1. Cycloconverters are static frequency changers (SFCs) designed to produce adjustable voltage
adjustable frequency AC power from a constant voltage constant frequency AC source without
any intermediate DC link.
Cyclo-converters are constructed using naturally commutated thyristors with inherent
capability of bidirectional power flow. These can be single phase to single phase, single phase to
three- phase and three-phase to three phase converters.
Applications of Cycloconverters
 Cement mill drives
 Ship propulsion drives
 Rolling mill drives
 Ore grinding mills
 Mine winders
 Synchronous Motors
 Variable- speed, constant-frequency{VSCF} power generation for aircraft 400 Hz power
supplies.
Advantages of Cycloconverters
 No intermediate DC state is required for AC-AC conversion.
 Extremely attractive for large power, low speed drives.
 Capable of power transfer in either direction between source and load
Limitations of Cycloconverters
 Large number of thyristors are required.
 The output frequency is limited to one-third of the input frequency.
Basically, cycloconverters are of two types:

2. i) Step-down cycloconverters: The output frequency fo is lower than the supply
frequency fs
ii) Step-up cycloconverters: The output frequency fo is more than the supply frequency
fs .
In case of step-down cyclo-converter, the output frequency is limited to a fraction of input
frequency, typically it is below 20Hz in case 50Hz supply frequency. In this case, no separate
commutation circuits are needed as SCRs are line commutated devices.
But in case of step-up cyclo-converter, forced commutation circuits are needed to turn OFF
SCRs at desired frequency. Such circuits are relatively very complex. Therefore, majority of
cyclo-converters are of step-down type that lowers the frequency than input frequency.
Step-down cyclo-converter circuits can be further classified into following types.
• Single-phase to single-phase cyclo-converters
• Three-phase to single-phase cyclo-converters
• Three-phase to three-phase cyclo-converters
Basic Principle of Operation of Cyclo-converter
The equivalent circuit of a cyclo-converter is shown in figure below. Here each two quadrant
phase controlled converter is represented by a voltage source of desired frequency and
consider that the output power is generated by the alternating current and voltage at desired
frequency.
The diodes connected in series with each voltage source represent the unidirectional
conduction of each two quadrant converter. If the output voltage ripples of each converter are
neglected, then it becomes ideal and represents the desired output voltage.If the firing angles
of individual converters are modulated continuously, each converter produces same sinusoidal
voltages at its output terminals.

3. So the voltages produced by these two converters have same phase, voltage and frequency.
The average power produced by the cyclo-converter can flow either to or from the output
terminals as the load current can flow freely to and from the load through the positive and
negative converters.
Therefore, it is possible to operate the loads of any phase angle (or power factor), inductive or
capacitive through the cyclo-converter circuit.
Due to the unidirectional property of load current for each converter, it is obvious that positive
converter carries positive half-cycle of load current with negative converter remaining in idle
during this period.
Similarly, negative converter carries negative half cycle of the load current with positive
converter remaining in idle during this period, regardless of the phase of current with respect
to voltage.
This means that each converter operates both in rectifying and inverting regions during the
period of its associated half cycles.
The figure
below
shows ideal
output
current and
voltage
waveforms
of a cyclo-
converter
for lagging
and leading
power

4. factor loads. The conduction periods of positive and negative converters are also illustrated in
the figure.
The positive converter operates whenever the load current is positive with negative converter
remaining in idle. In the same manner negative converter operates for negative half cycle of
load current.
Both rectification and inversion modes of each converter are shown in figure. This desired
output voltage is produced by regulating the firing angle to individual converters.
SINGLE-PHASE TO SINGLE-PHASE CIRCUIT-STEP-UP CONVERTER

