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Single phase cycloconverters

- 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.