Cyclo-converter, using two three-phase half-wave converters
Cycloconverter using two three phase half wave converters:
Fig: Three-phase to single phase cycloconverter (two three-phase half-wave converter)
- A three-phase to single-phase cyclo-converter, using two three-phase half-wave converters.
- Each thyristor conducts for around 120°, with the thyristors in each converter triggered in sequence, i.e. 1, 2 & 3, whether it is P-type or N-type.
- The thyristors, 1, 2 & 3 are connected to the phases, A, B & C, respectively, in series with the load impedance. The ripple frequency is 150 Hz, three times the input frequency of 50 Hz, as this converter is a three-pulse one.
- So, the inductance in the inductive (R-L) load must be high, as compared to one used in the earlier case, to make the current continuous.
- This inductance acts as the filter for the output (load) current.
- The mode of operation here is non-circulating current one.
- The harmonic content, both in the output voltage and current waveforms, is higher than those present in the earlier case using two three-phase full-wave bridge converters. This is, because six pulses are used in a cycle for the earlier one, the ripple frequency being 300 Hz.
- Also three thyristors are used in each converter, i.e., a total of only 6 devices for two converters are needed, whereas earlier, six thyristors are used for each bridge converter, needing a total of 12 devices.
- This means that the cost is much lower, as also the control circuit in this case is much simpler and cheaper, as only three pulses per each converter are needed.
- This may be preferred, as only the harmonic content is more, which may not be a demerit in most of the applications. If it is used to drive ac motors, the high impedance at the ripple frequency is expected to make the output current near sinusoidal one, with the result that no additional filtering component is needed.