5. These are rarely used in practice; however, these are required to understand fundamental
principle of cyclo-converters.
Mid-point cycloconverters
It consists of a single phase transformer with mid tap on the secondary windings and four
thyristors. Two of these thyristors P1 and P2 are for positive group and the other two N1 and
N2 are for the negative group. Load is connected between secondary winding mid-point 0 and
terminal A as shown in fig.
The load which is assumed to be an R Load is connected as shown.
In fig. 5, during the positive half cycle both SCRs P1 and N2 are forward biased from wt=0 to
wt=π. As such P1 is turned on at wt=0 so that load voltage is positive with terminal A positive
and 0 negative. The load voltage now follows the positive envelope of the supply voltage, fig. 5.
At instant wt1 P1 is force commutated and forward-biased thyristors N2 is turned on so that
load voltage is negative with terminal 0 positive and A negative. The load or output voltage now
traces the negative envelope of the supply voltage. At wt2, N2 is force commutated and P1 is
turned on, the load voltage is now positive and follows the positive envelope by supply voltage.
The cycle continues till wt=π.

6. After wt=π, terminal b is positive with respect to terminal a; both SCRs P2 and N1 are therefore
forward biased from wt=π to 2π. At wt=π, N2 is force commutated and forward biased SCR P2 is
turned on. In this manner, thyristors P1 and N2 for first half cycle and P2 and N1 for the other
half cycle and so on are switched alternately between positive and negative envelopes at a high
frequency. We observe that fo=6fs and hence a step up Cycloconverter.
BRIDGE - TYPE CYCLOCONVERTERS
This Cycloconverter consists of eight thyristors, P1 to P4 for positive group and the remaining
four for the negative group. During the positive half cycle of the supply voltage, thyristors pairs
P1, P4 and N1, N4 are forward biased from wt=0 to wt=π. At wt=0, P1 and P4 are turned on
together so that the load voltage is positive with terminal A positive with respect to O. and thus
load voltage traverses the positive envelope of supply voltage. At wt1 P1 and P4 are force

7. commutated and the pair N1 and N4 is turned on. And thus the cycle repeats. At wt=π,
thyristors pairs P2, P3 and N2, N3 are forward biased and thus the cycle repeats for wt=2π. In
this manner, a high frequency turning-on and force commutation of pairs P1P4, N1N4 and pairs
P2P3, N2N3 gives a carrier frequency modulated output voltage across load terminals
SINGLE-PHASE TO SINGLE-PHASE CIRCUIT-STEP-DOWN CONVERTER
A single-phase to single-phase Cycloconverter is shown in fig. 6.Two full-wave fully controlled
bridge converter circuits, using four thyristors for each bridge, are connected in opposite
direction (back to back), with both bridges being fed from ac supply (50 Hz). Bridge 1 (P –
positive) supplies load current in the positive half of the output cycle, while bridge 2 (N –
negative) supplies load current in the negative half. The two bridges should not conduct
together as this will produce short-circuit at the input. In this case, two thyristors come in series
with each voltage source. When the load current is positive, the firing pulses to the thyristors of
bridge2 are inhibited, while the thyristors of bridge 1 are triggered by giving pulses at their
gates at that time.
Similarly, when the load current is negative, the thyristors of bridge 2 are triggered by giving
pulses at their gates, while the firing pulses to the thyristors of bridge 1 are inhibited at that
time. This is the circulating-current free mode of operation. Thus, the firing angle control
scheme must be such that only one converter conduct at a time, and the changeover of firing
pulses from one converter to the other, should be periodic according to the output frequency.
However, the firing angles the thyristors in both converters should be the same to produce a
symmetrical output.

