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Basic Principle of Operation

The basic equivalent circuit of cycloconverter is shown in fig.1.

• Each two-quadrant converter (phase-controlled) is represented as an alternating voltage source, which corresponds to the fundamental voltage component obtained at its output terminals.
• The diodes connected in series with each voltage source, show the unidirectional conduction of each converter, whose output voltage can be either positive or negative, being a two-quadrant one, but the direction of current is in the direction as shown in the circuit, as only thyristors − unidirectional switching devices, are used in the two converters.
• Normally, the ripple content in the output voltage is neglected.

Fig. 1: Equivalent circuit of cycloconverter

The control principle used in an ideal cyclo-converter is to continuously modulate the firing angles of the individual converters, so that each produces the same sinusoidal (ac) voltage at its output terminals.

Thus,

• the voltages of the two generators have the same amplitude, frequency and phase,
• voltage of the cyclo-converter is equal to the voltage of either of these generators.

It is possible for the mean power to flow either ‘to’ or ‘from’ the output terminals, and the cyclo-converter is inherently capable of operation with loads of any phase angle − inductive or capacitive.

Because of the uni-directional current carrying property of the individual converters, it is inherent that the positive half-cycle of load current must always be carried by the positive converter and the negative half-cycle by the negative converter, regardless of the phase of the current with respect to the voltage.

This means that each two-quadrant converter operates both in its rectifying (converting) and in its inverting region during the period of its associated half-cycle of current.

The output voltage and current waveforms of cycloconverter are shown in the fig a and b.

The displacement angle of the load (current) is zero in fig a.

In this case,

• Each converter carries the load current only, when it operates in its rectifying region, and it remains idle throughout the whole period in which its terminal voltage is in the inverting region of operation.

The displacement angle of the load is lagging in fig b.

• During the first period of each half-cycle of load current, the associated converter operates in its rectifying region, and delivers power to the load.
• During the latter period in the half-cycle, the associated converter operates in its inverting region, and under this condition, the load is regenerating power back into the cyclo-converter output terminals, and hence, into the ac system at the input side.

Any other case, say capacitive load, with the displacement angle as leading, the operation changes with inverting region in the first period of the half-cycle as per displacement angle, and the latter period operating in rectifying region.

Single-phase to Single-phase Cyclo-converter

The circuit of a single-phase to single-phase cyclo-converter is shown in Fig 2.

Fig. 2: Single-phase to single-phase cycloconverter (using thyristor bridges)

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
• 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 bridge 2 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.  When a cyclo-converter 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.

Resistive (R) Load:

• For this 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 (0).
• Thus, the conducting thyristor pair in one of the bridges turns off at that time, i.e. the thyristors undergo natural commutation.

Fig.4: Input (a) and output (b) voltage waveforms of a cyclo-converter for resistive (R) load

The following points may be noted.

1. The firing angle (α) of the converter is first decreased, in this case for second cycle only, and then again increased in the next (third) cycle, as shown in Fig.4.b. This is, because only three cycles for each half cycle is used.
2. If the output frequency needed is lower, the number of cycles is to be increased, with the firing angle decreasing for some cycles, and then again increasing in the subsequent cycles.
3. To obtain negative output voltage, in the next three half cycles of input voltage, bridge 2 is used.
4. 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 & N₃conducts, 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 and current are now negative.
5. 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.
6. The ripple frequency of the output voltage/ current for single–phase full-wave converter is 100 Hz, i.e., double of the input frequency.

Only one of two thyristor bridges (positive or negative) conducts at a time, giving non-circulating current mode of operation in this circuit.

Inductive (R-L) Load:

For this load, the load current may be continuous or discontinuous depending on the firing angle and load power factor.

