Subject : Power Electronics
Unit : AC to AC Voltage Converter
Single-phase to Single-phase Cyclo-converter Resistive (R) Load
Single-phase to Single-phase Cyclo-converter Resistive (R) Load:
Fig: Single phase to single phase cyclo-converter (using thyristor bridge) Fig:Input (a) and output (b) voltage waveforms of a cyclo- converter (R) load
- 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. 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. 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 (1α), 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 (f_{1}=50 Hz). From three waveforms, one combined positive half cycle of output voltage is produced across the load resistance, with its frequency being one-third of input frequency (f_{2}=f_{1} / 3 = 16 2/3 Hz) .
- 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. This is, because only three cycles for each half cycle is used. 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, as described earlier.
- 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 and current are now negative.
- The ripple frequency of the output voltage/ current for single–phase full-wave converter is 100 Hz, i.e., double of the input frequency. The load (output) current & voltage is discontinuous.