lagging, leading, or unity
power factor loads while its input is always lagging. A synchronous machine can
draw any power factor current from the converter. This characteristic operation
matches the cycloconverter to the synchronous machine. On the other hand,
induction machines can only draw lagging current, so the cycloconverter does not
have an edge compared to the other converters in this aspect for running an
induction machine. However, cycloconverters are used in Scherbius drives for
speed control purposes driving wound rotor induction motors.
Cycloconverters produce harmonic rich output voltages, which will be discussed in
the following sections. When cycloconverters are used to run an ac machine, the
leakage inductance of the machine filters most of the higher frequency harmonics and
reduces the magnitudes of the lower order harmonics.
Applications of Cyclocontrollers

Cement mill drives

Ship propulsion drives

Rolling mill drives

Scherbius drives

Ore grinding mills

Mine winders
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