Mode of Operation of a Three phase to Single Phase Cycloconverter
Blocked Mode of operation:
Fig: Blocking mode operation waveforms
When the load current is positive, the positive converter supplies the required voltage and the negative converter is disabled. 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:
Fig: Cycloconverter (circulating current mode) with Inter-group(IG) reactor
- Both for single-phase to single-phase and for single-phase to three-phase cyclo-converters, circulating current-free or non-circulating current mode of operation was described, wherein only one of two bridge converters conducts at a time, but not both, in which case the converters would be short-circuited.
- The positive (P) converter conducts, when the current is in the positive half of the cycle, whereas the negative one conducts with the current flowing in the negative half.
- But, in circulating current mode of operation of the cyclo-converter, both the converters would conduct at a time, with an inter-group reactor (IGR) between the positive and negative groups. Though the output voltages of two converters in the same phase have the same average value, but their output voltage waveforms as a function of time are, however, different, and as a result, there is a net potential difference (voltage) across two converters.
- Due to this voltage, the reactor is inserted to limit the circulating current. The main converter − positive/negative, as the case may be, acts in the rectifier mode, and the other one acts in the inverter mode, with the average value along with the sign, of the output voltage being same.
- The sum of their firing delay angle must be 1800.
- If αp and αn are the firing angles for positive and negative group of converters, respectively, then these firing angles must be controlled so as to satisfy the condition.
- The continuous current of each group in the circulating current mode imposes a higher loading on each group compared to the non-circulating current mode of operation.
- This mode, i.e. circulating current one, would only be used, when the load current is low, so that continuous load current with a better waveform can be maintained.
- At the higher levels of load current, the groups would be blocked to prevent circulating current. Control circuits would be used to sense the level of the load current, allowing firing pulses to each group at low current level, but blocking firing pulses to one or the other group at the higher current levels.
- The reactor would be designed to saturate at higher current levels, when the cyclo-converter is operating in the non-circulating current mode, thus permitting a smaller one.