8. When a cycloconverter operates in the non-circulating current mode, the control scheme is
complicated, if the load current is discontinuous. The control is somewhat simplified, if some
amount of circulating current is allowed to flow between them. In this case, a circulating
current limiting reactor is connected between the positive and negative converters, as is the
case with dual converter, i.e. two fully controlled bridge converters connected back to back, in
circulating current mode. This circulating current by itself keeps both converters in virtually
continuous conduction over the whole control range. This type of operation is termed as the
circulating-current mode of operation.
Operation of the cyclo-converter circuit with purely resistive (R) Load:
With resistive load, the load current (instantaneous) goes to zero, as the input voltage at the
end of each half cycle (both positive and negative) reaches zero. Thus, the conducting thyristor
pair in one of the bridges turns off at that time, i.e. the thyristors undergo natural
commutation. So, operation with discontinuous current takes place, as current flows in the
load, only when the next thyristor pair in that bridge is triggered, or pulses are fed at respective
gates.
Taking first bridge 1 (positive), and assuming the top point of the ac supply as positive with the
bottom point as negative in the positive half cycle of ac input, the odd-numbered thyristor pair,
P1 & P3 is triggered after phase delay (α1), such that current starts flowing through the load in
this half cycle. In the next (negative) half cycle, the other thyristor pair (even-numbered), P2 &
P4 in that bridge conducts, by triggering them after suitable phase delay from the start of zero-
crossing. The current flows through the load in the same direction, with the output voltage also
remaining positive. This process continues for one more half cycle (making a total of three) of
input voltage (f2=f1/3= Hz).
To obtain negative output voltage, in the next three half cycles of input voltage, bridge 2 is
used. Following same logic, if the bottom point of the ac supply is taken as positive with the top
point as negative in the negative half of ac input, the odd-numbered thyristor pair, N1 & N3
conducts, by triggering them after suitable phase delay from the zero-crossing. Similarly, the
even-numbered thyristor pair, N2 & N4 conducts in the next half cycle. Both the output voltage

9. and current are now negative. As in the previous case, the above process also continues for
three consecutive half cycles of input voltage. From three waveforms, one combined negative
half cycle of output voltage is produced, having same frequency as given earlier.
The pattern of firing angle − first decreasing and the increasing, is also followed in the negative
half cycle. One positive half cycle, along with one negative half cycle, constitute one complete
cycle of output (load) voltage waveform, its frequency being Hz as stated earlier. The ripple
frequency of the output voltage/ current for single–phase full-wave converter is 100 Hz, i.e.,
double of the input frequency. It may be noted that the load (output) current is discontinuous,
as also load (output) voltage.

10. Operation of the cyclo-converter circuit with R-L Load:
For R-L load, the load current may be continuous or discontinuous depending on the firing
angle and load power factor. The load voltage and current waveforms are shown for continuous
and discontinuous load current in Fig. 8 and 9 respectively. In this case, the output frequency is
¼ times to that of the input frequency. So, four positive half cycles, or two full cycles of the
input to the full-wave bridge converter, are required to produce one positive half cycle of the
output waveform. Here the current flows even after the input voltage has reversed (after θ=π),
till it reaches zero at (θ=β1) with (π+α2) > β1 > π, due to inductance being present in series with
resistance, its value being low.

11. MODE OF OPERATION OF CYCLOCONVERTERS
Blocked Mode of Operation
The operation of the Cycloconverters is explained above in ideal terms. When the load current
is positive, the positive converter supplies the required voltage and the negative converter is
disabled. On the other hand, when the load current is negative, then the negative converter
supplies the required voltage and the positive converter is blocked. This operation is called the
blocked mode operation, and the Cycloconverters using this approach are called blocking mode
Cycloconverters.
Circulating Current Mode of Operation
However, if by any chance both of the converters are enabled, then the supply is short-
circuited. To avoid this short circuit, an intergroup reactor (IGR) can be connected between the
converters. Instead of blocking the converters during current reversal, if they are both enabled,
then a circulating current is produced. This current is called the circulating current. It is
unidirectional because the thyristors allow the current to flow in only one direction. Some
Cycloconverters allow this circulating current at all times. These are called circulating current
Cycloconverters.

12. Three-Phase to Single-Phase (3 -1 ) Cycloconverter:

14. 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 3f-3f 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.

16. Matrix Converter:
The matrix converter is a fairly new converter topology, which was first
proposed in the beginning of the 1980s. A matrix converter consists of a matrix
of 9 switches connecting the three input phases to the three output phases
directly as shown in Fig. 12. Any input phase can be connected to any output
phase at any time depending on the control. However, no two switches from the
same phase should be on at the same time, otherwise this will cause a short
circuit of the input phases. These converters are usually controlled by PWM to
produce three-phase variable voltages at variable frequency.