• Discontinuous load current

Fig. 5: Input (a) and output (b) voltage, and current (c) waveforms for a cyclo-converter with discontinuous

1. The load current in this case is discontinuous, as the inductance, L in series with the resistance, R, is low. This is somewhat similar to the previous case, but difference also exists.
2. Here, also non-circulating mode of operation takes place, with only one of the bridges − #1 (positive), or #2 (negative), conducting at a time, but two bridges do not conduct at the same time, as this will result in a short circuit.
3. Four positive half cycles, or two full cycles of the input to the full-wave bridge converter (#1), are required to produce one positive half cycle of the output waveform
4. The firing angle (α) of the converter is first decreased, in this case for second half cycle only, kept nearly same in the third one, and finally increased in the last (fourth) one, as shown in Fig.5b.
5. To obtain negative output voltage, in the next four half cycles of output voltage, bridge 2 is used.
6. The pattern of firing angle − first decreasing and then increasing, is also followed in the negative half cycle.

It may be noted that the load (output) current is discontinuous (Fig.5c), as also load (output) voltage (Fig.5b). The supply (input) voltage is shown in Fig.5a. One positive half cycle, along with one negative half cycle, constitute one complete cycle of output (load) voltage waveform, the ripple frequency remains also same at 100 Hz, with the ripple in load current being filtered by the inductance present in the load.

• Continuous load current

1. As given above, the load current is discontinuous, as the inductance of the load is low. If the inductance is increased, the current will be continuous.
2. To repeat, non-circulating mode of operation is used, i.e., only one of the bridges − #1 (positive), or #2 (negative), conducts at a time, but two bridges do not conduct at the same time, as this will result in a short circuit.
3. Also, the ripple frequency in the voltage and current waveforms remains same at 100 Hz. The output frequency is one-fourth of input frequency (50 Hz), i.e., 12.5 Hz.
4. So, for each half-cycle of output voltage waveform, four half cycles of input supply are required.
5. The firing angle (α) of the converter is first decreased, in this case for second half
6. 

   Fig. 6: Input (a) and output (b) voltage, and current (c,d) waveforms for a cyclo-converter with continuous load current.

   cycle only, kept nearly same in the third one, and finally increased in the last (fourth) one, as shown in Fig. 6b.

6. To obtain negative output voltage, in the next four half cycles of output voltage, bridge 2 is used. The current flows now in the opposite (negative) direction through the inductive load, with the output voltage being also negative. The current flows for about one complete half cycle.
7. Both the conducting thyristors turn off, as reverse voltage is applied across each of them. Both the output voltage and current are now negative. Also, the current flows for about one complete half cycle.
8. The pattern of firing angle − first decreasing and then increasing, is also followed in the negative half cycle.

It may be observed that the load (output) current is continuous (Fig.6c), as also load (output) voltage (Fig.6b).The load (output) current is redrawn in Fig.6d, under steady state condition, while the supply (input) voltage is shown in Fig.6a.

One positive half cycle, along with one negative half cycle, constitute one complete cycle of output (load) voltage waveform.

Advantages and Disadvantages of Cyclo-converter

Advantages

1. In a cyclo-converter, ac power at one frequency is converted directly to a lower frequency in a single conversion stage.
2. Cyclo-converter functions by means of phase commutation, without auxiliary forced commutation circuits. The power circuit is more compact, eliminating circuit losses associated with forced commutation.
3. Cyclo-converter is inherently capable of power transfer in either direction between source and load. It can supply power to loads at any power factor, and is also capable of regeneration over the complete speed range, down to standstill. This feature makes it preferable for large reversing drives requiring rapid acceleration and deceleration, thus suited for metal rolling application.
4. Commutation failure causes a short circuit of ac supply. But, if an individual fuse blows off, a complete shutdown is not necessary, and cyclo-converter continues to function with somewhat distorted waveforms. A balanced load is presented to the ac supply with unbalanced output conditions.
5. Cyclo-converter delivers a high quality sinusoidal waveform at low output fre-quencies, since it is fabricated from a large number of segments of the supply waveform. This is often preferable for very low speed applications.
6. Cyclo-converter is extremely attractive for large power, low speed drives.

Disadvantages

1. Large number of thyristors is required in a cyclo-converter, and its control circuitry becomes more complex. It is not justified to use it for small installations, but is economical for units above 20 kVA.
2. For reasonable power output and efficiency, the output frequency is limited to one-third of the input frequency.
3. The power factor is low particularly at reduced output voltages, as phase control is used with high firing delay angle.