Three-phase to Single-phase Cyclo-converter
Three phase to single phase cycloconverter:
Fig: Three phase to single phase cycloconverter
Fig: Output voltage waveforms for a three-phase to single phase cyclo-converter.
- In the circuit of three phase to single phase cycloconverters two three-phase full-wave (six-pulse) bridge converters (rectifier) are connected back to back, with six thyristors for each bridge, are used. The ripple frequency here is 300 Hz, six times the input frequency of 50 Hz. So, low value of load inductance is needed to make the current continuous, as compared to one using single-phase bridge converters with ripple frequency of 100 Hz.
- The non-circulating current mode of operation is used, where only one converter − bridge 1 (positive) or bridge 2 (negative), conducts at a time, but both converters do not conduct at the same time. Each thyristor conducts for about 120° (π/6), i.e., one-third of one complete cycle, whereas a particular thyristor pair, say 1& 2 conduct for about 60° (π/3), i.e., one-sixth of a cycle.
- The thyristors conduct in pairs as stated, one (odd-numbered) thyristor in the top half and the other (even-numbered) one in the bottom half in two different legs. Two thyristors in one leg are not allowed to conduct at a time, which will result in short circuit at the output terminals.
- The sequence of conduction of the thyristors is 1 & 6, 1 & 2, 3 & 2, and so on. When thyristor 1 is triggered, the conducting thyristor (#5) in top half, being reverse biased at that time, turns off. Similarly, when thyristor 2 is triggered, the conducting thyristor (#6) in bottom half, being reverse biased at that time turns off.
- This sequence is repeated in cyclic order. So, natural or line commutation takes place in this case. The procedure to be followed in the triggering of the thyristors in sequence in the two bridge converters has been briefly given earlier. The firing angle (α) of two converters is first decreased starting from the initial value of 900 to the final value of 00, and then again increased to the final value of 900.
- For positive half cycle of the output voltage waveform, bridge 1 is used, while bridge 2 is used for negative half cycle. The two half cycles are combined to form one complete cycle of the output voltage, the frequency being decided by the number of half cycles of input voltage waveform used for each half cycle of the output. As more no. of segments of near 60° (π /3) is used, the output voltage waveform becomes near sinusoidal, with its frequency also being reduced.